Bulletin No. 15. U. S. DEPARTMENT OF AGRICULTURE. DIVISION OF SOILS. S.23. s 593 ELECTRICAL INSTRUMENTS DETERMINING THE MOISTURE, TEMPERATURE, AND SOLUBLE SALT CONTENT OF SOILS. LYMAN J. BRIGGS, ASSISTANT CHIEF, DIVISION OF SOILS. WASHINGTON: O i ) V E R N M i: \ T I'RI .\" T I N < ; F F ICE. 1 899. ' JWWi . *V™ OlassJLm Book_i^-^ Digitized by the Internet Archive in 2011 with funding from The Library of Congress http://www.archive.org/details/electricalinstruOObrig Bulletin No. 15. U. S. DEPARTMENT OF AGRICULTURE, u DIVISION OF SOILS. sjrx ELECTRICAL INSTRUMENTS DETERMINING THE MOISTURE, TEMPERATURE, AND SOLUBLE SALT CONTENT OP SOILS. LYMAN J. BRIGGS, ASSISTANT CHIEF, DIVISION OF SOILS. WASHINGTON GOVERNMENT PRINTING OFFICE. l 8gg. M iontgrapfc A>" \ ^' <0 V UL 2 1907 D. of a LETTER OF TRANSMITTAL U. S. -Department of Agriculture, Division of Soils, Washington, I). C, March 30, 1899. Sir: I have the honor to transmit herewith a report upon electrical instruments for determining the moisture, temperature, and soluble salt content of soils, prepared by Mr. Lyman J. Briggs, assistant chief, and to recommend that it be published as Bulletin No. 15 of this division. Respectfully, Milton Whitney, Chief of Division. Hon. James Wilson, Secretary of Agriculture. CONTENTS. Page. Introduction 7 Electrical method of moisture determination : Soil hygrometer 10 Bridge wire 11 Balancing mechanism 11 Battery switch 12 Scale 12 Telephone receiver 11 Induction coil 11 Condenser 14 Current interrupter and battery 15 Method of operating the soil hygrometer 15 Location of faults in the instrument 17 Soil electrodes and compensating temperature cell 19 Adj ustahle temperature cell 20 Carbon electrodes 21 Adj ustable metal electrode 22 Installing the electrodes and temperature cell 23 Deep electrodes 23 Shallow electrodes 24 Installing cell 24 Connection of electrodes and cell to soil hygrometer 24 Wire 24 Splicing and insulating wires 25 Adjusting the readings of the hygrometer by means of the temperature cell 26 Electrical method of determining temperature: Electrical thermometer 27 Temperature coils 29 Use of the electrical thermometer 30 Location of faults 30 Adjustment of instrument 31 Standardization of temperature coils 32 Apparatus for determining the soluble salt content of soils : Electrolytic bridge '-- 32 Rotary switch 32 1 isc of the electrolytic bridge 34 Electrolytic cell 35 Location of faults 35 5 ILLUSTRATIONS. Page. Fig. 1. Soil hygrometer closed, ready for use, showing binding posts, scale and central plunger, operating ha.ttery switch 10 2. Soil hygrometer open, showing battery, condenser, bridge-wire, and hattery switch 11 3. Sectional view of the soil with electrodes and cell properly connected to the hygrometer 16 4. Diagram of interior connections of hygrometer 18 5. Longitudinal section of adjustahlo temperature compensating cell 20 6. Rectangular carhon electrode, showing saw-cut contact with wire 21 7. Carhon electrode with longitudinal section to show construction 21 8. Splicing and insulating wires - 25 9. Resistance-temperature curve for the iron wire used in the tempera- ture coils 29 10. Diagram ol interior connections of electrical thermometer 31 11. Diagram of interior connections of electrolytic bridge . ■ 33 12. Electrolytic cell and mercury cups 34 6 ELECTRICAL INSTRUMENTS FOR DETERMINING THE MOISTURE. TEMPERATURE, AND SOLUBLE SALT CONTENT OF SOILS. INTRODUCTION 1 . The determination of the water content of soils in place by means of the variation in the electrical resistance was first investigated by Prof. Milton Whitney in 1887. He first attacked the problem by meas- uring the variation in the internal resistance of an earth battery, con- sisting - of alternate copper and zinc plates buried in the soil. This method was found not to be satisfactory on account of the polarization of the plates. Measurements of the resistance between large copper plates buried at the desired depth were next made with a direct cur- rent, but polarization again interfered seriously. The Kohlrausch bridge method, employing alternating currents, was finally successfully used, and various forms of electrodes were tested, difficulty being experienced at that time in maintaining good contact between the electrodes and the soil. In 1895 the method was further developed in the Division of Soils by Professor Whitney and Dr. F. A. Wolff. The resistance-temperature coefficient of the soil, which in the preliminary work had not been con- sidered, was determined in order that the measurements might be cor- rected for temperature. An instrument for field measurements was designed, which could also be used for determining the temperature of the soil by measuring the resistance of a small hermetically sealed cell containing an electrolyte, the temperature coefficient of which was known. In 1890 the writer eliminated the effect of temperature on the meas- urements by balancing the soil resistance against an electrolyte hav- ing the same temperature coefficient as the soil, placed in a sealed cell near the electrodes, so as to acquire the same temperature. The method of moisture determination as thus developed, together with methods of determining the temperature and soluble salt content, were published in 1897 in bulletins jSTos. 6, 7, 8, and 12 of the Division of Soils. It is the object of this bulletin to describe the instruments and meth- ods at present employed by this division in investigating the moisture and temperature of soils in the field, together with a convenient field apparatus for investigating the soluble salt content of soils. Several important modifications in the instruments and methods, as previously described in other bulletins of the division, have been made. A special instrument is now used for each of the three classes of determinations, instead of a single instrument as heretofore. This change greatly sim- plifies the instruments, makes them easier to operate, materially lessens their cost, and, in the case of the moisture and temperature instru- ments, permits the use of direct reading scales, thus avoiding, except in cases where more than ordinary accuracy is desired, the necessity of any reduction of the results obtained. The necessary information regarding the water content of the soil can of course be obtained by the ordinary method of sampling and deter- mining the loss in weight through drying, and when carefully done probably no more accurate method exists. It was for the purpose of avoiding the laborious sampling, weighing, and drying, with certain errors commonly incident thereto, and for the advantage of being able to make determinations frequently and quickly in the field, that the electrical method of moisture determination was devised. While equally valuable experimentally, this method is also capable of certain economic applications. It is believed that an important use of the moisture apparatus is to be found in connection with irrigation operations, in determining when there is sufficient water present in the soil and when it becomes necessary to add more water. It is obvious that a knowledge of the water content of the soil at any desired distance below the surface, even though it be only approximate, must prove to be of great service in the intelligent application of water. With a knowledge of the water content of the soil at various depths, and an understanding of the