Conditions of Sensibility of Photo -Electric Cells with Alkali Metals and Hydrogen TJ'sriV.QF CONDITIONS OF SENSIBILITY OF PHOTO-ELECTRIC CELLS WITH ALKALI METALS AND HYDROGEN BY JACOB GARRETT KEMP A. B. University of Illinois, 1906 A. M. University of Illinois, 1910 THESIS Submitted in Partial Fullfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY IN PHYSICS IN THE GRADUATE SCHOOL OP THE UNIVERSITY OF ILLINOIS 1912 11131 UNIVERSITY OF ILLINOIS THE GRADUATE SCHOOL May 11 , 19 12 , 1 HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Jacob Garrett Kemp ENTITLED "J^.PI^^^^J^io 9^ .3..?^^ i.^ i IjAl. 9^" ® '^J^^^^ ri th Alakli MstaLls and Hydrogen BE ACCEPTED AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF Cog tor of E!!?..i...^.o..?9.P.^.Y. ^J' Head of Department Final Examination Committee on UlUC 1 . "Conditions of renflihility of Photo-El ec trie Cells with Alkali "et.ils and Hydrogen." Introduction. Under certain con-Utions all metals emit electricity v;hen acted upon by light waves. This pheno/r.enon is called the "Photo-Electric Effect", i.e., the Light-Electric Effect. The intensity of this ef- fect is largest for those metals vhich are most electro-positive; i.e., those metals which give off electrons m^st readily. The order of photo-electric strengths for the metals as given by J.J. Thomson^ is as follows:- caesium, rubidium, potassium, potassium-sodium alloy, sodium, lithium, magnesium, thallium and zinc. For copi^er, platinum, lead, iron, cadmium, carbon and mercury tlie photo-electric effect is very weak. The order of the metals for this e^'fect is precisely the same as that in the "^^olta series for contact-electricity. Ey means of I>Jaxwell's electromagnetic theory of light, the elec- tric action of a beam of light incident upon a metal surface may be partly explained. In a beam of light plane polarized at right angles to the plane of incidence, there is an electric force vvith a compo- nent normal to the reflecting surface, ^len light is polarized in the plane of incidence, the electric force is parallel to the re- fleeting surface. Elster and Geit^l*^ made the very interesting dis- covery that v/hen the plane of polarization is at right angles to the plane of incidence the photo-electric current is a maxiinum for any given ar^gle of incidence. Furthermore, they showed that when the angle of incidence is about 60° the effect is a maximum; and, when 1 . "Conduction of Electricity through Cases", pac^e 351. ^, Weid.Ann. LII p.433,1824; I,T p.684, 1895; LXI p. 445, 1807 . Digitized by the Internet Archive in 2013 http://archive.org/details/conditionsofsensOOkemp the quantity of li^ht ataorbed is a n,aximurri the effect ie a maximui:.. r.Fohl^, and R.Tohl n .1 r . Tr ing&hei rr,^, have shown in two papers that there are two effects, a normal and a selective effect, ouper- imposed upuii each other wh^n X)olari7.ed light acts upon a metal. In the nori.ial effect the current incr pases with the frequency of the in- cident light and the orientation of the plane of polarization is of practically no influence except in so far as the absorption of light depends upon it. The selective e'^fect Joes net appear by itself, but is always superimposed upon the norrnal effect. The curves in Fig. 1, below show the two effects as they are superirapoKed upon each oth^r for a particular angle of orientation of the plane of polariza- tion. A 4-00/^ FIO. 1. Current is plotted as ordinates and wave lengths of incident light as abscissae. The curve S shows the selective effect and the curve TJ shows the normal effect. The selective effect has the following features:- 1) it is restricted to a short interval of wave length. the pho to-^elec trie current reaches a maximum for a particular wave length independent of the orientation of the plane of 1. R.Pohl. "^T-erh-der Phy.G. p,350, 19 09. 2. K.Pohl ^ P,Pringsheim, "^'erh-der Phys.O. p. 474, 1911. 3 polarization. 3) the Gurr'^nt la mainly determined by the orientation of the plane o f polarization . Thus it is seen that the oriantation of the plane of polririza tion does not chanse the position of the curve IT, but it does cause the curx-e R to be shifted parallel to itself in the ^rsrtical direction. The highest position of the curve P! above Ihe horizontal exists for the plane of polarization perpendicular to the plane of incidence. The curve IT remains in about the same position except for slight variations due to the change in the amount of light absorbed for different conditions. Thus it is seen that when the plane of polar- ization is perpendicular to the plane of incidence, the photo-elec- tric current is a maximum and due mainly to the selective effect. Therefore, the photo-electric current is a maximum for any given condition when the electric component of the incident beam of light normal to the metal surface is in the region of its maximum value. The intensity of light is the amount of energy passing through unit area normal to the direction of propagation in unit time; i.e., it is proportional to the mean square of the amplitude, therefore, to the mean square of the electric component. Lenard"^ found that the velocity of projection of electrons from metals due to incident ultra-violet light is independent of the intensity. However, he found that the number of electrons emitted proportional to the in - tensity of the incident light while the velocity of each electron depends only upon the nature of the illuminated s\irface. In other words, the velocity of the emitted electrons is a function of the energy absorbed or is proportional to the square root of the maximum voltage to v;hich the mietal rises when light is incident upon it. 1. Ann.der Phys. "IT. p. 149, 19 03. And the nuniber of electrons emitted per unit time, or the current, is proportional to the mean square of the normal component of the elec- tric force in the incident light waves acting upon the metal surface. Ladenburg^ has shown that the photo-electric effect varies with the thickness of the metal layer until 10""* cm. is reached when the effect is independent of the thickness. From the abo^^e value of 10""* cm. J .J . Thomson^ has calculated, that if a photo-electric s+.ream of 1C~^^ coulorubs per