' . ■ - THE EFFECT OF SHORT ELECTROMAGNETIC WAVES ON A BEAM OF CATHODE RAYS BY CLAUDE JEROME LAPP A.B. Albion College, 1917 A.M. University of Illinois, 1920 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN PHYSICS IN THE GRADUATE SCHOOL OF THE UNIVERSITY OF ILLINOIS, 1922 URBANA, ILLINOIS Digitized by the Internet Archive in 2015 https://archive.org/details/effectofshorteleOOIapp UNIVERSITY OF ILLINOIS THE GRADUATE SCHOOL May 192JL I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SUPERVISION BY C LAU DE JFDn;.:^ lap? ENTITLED,. THE,„_EFEUCT OF. SHORT ELECTRO.;! AO NET I C W AVE S 0 0 A eea:: of cathode BE ACCEPTED AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IT PHYSICS Recommendation concurred in* Committee on Final Examination* TABLE OF CONTENTS I INTRODUCTION 1 II DESCRIPTION OF APPARATUS 2 III OPERATING CONDITIONS 14 IV PHOTOGRAPHIC MANIPULATION 17 V MEASUREMENTS IS VI RESULTS 30 VII DISCUSSION 41 VIII SUMMARY 46 I INTRODUCTION Some years ago J.J. Thomson 1 * 3 advanced a theory of light which had properties characteristic of both the emission theory and the usual form of the undulatory theory. While lecturing: in 1911, he proposed as an experimental test to the theory that if a stream of electrons had a strong beam of light thrown directly across their path slight deflections of the electrons might be expected. C.T. Knipp^ attempted the experiment in the following year, using a cathode beam twisted into a spiral, by means of a magnetic field, wmcn fell on a photographic plate leaving a trace in the form of a circle. Although much work was done at that time in the laboratory by Knipp, and later by O.A. Randolph (1513) and also C.F. Hill (1915) ; yet the difficulties of obtaining high vacua, together with the great mechanical complications, prevented satisfactory results from being obtained. Owing to the fact that since that time some prominent physi- cists have modified their views concerning the electromagnetic 1. J.J.. Thomsen, "Electricity and Matter" , Ch.O, pp 50-70; Phil. Mag. , V. 235, Feb. 1910. 3 C.T. Knipp, Phys.Rev. , V. 04, p 477. 3. H. Bateman, Phil. Mag. , V. 350, p 405, 1917. A. Einstein, Phys. Zeitschr. , V.18, p 131, 1517. See also J. H, Poynting, Phil. Trans., V. 171, p 377. W. Wei n, Ann. Phys. Chem. , Ba. 47, p 027. K. A. Lorentz, Encyklopadie der Mathemat iscnen Wissenschaf ten, Bd.47, p 327. Sir J. Larrnor, Proc. Inst . Congr. of Math., Cambridge, V.l, 1313. E. Cunningham, "The Principle of Relativity", Ch. 15, 1914. D.K. Mallik, Phil. Mag. , p 144, July, 1913. H. Bateman, Phil. Mag. , Oct. 1913, han.1914; "Messenger of Mathe- matics", May, 1915; Amer.Journ. of Math., Apr. 1915. W. G. Brown, Phil. Mag. , p 283, Aug. 1315. beam of Cathode rays 5 Plate 5). A McLeod gauge capable of measuring 0.00001 mm. of mercu- ry pressure with a difference in level of 1 mm. determined accurate- ly tiie lower limit of tiie vacuum while tne apparatus was in operation ‘fne electron discharge chamber (See Plate 5, Fig.l) was con- structed from a cylindrical glass jar 9.3 cm. in diameter anu ?6 cm. long, inside measurements. Three holes, each 3.5 cm. in diameter, were drilled through the jar, one on the bottom and one on each side, tneir centers being 3.5 cm. and 6.5 cm., respectively from the bot- tom. A P 3 O 5 bulb with ground joint connection was sealed into the bottom with Bank of England sealing wax. The seat or a ground joint, the plug of which carried a Wehnelt Cathode (See Plate 3) was sealed into the right side, while a tube lo cm. long closed by a quartz window at the outer end, and silvered on tne inside was sealed into the left side. Tne axes of these two tubes were parallel. A charcoal bulb and also the exhaust tube branched from the silvered tube near the seal. The radiation used in the experiment was admit- ted through tne quartz window into this tube, through which it was : conducted into tne electron discharge chamber. Six hundred and forty turns of wo. 13 copper wire were uniformly distributed in two layers over the discharge chamber in a space or 81 cm. The radius of the winding was increased over tne wax seals by carrying tne wire on a wooden support, wnicn extended with tne coil over the end of tne chamber. This arrangement gave ventilation to the wax seals, kept them from melting, and also gave opportunity to view any phe- nomena inside tne chamber. A source of electrons suitable for the experiment nad to be developed. (See Plate 3), A small beam obtained from a large one by means of a platinum diaphragm could not be used because of tne . . . ■ . ■/ ' * . . « » - V. 4 .4 1 | •' ’ . - . . . . . . Cathode: Ray Exp P late t\ s Z ft Oj flu/ib Quarfz Plait Anode Camera ■Solenoid binding /To Air Pumps' / \ n3/; Charcoal Bulb Spring Coupling Electron Discharge Chamber Cathode Ray Exp P/ote: ti-3 8 presence cf a strong magnetic field which twisted the beam into a spiral. These who had worked on the experiment before had used a hot lime cathode made by placing a speck of sealing wax on a strip of platinum heated by an electric current. This source of elec- trons had two faults; first, it