peculiarities of the crop grown, water can generally be supplied only as needed and all excess of water, resulting in seepage with the attendant translocation of salts which has proved to be so injurious, can probably be largely avoided. This excess of water can thus be saved to be applied in a useful way, and the deej>er rooting of the plants, resulting from the more judicious application of water, will render them less sensible to conditions approaching drought. The moisture instrument is also generally well adapted to indicate the water content of the beds of commercial greenhouses and has been used successfully in this connection in some large violet houses in the suburbs of Washington. Since the water content of the soil, however, is only one of the several factors which must be considered in success- ful greenhouse management, the indications of the instrument as apply- ing to the treatment of the plant must always be considered in connec- tion with the temperature, humidity, and amount of sunshine. When used in this manner it is believed that the apparatus may prove to be of much service in the commercial greenhouse. Used as above mentioned little more is required of the moisture apparatus than to indicate when certain limits representing drought and excess of water, respectively, have been reached, and to show at any time the state of the water content with, reference to these limits. In other words, it is a knowledge of the departure of the water content from the optimum condition rather than the absolute percentage of water that is desired. In order, however, that the percentage of water may be obtained when desired, the instrument is provided with a special scale from which the approximate water content, expressed in percent- age of the dry weight of the soil, may be determined directly, after a single standardization to give the relation between the readings of the instrument and the actual water content has been made by drying sam- ples of the soil. Since the relation betweeu the electrical resistance and the water content is not strictly the same for all soils, in case espe- cially accurate moisture determinations are required, as in the com- parison of cultural methods, it is advisable to make careful moisture determinations from time to time by sampling and drying, in order to determine the corrections, if any. to be applied to the instrument readings at different parts of the scale for the particular soil under investigation. ELECTRICAL METHOD OF MOISTURE DETERMINATION. The electrical method of moisture determination is based upon the principle that the resistance offered to the passage of au electrical current from one electrode to another buried in the soil, varies with the amount of water present in the soil. In nearly all soils throughout the range of water content favorable to plant development, it has been found by Whitney and Gardner 1 that the electrical resistance is very nearly inversely proportional to the square of the water content expressed in percentage of the dry weight of the soil. The soluble salts of the soil form with the moisture a salt solution, and it is the amount, concentration, and temperature of this salt solu- tion that determines the resistance of the soil. In order to employ the electrical resistance as a means of measuring the water content, it is necessary to maintain the temperature and the amount of salt in solu- tion constant, or to correct for their variation. Changes in resistance due to temperature are corrected by balancing the resistance betweeu the soil electrodes against the resistance of an electrolytic cell, which is buried near the soil electrodes and which is tilled with a salt solution having the same temperature coefficient as the soil itself. As regards the quantity of soluble salts present, it can be said that no gradual variation in the resistance due to a change in the salt content seems to take place during a seasou. In a few instances a sudden change in the position of the moisture curve, following very heavy rains, indi- cated that a movement of the soil or a leaching of salts had taken place. Such a change, however, will at once be apparent and can be easily corrected by restandardizing the instrument. The electrodes are buried in the soil at the desired depth at the 1 Bulletin No. 12. Division of Sails, p. 15. 10 beginning of the growing season and remain undisturbed during the development of the plant. From these electrodes insulated wires lead to the measuring instrument, which may be located at any convenient place. By this method of moisture determination one is enabled to investigate the changes in water content in the same portion of soil throughout the season, instead of dealing with different samples, as is necessitated in the tube method of sampling and drying. The advan- tage of always working with the same portion of soil is emphasized by the fact that a difference in water content of 2 or 3 per cent in dupli- cate samples is not an uncommon experience with those who have used the tube method. SOIL HYGROMETER. The moisture instrument, by means of which the resistance between the soil electrodes is measured and to which the name "soil hygrome- Fi«. 1. — Soil hygrometer closed, ready for use, showing binding posts, soalt operating battery switch. and central plunger ter" has been applied, is an adaptation of the well known Wheatstone bridge method of measuring electrical resistances. The instrument, which is illustrated in figs. 1 and 2, consists of a slide-wire bridge, provided with an induction coil, current interrupter, battery, and tele- phone receiver, suitably arranged for electrolytic measurements and inclosed in a small wooden case 8 inches long, 7 inches wide, and 4| inches high. The inside of the case is partitioned off in such a man- ner as to provide a space for the battery and also to form a small box for the reception of the current interrupter and induction coil. In the top of a partition running lengthwise through the middle of the box a series of steps is cut, on which the two springs of the battery switch are fastened in such a manner as to bring the ends of the springs directly beneath a plunger in the balancing mechanism. The central partition carrying the battery switch, together with the small case inclosing the current interrupter, can be seen in fig. 2. 11 BRIDGE MIRE. The bridge wire consists of a No. 24 B. and S. platinoid wire about 64 cm. long'. This wire is mounted upon the periphery of a wooden disk on the inside of the cover of the box in such a manner as to permit contact with a movable slide. The wire on the disk is so adjusted that there remains free at one end a portion about 16 cm. long, and at the other end a portion about 8.5 cm. in length. These are so coiled as to prevent short circuiting and are carefully soldered at their free ends to the left and right hand binding posts, respectively, as shown in the illus- tration. The wooden disk which carries the bridge wire is constructed of well-seasoned cherry and thoroughly covered with shellac. The periphery of this disk is smoothly polished and has a slight groove cut Fig. 2.- -Soil hygrometer open, showing battery, screw for adjusting condenser, bridge win battery switch. and in its surface one-fourth inch from the exposed face, the groove being of sufficient depth to retain the bridge wire in place and yet not interfere with the action of the slider in making contact with the wire. The bridge wire is stretched tightly in the groove and secured to the disk by soldering to two small pins driven radially into the disk about one- half inch apart. BALANCING MECHANISM. The balancing of the bridge is secured by the rotation of a shaft, working through a bushing inserted through the cover of the box and the bridge-wire disk, the whole being coaxial with the periphery of the disk. The bushing is retained in place by a brass collar, which is 12 soldered to its upper end and secured to the cover of the box. This collar also assists in keeping in place the celluloid cover of the paper scale. From the lower end of the shaft, passing through the cover of the box and the bridge- wire disk, extends an arm three-eighths inch wide, parallel to the face of the wooden disk supporting the bridge wire. Just beyond the periphery of the disk the arm bends upward at right angles and carries the bridge- wire contact spring, which consists of a piece of No. 32 B. and S. spring brass one-eighth inch wide, doubled back upon itself in such a manner as to bring the contact under the arm, thus protecting the contact from mechanical injury. The shaft is held in position in the bushing by means of a collar which carries the scale pointer, and is secured by a set-screw. The collar thus furnishes means of adjusting both the shaft and the pointer. Through the center of the shaft of the balancing mechanism there operates a plunger for closing the battery circuit. This plunger is keyed in such a manner as to prevent rotation within the shaft, while at the same time moving freely in the line of its axis. To the upper end of the plunger is fastened a circular handle of hard rubber which serves both to operate the plunger and rotate the balancing mechanism. The lower end of the plunger is cut down for a short distance to a smaller diameter, which works through a corresponding hole in the lower part of the shaft. A weak coiled spring, just sufficient to raise the plunger when released, is placed between the shoulder of the plunger and the bottom of the shaft. To the lower end of the plunger a small disk of hard rubber is fastened by means of a countersunk screw. This retains the plunger in place and also serves to insulate it from the battery switch. BATTERY SWITCH. The battery switch consists of two platinum-tipped springs so mounted upon the partition running through the center of the box that when the plunger is depressed the upper spring is forced down against the lower one, thus closing the battery circuit. This switch has the advantage of a rubbing contact, since the plunger has a move- ment sufficiently great to depress the lower spring for a short distance, thus causing a rubbing movement between the two springs and insur- ing a perfect contact. The switch is also highly advantageous from the fact that it thoroughly protects the battery of the instrument, since the moment the hand is taken from the handle of the balancing mechanism the plunger is released and the circuit is opened. Thus the battery is used oidy during the actual observation, and consequently has a much longer life. SCALE. The soil hygrometer is provided with a direct reading scale, the read- ings of the instrument being expressed in terms of the percentage of some definite water content, taken as a standard. In order that this may be accomplished it is necessary at the time the instrument is 13 installed to adjust either the electrodes or the temperature cell so that the reading - of the instrument may correspond with the water content of the soil at that time. The manner in which this is accomplished will be considered later. The direct reading scale is based upon the fact that within the limits between which moisture observations are usually made the electrical resistance of the soil is inversely proportional to the square of the water content, expressed in percentage of the dry weight of the soil. 1 This relation does not hold strictly for all soils as we approach the limits, but the agreement is so close that for ordinary observations it was not deemed necessary to provide the instrument with special scales for different soils. If we take 100 on the scale to express some particular percentage of water, which should be chosen to agree as nearly as possible with the most favorable water content of the soil, then 75 expresses the condi- tion of the soil when it contains but 75 per cent of the optimum water content, and 125 expresses the condition when it contains 125 per cent of the amount present in the optimum condition, and so on. The mid- dle point of the bridge wire is chosen as the 75 point of the scale. If we assume the resistance at this water content to be 1,000 ohms, for convenience in computation, the corresponding resistance for any other point of the scale can be calculated from the following formula: A\ 75" -"75 — 2 in which x represents the point on the scale, the corresponding resist- ance of which we wish to determine, E- 5 represents the resistance chosen for the 75 point, in this case 1,000, and R K represents the required resistance corresponding to the point x. For this formula the follow- ing calibration table has been prepared. Hygrometer calibration table. Scale Resistance X. fix. 140 287 135 309 13U 333 125 360 120 391 i 115 425 110 465 105 510 100 563 95 623 90 694 80 879 85 779 80 879 75 1,000 70 1, 14s 65 1,331 60 l. 56:; 55 1,860 50 2, 250 45 2,778 40 3,516 35 4,592 30 6. 252 Bulletin No. 12, Division of Soils. 14 To calibrate the hygrometer a resistance of 1,000 ohms is inserted between the middle and left-hand binding posts. The resistances given in the table are then inserted successively between the middle and right-hand binding posts. When the bridge is balanced against any of these resistances the table gives the corresponding scale reading. To avoid error, due to lack of uniformity in the bridge wire, the scale is calibrated for every 10, or better, for every 5 divisions. The bridge wire is chosen of such a length that the graduation extends through the whole available space on the scale, so as to make the scale as open as possible. The scale is protected by a sheet of transparent celluloid, the edges of which are held in place by a nickel-plated ring secured to the top of the box, as shown in the illustration. TELEPHONE RECEIVER. The telephone receiver used in balancing the bridge is of the watch- receiver form, with a hard-rubber case to minimize the danger of a stray circuit from the receiver to the earth. One terminal of the receiver is connected with the balancing mechanism by means of a spring brass friction contact between the bridge-wire disk and the outer bushing of the balancing mechanism. The other terminal is con- nected to the middle binding post of the instrument. INDUCTION COIL. In order to prevent polarization of the soil electrodes it is necessary to employ an alternating current for measuring purposes. This is secured by the use of a small induction coil, 2f inches in length, with a laminated core about one-half inch in diameter, built up of No. 15 B. and S. soft iron wire. The primary of the induction coil consists of four layers of No. 24 B. and S. D. 0. O. copper wire and the secondary of three layers of No. 36 copper wire. This coil is placed in the bottom of the small case which holds the current interrupter, and is held in position