gave a very large beam and second, it was very short lived, sometimes lasting only a few seconds. A source, to be successful for the work, had to give a very small, compact, permanent beam of electrons. A strip of platinum 0.5 mm. wide was cleaned with nitric acid and amonium hydroxide. A tiny drop of strontium hydroxide was placed on the strip after which it was dried by gently heating with an electric current. After the second application the strip was heated to 500° C to harden the deposit. A small, almost microscopic piece cf barium re3inate was then placed centrally on the spot and the whole carefully heated so as- to evaporate the resin and leave barium oxide. After two or three coats cf barium oxide the strip was glowed to cherry red for several minutes in order to drive off all organic material. A coating of approximately 0,1 mm. in diam- eter was thus obtained which gave an intense and compact permanent beam cf electrons without the use of a diaphragm. The temperature at which the organic material is driven off is very important. 'Too high a temperature causes some sort of chemical change, leaving a dark deposit which does net produce a good electron beam. If the process is carried on slowly under a microscope, the heating current can be regulated so as to leave a white deposit which is most desirable. A good beam has been obtains 6. C.T. Knipp, Phys.Rev. , V. 34, p 58. Nellie Horner, Am. Jour n. Science, p 591, 1513. . . . . . . . 9 from tne deposit left by one speck of barium resmate alone. Tne anoae consisted of a small piece or wire brought into tJae discharge chamber through tne exhaust tuoe, ending at tne side about 5 cm. in front of tne catnode. A potential dinerence of about 3OU0 volts was maintained be- tween tne electrodes, during tne operation of tne experiment, oy one tnousand small storage cells, These were connected tarougn two water resistances and a paraffine switch for protective purposes. A camera was placed within tne discharge chamber at the end opposite tne catnode. Tne chamber was closed by a tnick plate glass, carrying a winch connected to tne camera plate holder (See Plates 1 and o). The camera used was a small brass cylindrical box. (See Plate 1 and Fig. 3). Tne body was 81 cm. in diameter, closed at one end except for a circular opening 3.8 cm. in diameter and 3 cm. off center tnrough which the photographic plate was ex- posed. a second cylinder with two sets of cross supports just fit- ted into tne first. The plate holder, a fiat brass disn ? cm. in diameter, was secured to a ©haft which extended tnrough tne cross support of tne second cylinder, leaving tne nolder free to rotate. The photograpnic plate was stuck to the nolder witn half and half wax, after which it was made circular with a diamond glass cutter. When the parts were assembled, the photographic plate was pressed snugly against tne face of tne camera with only a small portion ex- posed. The shutter, a disn of aluminum with a hole in one 9ide 2.8 cm. in diameter and 2 cm. off center, so tnat it exactly matched the hole in the face of the camera, was mounted inside a cap which fitted over the face of the camera. This cap carried a system of levers which held the shutter in place, except when it was released . . • . • . . t . • ' : ■ fl . . . , 13 by a Biagnet on the outside of the discharge chamber, acting on a small piece cf iron attached to one of the levers. The shutter was given mechanical motion by a double spring which rotated it once around each time it was tripped. A willemite phosphorescent screen was deposited on the shutter in such a position that it was exposed when the shutter was at rest. This enabled the operator to see the configuration that would be gotten on the plate as a picture, b ef ore the picture was taken. After a little experience the operator could adjust the image on the screen to any desired size by slightly ro- tating the cathode and regulating the current through the solenoid. Several kinds of photographic plates were tried. The electron sensitivity of a photographic plate appears to be in no way con- nected 'with the light sensitivity. The plates finally adopted were Imperial (special) Lantern Plates, manufactured by "The Imperial Dry Plate Co. Ltd. , Crickelwood, London. They had an exceedingly smooth gelatine surface and a lew sensitivity to light. Two baffling plates were equally spaced in the discharge chamber between the cathode and the camera in order to shut off any stray light effects which might darken the plates. The holes cut through these plates to allow the spiral beam to pass were 2,5 cm. in diameter. Two sources of electromagnetic radiation were used; a ninety degree carbon arc, and a Ccolidge X-Ray tube (See Plate 4). The arc using white flame carbons and 30 amperes current was placed in- side a light-tight box 33 cm. from the quartz window at the end of the silvered tube. The arc was then about 43 cm. from the beam of electrons upon which it was to fall. No lenses were used in most of the work, hence, a very intense beam of radiation rich in ultra . ' . . ' . 