by means of cork wedges placed at the ends. The terminals of the secondary coil are soldered to the side hinges. These hinges are in turn connected to the right and left hand binding posts. The hinges thus form very convenient flexible contacts, combining both compact- ness and durability. CONDENSER. Owing to the fact that a small but appreciable capacity may exist between the two electrodes buried in the soil, it has been necessary, in order to secure sharp readings, to place a small condenser parallel with that arm of the bridge adjacent to the soil resistance, which would throw the two capacities on opposite sides of the bridge with respect to the telephone receiver (see fig. 4). In this way the disturbing capacity in the soil can be entirely eliminated, and the minimum in the receiver becomes sharp and distinct. Since this capacity between the 15 electrodes in the soil varies with different moisture contents of the soil, and under certain other conditions which have not yet been fully deter- mined, it is necessary that the condenser be capable of adjustment. A small condenser has accordingly been devised which is capable of continuous adjustment throughout the range of capacity necessary to balance the capacity effect between the electrodes in the soil. This condenser depends upon the principle that the electrostatic capacity of a series of insulated plates varies greatly with the distance between the plates, the capacity being inversely proportional to the square of this distance. Spring brass strips, 2 inches wide and 3 inches in length, are given a slight curvature, so that wheu the plates are stacked up between thin sheets of mica with the alternate plates projecting at opposite ends of the pile the plates and mica do not form one compact mass, but are separated to a greater or less degree, depending upon the curvature of the plates and their thickness. To the ends of the plates projecting from one end of the pile is soldered a common terminal, while a similar terminal is soldered to the plates at the other end. These two sets of plates are in this way thoroughly insulated from each other by means of the pieces of mica, while the distance between them can be greatly varied by simply compressing the pile. A small hard rubber plate is placed upon the top of the condenser as thus built up, and the whole is placed in a frame which has a small compression screw, the end of which works in a corresponding depression in the center of the hard- rubber plate. Upon changing the position of this compression screw we compress or release the condenser, and so vary its capacity at will. The terminals of the condenser are connected by means of the hinges with the middle and right-hand binding posts. CURRENT INTERRUPTER AND BATTERY. The current interrupter consists of a small pocket buzzer giving about 120 interruptions per second. This buzzer is placed in a small case and packed with cotton wool, which serves to inufrle all sound from the buzzer and thus facilitates the discernment of slight sounds in the receiver. This form of interrupter is of great convenience in such an instrument, as it is protected from injury and permits rough treatment without requiring readjustment. The case for the battery is of such dimensions as to allow any dry cell of standard size to be used. This enables one to secure without difficulty a dry cell adapted for use in the instrument when it becomes necessary to supply a new cell. The battery is held in its case by means of cork wedges, which permit it to be easily and quickly removed. METHOD OF OPERATING THE SOIL HYGROMETER. In using the soil hygrometer in the held it is customary to provide a small platform, consisting of a stake with a board nailed across its top, upon which to rest the instrument. The arrangement of the connec- tions is illustrated in tig. 3. The wire leading from the temperature 16 cell is connected to the left-hand binding post of the instrument; the wire which is common to both cell and electrode is attached to the middle binding post, while the third wire is attached to the right-hand binding post. The ends of the wires should be scraped bright with a dull knife, in order to insure good electrical connections with the bind- ing posts. These connections being made, the telephone receiver is pressed tightly against the ear and the handle of the instrument pushed down, when a buzzing sound will be heard in the receiver. Holding the .fi'iG. 3. — Sectional view of the soil, with electrodes and cell properly connected to the hygrometer. handle down so as to keep the battery switch closed, the pointer is rotated to either right or left until the position is found at which the note in the telephone receiver is no longer heard. On rotating the pointer to either side of this position the sound in the receiver should gradually increase. In case difficulty is found in locating the exact position of balance, it will be found to be of assistance to rotate the pointer rapidly back and forth over the position of no sound, locating points of equal sound intensity on either side. The mean position 17 between these two points gives the position of balanee, and the number opposite the pointer gives the desired reading. In case it is not possible to obtain a well-defined minimum in the sound in the receiver, recourse should be had to an .adjustment of the screw of the condenser inside the box. Upon compressing or releasing the condenser a position of the compression screw will be found at which the minimum in the receiver is much more distinct. In making the preliminary readings with the instrument it will usually be found more satisfactory to release the condenser as much as possible by loosening the compression screw. If a number of electrodes are to be read by the same instrument, it may be found necessary to adjust the condenser for the various electrodes. Usually, however, a common adjustment of the condenser can be found which will be satisfactory for all readings. If the instrument is to be shipped, it is advisable to compress the condenser in order to prevent possible disarrangement of the condenser plates. It is also necessary in such cases to place a washer between the head of the plunger and the end of the shaft, or else disconnect one terminal of the battery, in order to prevent the closing of the battery circuit during transportation. LOCATION OF FAULTS IN THE INSTRUMENT. It may sometimes happen that during a journey or through careless handling an instrument may get out of adjustment or fail to work properly. Some of the faults which are likely to occur will conse- quently be discussed. In case there is no sound in the telephone receiver when the battery switch is closed, the failure may be due to (1) a run-down battery; (2) lack of contact between the two springs of the battery switch, due to dirt on the platinum contacts; (3) improper adjustment of the current interrupter; (4) broken connections, either in the primary or secondary circuits of the induction coil; (5) failure of the contact spring of the balancing mechanism to make contact with the bridge wire; (G) broken receiver circuit. The question as to whether the difficulty exists in the bridge connec- tions or is duo to the current interrupter can generally be decided by closing the battery switch and placing the ear close to the current- interrupter box, when, if the interrupter is working, a slight