4 Hu wyv- llo V.D.C. Jolenoid Circuit t ySolenoid Hi riding] 4/o Its St.Q. it 1 Note: llo l/olts A C used for Pump Motors, Heating Coils etc . Pc n e r u 'Shutter Circuit Zo/olts Wiring Layout Cathode Ray Lxp Plate /V e 4. 14 violet reached the discharge chamber. The X-Ray tube used was the Universal Type Coolidgs tubs with a broad focal spot. This was excited by a 6 inch spark Klingelfuss induction coil operated by a Wehnelt interrupter on 110 volts P. C. This tube was mounted inside a heavy lead box so that the target was 21 cm. frojp the quarts window and 3? cm. from the beam of electrons. Ill OPERATING CONDITIONS Pus to the fact that the experimental operations of this re- search were very critical, the exact conditions under which the re- sults were obtained are definitely stated. The vacuum was always 0.00001 mm. of mercury or less when the exposure was started. At the end of a series of exposures the pressure was measured and it was seldom higher than 0,00001 mm. The discharge chamber was freed cf water and mercury vapors by a'P 30 g bulb, a large cocoanut char- coal bulb, and a liquid air trap, tne last two being immersed in liquid air (See Plate 5). Liquid air was never applied until the pressure was 0.00003 mm. cf mercury, sc that the absorbing capacity cf the charcoal was saved to remove any gases given off by the hot cathode while photographs were being taken. The Wehnelt cathode was heated to a degree of hoc ness gained by experience until a beam cf electrons of sufficient intensity was obtained to make an impression on the photographic plate. Richard- 7 son has shown that the number cf electrons emitted from a hot body is a function of the temperature, the emission current being given ?. O.W, Richardson, "Emission of Electricity from Rot Bodies'', Chap. 2, pp 32 and SO; Phil. Trans., A. ,V,20i, p 530. . . . . . 15 by the following formula: I = CT 2 e when 0 and. d are constants, T is tne absolute temperature, and e Is tne natural logaritnmic base. Their velocity, however, depends largely upon the potential gradient through thicn tney fall®. A very low voltage acting against the cathode will prevent tne escape Q of electrons,'' even though the cathode may oe at a very nigh tem- perature. Four degrees of hotness were recognized and recorded. They were cnerry red, hot, very hot, and white neat, the correa- pondmg temperatures being approximately 770, SOO, 1150 and 1550 degrees centigrade respectively. Because of tne high vacuum used and the absence of any track of mercury vapor it was sometimes very difficult to start tne discharge even cn the application of 2000 volts. It could, however, usually be induced to start by heating the cathode very hot for an instant. When once tne beam was start- ed, it invariably started readily thereafter and at lower poten- tials.*^ It was found that a trace of mercury vapor caused the discharge to start very easily. After the beam was started the cathode was rotated until it was projected against the side of tne tube. When tne current was turned on m the solenoid circuit around the discharge chamber, tne beam was caugnt m a magnetic field of approximately ISO gausses ana wound into a spiral which 8. O.W. Richardson, "Emission of Electricity from not Bodies", Ch. 5, p. 15b, 9. O.W. Richardson, "Emission of Electricity from Rot Bodies" ,Ch. 5, pp. 169-171. 10. Nellie N. Horner, Am. Jcurn. Sci. , V.33, p 596, Dec. 1913. . . . , . . . - . . , . . . . . IS traversed the length of the discharge chamber, striking on the willemite screen on the outside of the shutter (See Plates 1 and 4). The phosphorescent spot was moved by means of a focusing magnet placed on the outside of the discharge chamber until it was central- ly located on the screen before exposures were made. The electron beam from the cathode could be easily seen in spiral form within the discharge chamber for pressures in the neigh- borhood of 0.001 mm. of mercury. The pitch and diameter of the spiral could be changed at will by rotating the cathode and regulat- ing the solenoid current. With pressures of 0.0001 mm. and lower the beam couid no longer be seen and only an estimate could be made concerning the pitch of the spiral. While the electron beam was passing in front of the tube through which the radiation entered it was subjected to any effect the radiation might have upon it. The light radiation in the form of a beam 2.3 cm. in diameter was throwr at an angle of 90° acrbss the path of the electron beam, hence, any action on the electrons, due to the radiation, took place during the time the electrons were passing through a space of about 2.2 cm, When X-rays were used, due to the size of the slit in the lead box around the Coolidge tube, the space filled with radiation through which the electrons passed was 1.2 cm. Nc radiation was permitted to fall on the Wehnelt cathode. The magnet operating the camera shutter, being only 13 cm. away from the photographic plate, had a small displacement effect on the electron beam at tne instant the shutter was tripped. The revolving s nut ter, however, had a time lag of about 0.2 second between the time it was tripped and the time it opened to take the photograph. The shutter magnet current was operated by a tapping . . 