note will be heard. If the interrupter does not work, it should be first examined. This is done by taking the cover off from the small case containing the interrupter, which will be found packed in cotton. The current inter- rupter should be taken out and the metal cover removed. Keeping the battery switch closed, it should now be determined whether the interrupter can be made to work by some simple adjustment of the contact. If this can not be done, the connections with the battery and induction coil should be examined. If these connections are found to be perfect, it is probable that the battery is defective and should be replaced by ;t new one. 19805— No. 15 L> 18 In case the current interrupter works satisfactorily and still no sound can be beard in the receiver, it is evident that a broken circuit exists This can generally be found without difficulty by carefully examining the connections, which should be in accordance with the accompanying diagram (fig. 4). If the difficulty appears to be in the bridge connections, the bridge- wire slider should be first examined, to see whether it makes contact ^\0GE_JV/^ BRIDGE WIRE CONTACT TO TELEPHONE RECEIVER BATTERY BATTERY SWITCH Fig. 4.— Diagram of interior connections of hygrometer with the bridge wire, and adjusted, if necessary, by carefully bending the contact spring up or down until the slider makes contact for all positions of the scale. This is the cause of the trouble when a note can be heard in the receiver for certain parts of the scale only. In case the bridge wire has become loosened, it should be unsoldered from one of the pins in the periphery of the disk, carefully placed in the groove in the disk, drawn taut, and resoldered. ! 19 In case the faults seem to be in the receiver circuit, the connections inside the box should be examined, as well as the screws binding the cord terminals to the telephone receiver. If necessary, the face of the receiver can be removed by unscrewing it and the inside connections examined. A pair of test coils accompany each instrument. These coils are to be connected Avith the three binding posts in such a manner that the com- mon terminal of the two is connected to the middle post and the single terminals to the right and left hand binding posts. When these two coils are connected to the instrument in the manner described, a balance of the bridge should be obtained at the 75 point of the scale. In case the balance can not be obtained at or near this point, the instrument is either oat of adjustment or else broken connections or short-circuiting exists. If no balance of the bridge can be obtained by means of this coil, all of the connections should be carefully examined to see if they correspond with those given in the diagram (fig. 4). The trouble might also be due to a short-circuiting in the condenser, which would connect directly the middle and left-hand binding posts. In such cases the connection of the condenser with the middle hinge should be broken in order to see if this remedies the trouble. If a balance is obtained at a position differing by two or three divi- sions from the 75-poiut mark, the instrument is out of adjustment. This can be remedied by loosening the screw in the pointer collar, and, hold- ing the contact arm rigidly at the position of balance, bringing the pointer to coincide with the 75-ppint position, and then tightening the screw in the collar. In case it should be necessary to remove the shaft of the balancing mechanism for the purpose of repairing or renewing the plati- num bridge-wire contact, this adjustment should always be made after the instrument is once more assembled. The adjustment of the instrument should always be determined by the use of these test coils before beginning field work. A failure to secure a balance of the instrument when connected with the soil elec- trodes in the field, provided the instrument is already in adjustment, will be discussed later (p. 2G). SOIL ELECTRODES AND COMPENSATING TEMPERATURE CELL. It has been mentioned above that the effect of varying temperature on the soil resistance is compensated by balancing this resistance against the resistance of an electrolytic cell containing a solution at the same temperature and having the same temperature coefficient as the soil. The resistance of different soils, however, varies so greatly that in order to bring the readings upon the most sensitive portion of the instrument scale it is necessary that the ratio of the soil resistance to that of the temperature cell be capable of adjustment. This can be accomplished by varying the resistance of the compensating cell or by changing the area of the soil electrodes. Both methods have been employed, but the former is more convenient and is the one generally recommended. 20 ADJUSTABLE TEMPERATURE CELL. The adjustment of the instrument reading by changing the resistance' of the temperature cell instead of changing the area of the soil elec- trodes lias the important advantage of obviating the necessity of dis- turbing the electrodes. This also permits the use of carbon instead of metal electrodes, the former seeming to possess some advantages over over the latter. The adjustable temperature cell is illustrated in tig. 5, which, shows a longitudinal section. It is constructed of brass and hard rubber, and is made in two parts, one of which is capable of adjustment within the other. The cell is used in a vertical position, the cylindrical bulb being at the top. The lower part of the outer portion consists of a piece of hard-rubber tubing, 3 inches long and five-sixteenths inch inside diameter, with walls one sixteenth inch thick. Into the lower end an unburnished, nickel-plated, brass plug is sealed, which forms one electrode. The brass cyl- inder, nickel-plated on the inside, constitutes the other electrode. Suitable insulated connecting wires are soldered to the two electrodes and secured to the sides of the cell with wax. The outer surfaces of both electrodes are insulated by thoroughly cov- ering them with insulating wax or paint. The ad- justable plunger consists of a hard-rubber rod, 5 inches long and one-fourth inch in diameter, which works through a short piece of closely fitting flex- ible rubber tubing attached to the tubulure on the end of the brass cylinder. The cell is filled to a point one-fourth inch above the upper end of the hard-rubber tubes with a salt solution which has the same electrical temperature coefficient as the soil. The solution consists of nine parts of a solution of four-fifths normal sodium chlo- ride and one part of 95 per cent alcohol. The deter- mination of the resistance-temperature coefficient of soils, which gave the data for the preparation of this solution used in the temperature cell, has been described in Bulletin No. 7 of this division. When the plunger of the adjustable cell is pushed down into the solution, a portion of the solution is displaced and the resistance of the cell is increased. By thus varying the position of this plunger we are enabled to increase continuously the resistance to about three times the resistance of the cell when the inner tube is drawn down entirely out of the lower portion of the cell. In adjusting the reading of the instrument the plunger is moved up or down until the desired reading is obtained. This form of compensating cell has sufficient Fig. 5. — Longitudinal sec- tion of adjustable