17 key which was never closed for prooably more than 0.01 of a second. Tnis gave ample time for any displacement effect on tne Dean, to dis- appear before tne picture was taken. IV PHOTOGRAPHIC MANIPULATION When tne camera had been placed in position on tne insiae of tne discharge chamber ana the vacuum brought to tne proper point, tne catnode was neatea; tne solenoid current turned on; and tne dis- charge potential placed across tne electrodes. At urst, a faint phosphorescent trace appeared on tne screen, wnicn rapidly increased in intensity until a circle or an arc of a circle was visible. The intensity, size, snape ana position of the phosphorescent spot could then oe changed by adjusting or regulating tne pitch of tne cathode ray spiral, the temperature of the hot catncue ana tne solenoid cur- rent. The focusing coil enabled tne final adjustment to be made, after which a succession of photographs were taken. This process, wnicn ordinarily took several minutes, usually caused a let down in tne vacuum, due to tne continued heating or tne platinum strip, of a few hundred thousandths of a millimeter. The vacuum, however, was quickly restored to below 0. 00001 mm. oy tne rapid acting pumps. After everything was in readiness the source of radiation was start- ed and the photographing began. Six phctcgrapns were taken on each plate. A practice was made oi taking tne odd numoered pictures without, and tne even numbered ones witn tne radiation falling on tne electron spiral. Between exposures tne screen controlling the radiation had to be operated ana tne photographic plate turned for- ward to its next position. The time between pictures was 5 to b seconds. The average time elapsing between the first and the sixth ' . . ■ . . * 18 exposure was 87 seconds. V MEASUREMENTS After the photographic plates had been developed and numbered, they were carefully examined to see which could be subjected to measurements. A plate, tc be of value for measuring, had to pos- sess certain qualifications adopted as standard. First, tne elec- tron trace had to be of sufficient intensity to be easily seen with the naked eye, since faint traces could not be seen at all under the microscope used in measuring tne photographs. All of the plates were under-exposed, hence, tne development nad to be forced, result- ing in many discolored plates. Second, the trace had to form an arc of a circle of sufficient length to measure its diameter. Third, the edges of the circle had to be snarp so tnat the error of measure- ment might be small. Traces that faded out along tne edges were of no value because no marks could be found on which to set tne measur- ing instrument. Fourth, tne six pictures on a plate had to be simi- lar so tnat the same measurement could be taken on each one. After the plate had been selected, a very fine line was drawn across each circle to indicate the diameters to be measured. This diameter was measured by means of a small dividing engine (See Fig. 3) tne screw of which was graduated to 0.001 cm. This screw was mounted in a rigid frame holding small strips of plate glass upon which the photographic plate was placed in sucn a way that light could be reflected through it. A needle point, ground to look sharp under twenty diameters magnification, and mounted on tne car- riage, was set alternately on tne edges of tne photographic circle at the ends of a given diameter and the readings of the micrometer . » . ' . . . . . . Cl- so taken. The difference between readings gave the diameter. A binoc- ular microscope of low power was used to set the needle point accu- rately. VI RESULTS A table has been prepared showing quantitatively for each plate tne four experimental quantities which affected the pictures most; the electrode voltage, the solenoid current, the cathode con- dition and the vacuum. The condition of the plate after it was developed and the effects evident on it are also shown in the table. Consecutive plates have been numbered 10, 20, 30, 40 , etc. , because there are supposed to be six pictures on each plate. On the fourth plate, then, tne third picture would be referred to as 43, the sixth as 46, etc. Due to lack of 3pace in a table containing so many columns abbreviations have been used in many cases; such as, st. for stained, F for fogged, ft. for faint, C. Ip. for traces im- perfect, e.p. for edges poor, exp. for experimental plate, and pos, for positive. When the even numbered circles - those taken with tne radi- ation turned on and numbered 2,4,6 - have a smaller average diam- eter than the odd numbered one3j those taken while radiation was off the effect is defined as positive. If the tendency seems clearly to be in that direction, but the results are not absolute, the re- sult is designated as "positive?". Another effect is undouotedly present, and that is a scatter- ing of the electrons or a diffusion of the electron beam. This ef- fect can be noted even when the traces are not circles. When it is present the table indicates tne fact by "yes" in the last column. 31 Plate Condi- Elec- Solenoid Cathode Vacuum Eft' ect Scatter- t icn trode Current Condi- in jng? No. of Volt- in t i on mm. Plat e age amperes Radia- fcion - Carbon used with condenser lens and glass plate 340 Good 3000 15.0 hot B0. 00002 Pos. ? Yes? 350 Exp. 360 Good 1935 12.5 V. H. B0. 00003 EO. 0003 Pcs. ? No | 3?0 Good 3070 13.0 V. H. BO. 00002 EO. 0002 Pcs. Yes? 360 Good 1900 13.0 V.H. BO. 00003 Pos. ? Yes 390 Good 1900 13.4 Whit e BO. 00002 Neg. ? Yes? Radiation - Carbon arc used with glass plate o o •* Good 2000 15.0 V.H. BO. 00001 EO. 0001 Neut . Yes? i 410 Good 1925 13.8 hot BO. 00001 Neg. No Radiation - Carbon arc used with quartz plats 430 E, P. C. Ip 430 Good 1950 18.0 hot BO. 00003 EO. 0002 N eut . Yes 440 Good 1975 15.7 hot BO. 00003 Neut . Yes 450 Good 1900 13.6 hot BO. 0002 EO. 0003 Pos. ? Yes 460 Good 1935 13.0 V.H. BO. 00006 EO. 0001 Neut. Yes? 470 C. Ip. 480 Good 1940 11.6 V.H. BO. 00001 EO. 0001 N eut . No 490 Good 2075 16.6 hot BO. 00001 Neut . No 500 C. Ip E.P • 510 St.C. Ip. 520 Ft .E.P. \ Plate No. Condi- Elec- tion trode of Volt- Plate age Solenoid Current in amperes Cathode Condi- tion Vacuum in mm. Effect 22 Scatter- ing? X-R&ye - First Series 530 Good 3000 15.5 hot BO. 00001 Pos. Yes 540 C Ip.E.P. 550 F. St.C.Ip Yes 560 C. Ip.E.P. Yes 570 Broken Yes 580 Good 3100 26.4 hot BO. 00001 Pcs . Yes 530 C. Ip. E.P. Yes 600 C. Ip. F. St. 610 C. Ip. E.P. Yes? X-Rays - Second Series 700 Ft. 710 Good 1800 15.0 hot EO. 00001 Pos. "V o o 730 Good 1900 16.3 V.H. BO. 00001 Pcs. Yes 730 Good 3100 16. 5 V.H. BO. 00001 Pos. Yes 740 Nothing 750 C. Ip.E.P. 2000 17.0 V.H. BO. 00001 Pcs. 760 Good 3000 18.8 V.H. BO. 00001 Pcs. Yes? 770 C. Ip. E.P. 2050 16.0 V.H. BO. 00U01 Pos. ? 780 Good 3000 30.5 V.H. BO. 00001 Pos. Yes 790 Good 2000 16.0 hot BO. 00003 Pos . Yes? 33 | If the tendency seems clearly to be in tnat direction, but if the indications are not absolute, tne result is designated as "yes?". Thirty three plates were exposed before the apparatus was brought under control, and one was obtained that could be measured. Of the next twenty eight, however, fifteen were perfect enough to measure and five others were examined for a scattering effect. Four types of radiation were used in this series of plates, hence they will be divided into groups depending on the radiation. The carbon arc wa3 used in tne first tnree groups with plates 340 to 390 inclusive. A condensing lens m3 used with it and a plate glass window covered tne end of the tube that conducted tne radi- ation into tne discharge chamber. Plates 400 and 410 were taken with tne lens removed. For the next group including 430 to 490 a quartz plate was substituted for tne plate glass window. This per- mitted a beam of light, rich in ultra violet, to act on tne elec- tron beam. Hard X-Ray3 were used with plates 50 to 61 inclusive. Plate wo. 610 ending tne last series was taken Aug. 11, 1921. No more were attempted until tne Christmas vacation 1931-1923. In the meantime the first series had been examined and marked effects had been found on tne plates. At the time tne photographs were measured, the data taken was put into graphical form in order that it might be more easily inter- preted. The logical way to plot the results would be to plot time as abscissae against the diameter of the measured circles, the time starting when the first exposure on the plate was made. The time between exposures was noted for tne plates 370 to 440 inclusive. The variation was so slight that it was not thought necessary to have an extra observer simply for tne purpose of noting time. For . . . . . . . . ♦ 24 this reason the abscissas used were simply the exposure numbers as they occurred on the plate. The curves for plates 370, 440, 480 and 490 were chosen as representative of all the results that were obtained when the carbon arc was used as a source of radiation. (See curve sheets for above plates). The slope of the curve is mainly due to the fact that the solenoid current slightly decreased as the resistance of the coil increased by heating. Changes in elope such as occur in Nol440 and 490 are probably due to slight variation in voltage a3 a 135 volt D. C. line supplied the current. Plate No, 370 apparently gives results that are distinctly posi- tive. This is the only one, however, as noted above, of the thir- teen examined of the series taken when arc light radiation fell on the electron beam that snows a decided positive result. Four were "positive" but the magnitude of the results were within the experi- mental error; six were neutral; one was slightly and another dis- tinctly negative. After measuring and examining this series of plates, the fol- lowing may be said. So far as the positive effect is concerned the only conclusion admissible here is that under the conditions of the experiment, if radiation of wavelengths from 8000 to 13000 Angstrom units falls across a stream of rapidly moving electrons, there may be a slight positive effect which is possibly less than the experimental