tem- perature compensating cell. 21 range to permit adjustment in all cases except when the electrodes are much too large or too small. In such cases it is necessary to increase or diminish the size of the electrodes and then make the final adjust- ment by means of the cell as above. CARBON ELECTRODES. Carbon forms a more satisfactory electrode than metal, since it seems to give a more perfect contact with the soil grains, due undoubtedly to the fact that the moisture permeates to some extent tlie carbon itself. A form of electrode which has been used extensively in connection with shallow depths consists of a rectan- gular strip of carbon 3 inches long, five eighths of an inch wide, and three-sixteenths of an inch thick. A series of diagonal cuts (see fig. 6) are made in the edge of the carbon, just, wide enough to allow the connecting wire to be forced into place, the contact being after- wards covered with insulating wax. A more substan- tial contact is made by boring a hole through the carbon strip near one end, in which is inserted an uulacquered, round headed, brass machine screw, provided with a nut. A finch 8-32 machine screw is suitable for this purpose, the hole beiug of such a size as to make a snug fit with the screw, which is drawn tight by means of the nut. The connecting wire is then soldered with rosin in the slot of the screw-head and allexposed metal covered with insulating paint. A form of electrode which is well adapted to both deep and shallow work is illus- trated in fig. 7, together with a longitudinal section, show- ing the manner in which con- tact with the wire is made. The electrode is one-half of an inch in diam- eter and 2£ inches long, the general form being that of a paraboloid of revolution. This form is given so that when the electrode is forced into a conical hole in the soil perfect contact will be made at every part of the electrode. A cylindrical cavity with corrugated sides is made in the upper end of the electrode, into which the end of the connecting wire is inserted, and which is then nearly filled with fusible metal which holds the connecting wire firmly in place and makes perfect contact Fig. (i. — Rectangu- lar carbon elec- trode, ahowiug saw -cut contact with wire. Pig; 7.— Carbon electrode, with longitudinal section to show con- struction. 22 with the carbon. The upper end of the electrode arid any exposed por- tion of the conducting wire are covered with insulating wax. A modification of the electrode just described cau be constructed in the laboratory from unplated dense electric-light carbons (preferably those made for inclosed arc lamps). These carbons, which are one-half of an inch in diameter, are sawed into 3-inch lengths and ground to a conical point on a stone. A three-sixteenths inch hole is then drilled in the other end to the depth of half an inch, which is afterwards cor rugated by means of a suitable turning tool. The end of the insulated wire, which has been scraped bright and clean, is bent into a loop and inserted in the corrugated cavity, around which is run a fusible metal. Wood's fusible metal, consisting of 1 to 2 parts of cadmium, 2 parts of tin, 4 parts of lead, and 7 to 8 parts of bismuth, which melts at 60° to 70° C, has been found suitable. It is advisable to heat the carbon about the cavity with a Bunsen flame before running in the metal, so that the latter will not be cooled by coming into contact with the carbon. The metal and wire should finally be thoroughly covered with insulating wax as before. ADJUSTABLE METAL ELECTRODE. When the adjustment is made by changing the area of the electrode, it is most convenient to use metal electrodes of nickel-plated copper wire of about No. 24 B. and S. gauge. Two of these wires, 4 or 5 feet in length, are attached at one end to two insulated connecting wires by means of the telegraph splice (see p. 25), the conducting wires being of sufficient length to reach from the point where the electrodes are to be buried to where the instrument is located. The splice is now thoroughly insulated with wax and tape (see p. 25), and the wires are buried at the desired depth, parallel to each other and from 1 to 2 feet apart, by excavating two very narrow ditches in the soil, after which the soil is thoroughly tamped back into place. When the ground about the electrode has become thoroughly settled, which generally requires two or three days, a small excavation is made in the soil at the free end of the wire, and small portions of the electrode are cut off until the desired reading is obtained. It is, of course, necessary to have the electrode entirely covered at the time the observation is made. This method of adjustment is not as convenient as that of the adjustable temperature cell, and is open to the rather serious objec- tion that the electrodes are not in a normal condition and are liable to undergo slight subsequent changes. If the investigation of the water content of a certain layer of the soil is desired, the wire is bent into a zigzag form of such dimensions as to traverse the depth of the layer under investigation. When this form of electrode is desired it is" not necessary to use the adjustable temperature cell, and it has been customary to use a form which is her- metically sealed, although the adjustable cell is equally well adapted to this use, and possesses the advantage that the final adjustment can be made by the cell instead of by means of the electrodes. 23 INSTALLING THE ELECTRODES AND TEMPERATURE CELL. In the form of moisture instrument previously used by tliis division a shelter box was placed near the plats of ground on which moisture investigations were being made, and the various electrodes and tem- perature cells were connected by underground lead-covered cables with these shelter boxes in which the moisture instruments were kept. The terminals of the various electrodes and temperature cells were brought up to a suitable switch board, which enabled connections to be quickly made with the moisture instrument. This arrangement is somewhat expensive, however, especially in case the different plats under investi- gation are some distance apart. It is therefore recommended that the three wires leading from the electrodes and the temperature cell be brought to the surface a short distance from the electrodes, and that the soil hygrometer be carried from point to point, and connected with these wires at the time of taking the observations. In installing the electrodes for moisture observations it is of course important that a typical portion of land be selected, special precaution being taken to select ground which is not depressed at that point, as any depression would tend to permit an excessive accumulation of water, and thus render the moisture observations inaccurate when applied to the conditions over the whole plot. It is also important that the electrodes should be located in such a position as not to be disturbed by cultivation. If the electrodes are less than G inches deep, the treading of a man or a horse on the ground immediately above the electrodes might so alter the compactness of the soil and its relation to water as to change materially the readings of the instrument. In such cases, therefore, both the electrodes and temperature cell must be so placed as to be out of reach of the culti- vator and the horse, and care must be taken not to tread on the earth