error. Concerning the scattering, the following was found: four were neutral; five showed slight indication of scattering, and four clearly showed a scattering effect. There seems, then, to be evi- dence that a scattering effect was present. ' . . The first X-Ray plate (when X-Rays fell across tne electron beam) measured, No.5o0, (See Curve sneet, 530 and Fig. 4) gave both a decided positive effect and a scattering. Picture No. 531 was com- > pletely darkened by togging, hence for this plate tnere are only five points on the graph sneets. The remainder of tne series were carefully examined, but only one was found. No. 580, (See Curve sneets 580 and Fig. 5) that would subject itself to measurement. This also gave large positive results. The last X-Ray series, 700 to 730, (the Nos. b20 to 630 were omitted) was taken to cneck tne previous worx. In order that each plate might be brought to account and excuses might not nave to be made for missing plates, the greatest care was taken in making the exposures. This, however, was too much to expect of apparatus so difficult to manipulate. Plate 700 was almost a blank. No. 740 was a complete blank, while tne eages were so poor and the traces so indistinct on No. 750 and 770 that they could be inspected only, and not measured. Since Plate 710 was too faint to measure under the microscope, a needle point divider was used. The error in measure- ment was hign, which accounts for the divergence of tne curves (See Curve sneet 710). The two effects, however, were plainly visiDle to the naked eye. Plate 720 snows a large positive effect. The probable error here is +0,008 cm. (See Curve sneet 720 ana Fig. 6). An examination of Fig. 6 will 8hov? tnat tne effect is easily visible to tne eye. Plate 7o0 (See Curve sneet 730 and Fig. 7) nas tne largest positive effect obtained. An examination of tne data in tne table shows tnat all conditions here were favorable for very hign velo- city electrons projected at a large angle witn tne axis of tne 26 solenoid. Hence we would expect a large effect, Plate 760 was measured along a radial diameter, and also along a diameter at ngnt angles. (See Curve sheet 760). Tne last two curves form a large angle with the first. This fact can only be explained on tne assumption tnat tne velocity distrioution in- creased as the plate was being exposed. Both sets of curves show a positive effect. Plate 770, although imperfect, was measured as carefully as possible. The results, although inconsistent, show a tendency to- ward a positive effect. No curve was plotted for this plate. Plate 780 shows a positive effect, larger tnan tne experi- mental errors, although not so large as the otners. ^See Curve sheet 780). On the last plate. No. 790, the last two pictures are entirely different from the ethers. Consequently, no direct comparison can be made. Pictures 792 and 794 are distinctly smaller tnan 791 and 796 This shows a positive effect. From the series just presented we see tnat from ten plates examined, all but one snowed a distinctly positive errect. This single plate had a positive tendency but was too imperfect to ex- amine accurately. Thirteen were examined for scattering. Ten showed distinct scattering and three were inclined in tnat direct io^. We may conclude, tnen, tnat under the conditions of the ex- periment, X-Hays cend to decrease tne velocity of an electron beam wnen tnrown across tneir patn; also tnat tne electron beam is dif- fused or scattered by the X-Pay3. . . w . . . . . 3? Plate No. 530 Fig. 4 This photograph snows *the traces made 'by tne electron beam when it was alternately exposed to hard X-rays (No. 2,4,6 exposed). Elec- trode voltage 3000 volts, solenoid current 15.5 ampere, vacuum 0.00001 mm. mercury. This plate shows the positive effect and also fja scattering. See Curve Sheet No. 530. ■ • i . • . Plate No, 580 Fig. 5 This photographs shows the traces made by the electron beam when it was alternately exposed to hard X-rays (No. 3, 4, 6 exposed). Electrode voltage 2100 volts, solenoid current 26.4 amp., vacuum 0.00001 mm. of mercury. See curve sheet No. 580. This Plate shows |a positive effect and also a scattering. Here the different diam- eters of the circles can be noted. 22 T31 o ate No. 720 Fig. 6 This photograph was in one taken in the second series for check-} ing purposes. Traces No. 2 , 4 and 6 were made by electrons which had been exposed to hard X-rays. Cathode voltage 1900 volts, solenoid current 16.2 amperes, vacuum 0.00001 mm. mercury. This photograph shows distinctly that the electrons moving in the spiral when ex- posed to hard X-rays as described in the text, are appreciably slewed down in velocity and hence under the strong magnetic field are twist- ed into a spiral of smaller diameter. This is clearly shown by the traces 2, 4, 6 which are smaller in diameter than traces 1, 3, 5 which ?/ere not exposed to X-rays. The scattering effect is also present . . . . . 30 Plate No. 730 Fig. 7 This photograph snows the san e as No. 730, Fig. 6 . It was taken under slightly different conditions. Cathode voltage 3100 volts, solenoid current 16.5 amp., vacuum Q . 00001 mm. mercury. Piute, yw Z 34 Plate P90 mo two lift) X /?ay Pkte 530 l%D S iViO mo 1200 IW im Picture Y) umbers. 3 1 S 36 '± 40a 41 * VII DISCUSSION The examination of the figures 4, 5, 6 and 7 raises a number of questions, some of which must for the present remain without satis- factory answers. Why does the electron beam "Spiral" down the dis- charge chamber? Why is there a continuous, almost circular trace, on the photographic plate since one would expect that a plane cross section of a spiral would give a point and not what is apparently a projection of a spiral? Why is the trace sharp and narrow on one side while it is wide and diffused on the other? Why is it not a circle? The following are answers to the above. Consider in Fig. 8 the vector OA to lie parallel to the axis of the discharge chamber. If an electron beam were projected along OB, its ve- locity could be resolved into the twc components OC and OA, where OC is at right angles to the magnetic field. The latter component would be convert e into a circle, the radius cf which we Fig. 8 might compute if we knew the original velocity, or, if we know the radius we can find the velocity from the equations or F = Hil = Hev = ma = He mv* wnere H is the intensity of the magnetic field, v is the velocity cf the electrons in the beam, m is the mass of the electron, _e is its charge, and r_ is the radius of the circle into which the beam is changed. ■ . ■ . 'I . 43 The component OA is unchanged, and our components are now like Fig. 9 B / \ / 1 V I \ / 'X ./ N Hence, the resultant motion is a spiral along OBA, which travels down the tube at a velocity OA. Consider the original beam OB, Fig. 8, as it comes from the 11 Fig. 9 oxide on the heated platinum. The velocities of the electrons in the beam are distributed according to Maxwell's distribution law. The electrons at this point are caught in the intense electric field and all are accelerated through the same change in velocity. This leaves the velocity difference between the slowest and the fastest the same as it was before the acceleration. Different velocities are twisted into circles of different radii by the magnetic field, and since all the electrons start at one spdt and initial ly have the same direction, the traces are all tangent tc the point A, Fig. 10, which corresponds to the emission point, equation we see that the radius of a circle into which a beam is twisted is directly proportional to tne vector velocity OC, # B i \ » / ‘ i \ / / \ < » y ✓ \ k 0 -‘ \ ■ l I x i \ i \ i \ i \ I \ / /\ ✓ v / » ' ! D B Fig. 11 11. O.W. Richardson, "Emission of Electricity from Hot Bodies", Ch.5,p 139-178, see p. 161 (1916); Phil. Trans. A. ,V. 301 ,p 503, (1903); Phil. Mag;. .Vcl. 18.p695 (1305). 43 If we consider the spiral in Fig. 11 we may assume that if a group of electrons started from the source at 0 at the same instant, the fastest of them would reach the plate A in a given time t. Some a little slower would have arrived at B and would still have one half turn to make before striking the plate. This would bring them to a point B on the trace in Fig. 10. Otner electrons with a still smaller velocity, being af the points C and C, Fig.}l, would fall at A and D respectively. Fig. 10. This explanation accounts entirely for tne sharp edge at A and the wide and diffused part between B and D. If we also consider that the outside of the trace is an envelope, tangent to arcs of a family of circles having a common point A, we can also readily see why the trace is not a perfect circle. A possible effect cn the results obtained might come from the action of the radiation on tne apparatus itself. A study of Plate 4 will show that the radiation after passing tne electron beam would strike the side of the discharge chamber, freeing some electrons. A positive charge would be built up on the glass across from the anode until the potential reacned a point such as to prevent tne escape of any mere electrons. This charge would leak off, in part at least, during the time a photograph was being taken when the radiation was turned off. If Crookes dark space is small compared, to the space between the cathode and the positive charge on the glass then an increase in the velocity of the electrons would re- sult when radiation was present, hence, an increased diameter of the trace would be expected instead of a decreased one, as was found. No formal attempt will be made to explain from a theoretical ■ . 