immediately above the electrodes when taking the observations. At lower depths such precaution is not necessary. For most held crops in Eastern soils the water content at a depth of 5 to 8 inches seems to give the most important information regardiug the moisture content of the soil. The depths at which moisture observations should be carried must necessarily vary to some extent with different soils and under different conditions, and the particular depths at which the electrodes should be placed will suggest themselves to the investigator. DEEP ELECTRODES. In installing the electrodes at the lower depths an inch auger hole is made in the soil down to the upper limit of the depth at which elec- trodes are to be placed. A stick, the end of which is in the form of a cone, three-eighths inch in diameter and 2 inches long, is then forced down until the base of the cone is even with the bottom of the original auger hole. The electrode is then carefully lowered by means of its connecting wires into this cavity prepared for it, and pushed firmly 24 into place with the other end of the stick. The hole is then filled with moist soil, care being taken to thoroughly tamp the soil with the rod during the process of refilling. The other electrode is buried in a similar way at a distance of 2 or 3 feet from the first. The resistance between the electrodes is practically confined to volumes of soil not exceeding G inches in diameter, with the electrodes as centers, so that there is no special advantage in separating them further, unless it is to avoid the effect of a possible lack of uniformity in the soil. SHALLOW ELECTRODES. In case the electrodes are not to be located deeper than the land was plowed the hole can generally be made by the stick alone. The elec- trode is then forced into place by means of the stick, and the hole filled with earth and thoroughly tamped as before. INSTALLING CELL. The adjustable temperature cell is filled with the salt solution (see p. 20) and buried in the soil somewhat to one side of a line joining the two soil electrodes and at a depth such that the lower part of the cell occupies the same layer of soil as the electrodes. This is conveniently done by boring a hole similar to that made for the electrodes in which the cell is placed, any vacant space being carefully filled with soil. To adjust the cell a small excavation is made above the cell to expose the plunger. In case the cell is located so deep that such an excavation is not practicable the plunger may be spliced to a stiff wire (No. 10 iron wire is satisfactory) which is cemented in the hole in the end of the plunger by means of the insulating wax. The wax should be allowed to become thoroughly hardened before attempting the adjustment. CONNECTION OF ELECTRODES AND CELL TO SOIL HYGROMETER. The wire leading from the upper electrode of the temperature cell is spliced (see p. 25)' to the wire leading from one of the soil electrodes, the common terminal, which is joined to the middle binding post of the soil hygrometer, being known as the combination wire. The other wire from the cell is known as the cell wire, and is joined to the left- hand binding' post, while the wire from the other carbon is connected with the right-hand binding post. (See fig. 4.) The wires used for connection should in all cases be provided with that grade of insulation known to the trade as " waterproof," either when used in the soil or when exposed to the weather. Where wires are protected from rain or excessive dampness a cheaper grade of in- sulation may be used. In making long connections a three-conductor lead-covered c?Jble, which cau be safely buried under ground, is con- venient and not expensive. The three conductors of this cable should be provided with insulation of different colors to facilitate connections. 25 SPLICING AND INSULATING WIRES. Care must be exercised in splicing - wires. Tlie best form of' connec- tion for field use is the telegraph splice, illustrated in fig. 8, a.^ In mak- ing this connection che insulation should be removed from each wire for a distance of at least 3 inches from the end and the wire scraped bright and clean. The wires are then brought together, their ends pointing in opposite directions and twisted together at a point about three-fourths of an inch from where the insulation begins. This forms the middle part of the splice. Grasping this firmly in a pair of pliers, the free end of each wire is wrapped tightly around the straight por- tion of the other, as shown. This splice should be used in all cases where it is necessary to lengthen wires. In temperature work these wires should be soldered if possible after being spliced. The method of joining one wire to the middle of another, as when Fifi.8. — Splicing and insulating wires: (a) Telegraph splice; (b) Side connection; (c) Taping the Bplice. making connection between the temperature cell and the soil electrode, is illustrated in fig. 8, b. The insulation is removed for a distance of li inches from the wire to which the splice is to be made. The end of the other wire is prepared as before. The wires are now tightly twisted together close to the insulation for about half an inch, and then the end of the free wire is wrapped helically about the other, as shown. All splices should be thoroughly insulated, both to prevent the con- tact from being impaired through subsequent exposure and to prevent the occurreuce of stray circuits. The exposed portions should be first covered with insulating wax, which is applied when hot. The prepara- tion known as "Chatterton's Compound" has been found very satisfac- tory. A thin coating of this will suffice, but care should be taken to see that all parts are covered. It is then advisable to wrap the con- nection with insulating adhesive tape, which is wound spirally about the splice, as shown in fig. 8, c. 26 ADJUSTING THE READINGS OF THE HYGROMETER BY MEANS OF THE TEMPERATURE CELL. After the soil about the electrode has been allowed to settle, which usually requires two or three days, and preferably after a heavy rain, the wires leading from the electrodes and the temperature cell are con- nected with the instrument in the manner described (see p. 24), and a reading of the instrument is taken by pushing down the plunger in the handle until a note is heard in the telephone receiver, and then rotat- ing the handle until a point is found at which the note vanishes. The inner rod of the temperature cell is then raised or lowered until the desired reading on the instrument is obtained. By pushing the inner rod into the temperature cell the reading on the instrument becomes higher; conversely, by raising the inner rod the instrument gives a lower grading. In case it is not possible to secure the balance on the instrument — that is, a point where there is no sound in the receiver for any position of the inner rod of the adjust- able temperature cell — it becomes necessary either to increase or dimin- ish the area of the soil electrodes. If the sound seems to be weaker when the inner rod of the compensating cell is pushed entirely down, and the pointer is at the lower number, then the soil electrodes are too small and must be made larger. If, on the contrary, the sound in the receiver diminishes in volume when the pointer approaches the higher numbers and the inner rod in the temperature cell stands in its highest position, then the