44 point of view the results obtained in this research. It seems, how- ever, that it would not be out of place to suggest possible lines along which explanations might be found. As was stated in the introduction, the experiment grew out of a remark made by J. J. Thomson. He suggested that if a diffused pattern in the electron trace was found when radiation was thrown across the path of the electrons, the result might be taken as indicative of the correct- ness of a theory of light which he had advanced. ^ C.T. Knipp, who was a student with Thomson at the time, saw the possibilities of such a research and soon after his return to Illinois, designed and built the apparatus with which the early work was done. It seems possible that if an electron were projected at an angle to the axis of the discharge chamber, through electromagnetic waves as they are considered in the usual form of the undulatofy theory, the electron would be set in a swaying motion as it advanced through electric and magnetic fields which periodically reversed in direction. If the fields should suddenly cease to exist, the elec- tron would continue in a line tangent to the path of its motion. This path in all probability, would be at an angle to the line of flight when it entered the field. It seems improbable, however, due to diffraction and scattering effects, that the electric and mag- netic fields of radiation have a sharp well defined boundary. On the contrary, it seems more likely that the fields diminish gradual- ly over a considerable space, when measured in the radiation wave length. The oscillatory motion of the electron would tnen slowly subside, and its final path would not be very different in directior 13. J.J. Thomsen, "Electricity and Matter", Ch. 3, p 53-70,(1906). Phil. Mag., Vol.19, p 335, Feb. 1910. c 45 from its original path. It would seem, then, that the scattering ef- fect, due to. continuous wave fronts would probably be too small to be detected. If, however, we postulate any kind of a wave theory in which the wave front is discontinuous as J.J, Thomson 1 ^ and A. Einstein 1 ^' have done, it is evident at once that an appreciable scattering ef- fect would be expected under the conditions of the experiment. Why the velocity of the electron should be decreased when it passes through short electromagnetic waves is difficult to see in th: light of our present theories. So far as is known the usual form of the undulating theory cannot give an explanation. Other research work should be done on the two phenomena dis- covered. Ts?o experiments might be suggested. First, a straight beam might be used and permitted to fall on a very small, movable slit behind which should be placed an insulated Faraday cylinder at- tached tc an electrometer. The rate of collection of charge could be measured for any given position of the slit with the radiation alternately off and on. When the slit was near the edge of the beam, if scattering occurred with radiation present, the charge would build up more rapidly than when radiation was absent. Second, the other experiment could be performed ’with a device similar to a Braun tube. The electron beam could be made very narrow by passing it through a hole in a diaphragm, after which it would pass through an alternating magnetic field. The electron beam would then produce a phosphorescent line on a will emit e or calcium, tungstate screen. The cross hairs of an observing telescope could then be set on the end of the luminous line. If the length of the line changed when 13. loc. cit. 14. A. Einstein, Phys. seitsohr, , V.18, p 131, ,1917. . . . . 46 " radiation was turned on, it would be evident that a velocity change bad occurred in the beam. VIII SUMMARY From the experimental work just presented under the operating conditions described above, the following conclusions may be drawn: 1. When a strong beam of radiation of wave lengths from 8000 to 1500 Angstrom units fell across a stream of rapidly moving elec- trons, there were indications of a slight decrease in the velocity of the electron. This effect, however, was smaller than the errors of measurement. 3. With the above radiation wave lengths the evidence is very strong that there was a scattering of the electrons in the beam. 5. When hard X-rays were used instead of the radiation given in 1, there was a distinct decrease in the velocity of tile moving electrons, as is shown by the decrease in the diameter of the elec- tron trace (Fig. 6 and 7). 4. It was also found that X-rays caused a decided scattering of the electrons in the beam. The author wishes to recognize the help received from the early rd of Professor C.T. Xnipp on this problem, and to express his thanks to him for his advice and aid throughout the research, and to Professor A.P. Carman for the facilities of the department. ■ . ■ ■ * ■ VITA Claude J erome Lapp was born June 24, 1693, near Smiths Creek, Michigan. He received his elementary education in the public schoolii of Saint Clair County, Michigan, and his secondary training in the Richmond High School, Richmond, Michigan. In September, 1313, he entered Albion College, where he received the degree of Bachelor of Arts in June, 1917. From September to December 1917 he was a scholar in physics at the University of Illinois, and from December, 1917, to December, 1916, served in the Aviation Section of the Bureau of Aircraft Production at the Bureau of Standards. In January, 1919, he returned to the University of Illinois and from it, in 1S20, re- ceived the degree of Master of Arts. He has held the position of Assistant in Physics, University of Illinois, 1919, 1919-3$ and In- structor in Physics in the Summer School, 1919, 1930. ' ■ . • .'.'A .