electrodes are too large and must be cut down. In case no difference can be perceived in either of the positions, it will be necessary to change the size of the electrodes and observe the effect. In all cases it is desirable to adjust the instrument so that 100 on tbe scale represents as nearly as i>ossible the optimum condition as regards water content. When the instrument is used in this manner the instru- ment readings represent the percentage of the optimum water content in the soil. The scale is consequently much superior to purely arbi- trary units, since it gives a definite idea of tbe amount of water present. In case it is desired to determine the percentage of water in the soil, a number of samples for moisture determination should be carefully taken at the depth of the electrodes at a time when the soil is in its most favorable condition, i. e., when the reading of the instrument is near 100. At the same time a careful reading of the instrument should be taken. The value of the 100 point of the scale in actual percentage of water can be determined from tbe following equation: in which P x is the actual percentage of the water as found from the samples, which correspond to the reading x on tbe instrument at tbe time the samples were taken, while P m is the required percentage of the water corresponding to the 100 point on tbe scale. 27 To take a concrete case: Suppose that the value of P, as found from sampling' and drying was 18.6 per cent, while the reading - x of the instrument was 95, we then have P m = 18.0 q^ = 19.0 per cent, or the water content corresponding to the 100 point on the scale is 19.0 per cent. Having determined the water content for the 100 point, the water content corresponding to any other reading can at once be found by multiplying the water content at 100 by the scale reading expressed as a percentage. To illustrate again : The water content when the instru- ment reads 80 in the above example would be 19.0 x .SO = 15.7 per cent; when the reading was 125, it would be 19.0 x 1.25 = 23.5 per cent, and so on. As has been mentioned before, the inverse square law upon which the moisture scale was constructed is not strictly applicable to all soils, and Avhen especially accurate results are desired, the departure from this law for the soil in question should be determined by comparing the instrument readings with careful moisture determinations, from which the correction to be applied to the instrument readings can be obtained. For example, suppose from sampling and drying, the water content when the instrument reads 80 was found to be 10.7 per cent instead of 15.7 per cent, as above, then, since the instrument reading was 1 per cent too low, at 90 it would be 0.5 per cent too low, at 95, 0.25 per cent too low, and so on. A similar determination should be made above the 100-point position to determine the departure on that side. ELECTRICAL METHOD OF DETERMINING TEMPERATURE. The change in the electrical resistance of metals with temperature enables us to employ this property as a thermometer by measuring variations in the electrical resistance of a suitable coil of wire located at the desired point. This method has the very important advantage of enabling the temperatures in very inaccessible places to be deter- mined, owing to the fact that it is only necessary to have a small tem- perature coil at the point where it is desired to make the measurements, while the measuring instrument can be located at any convenient place, connection with the temperature coil being made by means of wires. A convenient instrument for this purpose has been devised and lends itself readily to the measurement of temperatures in inaccessible places, such as the temperature of the soil at various depths, the temperature of fermenting tobacco heaps, of rooms, of tanks, and, in fact, at any point where it is desired to determine the temperature and where it is inconvenient to use mercurial thermometers. ELECTRICAL THERMOMETER. This instrument is of the same dimensions and general construction as the soil hygrometer, the essential difference between the two instru- 28 ments being that in the soil hygrometer two arms of the bridge — namely, the soil resistance and the temperature cell — are outside the instrument, while in the electrical tbermometei it is necessary to have only one arm of the bridge — namely, the temperature coil — outside the instrument. In other words, the compensating cell in the soil hygrometer has been supplanted in the electrical thermometer by the comparison coil within the instrument. The condenser is omitted, since the slight capacity of the temperature and comparison coils are practically equal and no disturbing effect is noticed. The comparison coil is made of manganin wire, the resistance-temperature coefficient of which is practically zero at ordinary temperatures. Different metals vary in the change in electrical resistance which they experience with the change in temperature. In order to make the thermometer sensitive it is important to use a metal having a high temperature coefficient. For this purpose iron has been selected, as it has the greatest temperature coefficient of any of the common metals and also possesses the advantage of a relatively high specific resist- ance, thus necessitatiug the use of only a small quantity of wire and enabling temperature changes to be taken up more quickly. The wire in the temperature coil is of No. 36 B. and S. gauge, drawn from iron as free from carbon as possible, and is covered with single cotton insula- tion. Since the temperature coefficient of iron seems to vary greatly with the amount of carbon present, it was deemed necessary to deter- mine the temperature coefficient. This was done by making an open coil of a suitable length of the wire, providing it with heavy copper lead- wires, and placing the whole in an oil bath which was gradually heated up to 160° F. and then allowed to cool slowly, the resistance of the wire being noted at 5 degree intervals as the bath cooled. The variation in resistance with tbe temperature, computed from the curves actually obtained, is given in the following table and is shown graph- ically in the accompanying curve: Calibration table for electrical thermometer. Tempera- ture (degrees F.). Resistance . (ohms). 77.8 10 80.6 20 83.3 30 86.0 40 88.7 50 91.5 60 94.3 70 97.1 80 100.0 90 102.9 100 105.9 110 108.9 120 112.0 130 115.2 140 118.4 150 121.5 160 124.7 29 The resistance-temperature curve of the iron wire used iu the tem- perature coils having been determined, the resistance of the temper- ature coil for every 10 degrees throughout the desired range can be found from the curve. The temperature instrument can then be cali- brated by balancing the instrument against the resistances correspond- ing to these temperatures. The usual range of temperature for each instrument is 100 degrees on the Fahrenheit scale, which may be selected from any part of the absolute temperature scale up to temper- atures which would destroy the temperature coil. This can be accom- plished by adjusting either the resistance of the comparison coil or the temperature coils. The former method is preferable, since the temper- ature coils can then be used in connection with any instrument. Owing to the fact that the variations in the resistance in the temper- O 10 20 30 y LBJe'07 LIBRARY OF CONGRESS ■■III 0D02fcfl34Sfia