second per square centimeter, which is one of more than average intensity, be flowing from a metal surface which is 10""^ cm. thick, more than 300 years would elapse before +he character of the surface would be changed. This calculation gives some idea of the constancy of the photo-electric property of a metal under the action of light. But there are other changes which are liable to take place, due to the influence of temperature changes and the action of remnant gases. Elster and Geitel^, working with potassium photo-electric cells, found the rate of discharge of electrons or the current, from the surface of the metal to be lirectly proper tional ' to the intensity of the incident light for small ranges of light intensities. Fdchtmeyer'* found that the current from a sodium surface at zero potential is strictly proportional to the light intensities between C.007 C.f. and 0.5 O.f. His cell was made of a glass tube with sodium upon one electrode and platinum wire for the other in a very high vacuum. In 1910, Elster and Geitel^ made photo-electric cells using the 1. Ann.der ?hy. XII. p. 558, 1903. 2. Conduction of Electricity through Gases, p. 278. 3. Ann.der Fhy, 48, p. 625, 1893. 4. Phys. Rev. 29, p. 71, 19 09. 5. Phys..Seitschr . 11. April, 1910. g metals caeoiun), rulidiuin, potauoiuin .'-■'Iju sodiu/n (aa the cathode) in hydrogen gtxQ , The electrode in contact with the metal, that is the cathode, was connected to thi- negative terminal of a battery of 3C0 or 400 volts while a resistance of 30OO ohrns arid a galvanometer was placed in series with the positive terminal of the battery and the anode of the cell. The pressure of the hydrogen gas in the cell was then reduced until the current -flowing between anode and cathode of the cell caused a faint glow to fill the whole tube. The metal sur- face which was very bright before the illumination appeared in the tube afterv/ard became colored, being brownish for sodium, bluish- violet for potassium, and light greenish for rubidium and caesium. These colors are due to the formation of a compound of the hydrogen and the metal which is called a hydride or an alkali-hydride. They found that after this forming process the cells were three to four times more sensitive than v/ith the pure metals. In 1211, Elster and Oeitel-^ investigated the stability of the sensitiveness of photo-electric cells with formed potassium cathodes and hydrogen. They found that the hydride formed on the surface of the metal was very unstable in an atmosphere of hydrogen, and further- more, that after about two months a decrease of about 85/o in the sensibility was observed. Ey forming the hydride surface and then replacing the hydrogen by one of the inert gases argon or helium the cells were found to be more permanent. It was found that wlien hydrogen was left in the tube after the hydride surface was formed the hydride coloring disappeared and the hydrogen was partly absorbed by the metal, Tnen argon or helium gas was placed in the tube a-^ter the metal surface was formed the colors kept quite the same while the sensibility remained prac- tically constant, jl. Phys.Zeitschr . . Aug... iCll. No quantitative itiea8ureni'=»ii ba hnv/^v^r, hnvn V.p'^n ir-arle for de- i termining the conditioijs for niaxinium sen^i ibili t y of plio to-cl ec tri o cells of alkali n.etals v/ibh hydrogen gas; while Elster and Oeitel's work hao been iLore of a qualitative nature yt the renults are very definite in fixing the g-^neral facts for this type of photo-electric cells. Turpose, The puri'ose of this in^^es tigation is to make a systernatio, quantitative study of the conditions of sensibility of photo-electric cells of alkali metals with hydrogen. This investigation necessarily involves the study of the effect of variations of the pressure of the gas, distance between electrQdes, area of metal illuminated, voltage applied to the electrodes, and the intensity of illumination. When light is incident upon the metal, which is the cathode of the cell, and a potential difference is acting between the electrodes then electrons are emitted from the metal at the cathode causing ionization of the gas contained in the cell. Therefore, some remarks concerning ionization will be made in order to make clear v;hy the study was made mainly along the lines of ionization. If a potential differ-^nce be maintained between two electrodes which are surrounded by a gas, a current will flow if some ionizing agent is caused to act upon the gas. The ionizing agent may be Roentgen rays, radioactive substances, ultra-violet light or- ordinary white or monochromatic light acting upon one of the electrodes. If the current flowing between the electrodes be plotted as ordinates, and the potential .iifferences between them be plotted as abscissae, then for a constant ga s pressure a curve as shown in Fig. 3 7 below will result. For email potenti-.tl Ji T?re is a deviation from this law until the curve becoiries parallel to the F.D. axis. This is called the condition of saturation; i.e., no increase of current for increase of potential difference. The satur- ation current ie proportional to the distance between the electrodes point for constant pressures of the gas. At the^b the curve begins to rise showing an increase of curr'^nt with the increase of rjotential dif- ference. This is a very important point on the curve since it in- dicates the minimum pot'-^ntial dif-ference required to cause sufficient potential gradient to give the negative ions velocities high enough to prod-:ce other ions by collision. This phenomenon indicates that to I'roduce an ion a certain minimum amount of energy is required. If this critical potential gradient be known, s-^y E, the charge 6 of a negative ion, and its mean free path 1, then this minimum energy is W = Eel. For conditions represented by the curve beyond the point b the positive ions reach velocities sufficiently high to produce ions by c 'llision. At the point d the curve begins to approach parallelism to the axis of ordinates. This indicates an exceedingly 8 lar^e increase o"^ current for an extreirely aniall increaBe in poten- tial differeiice indicating th \ the sparking potential has been 1 reached. Since the iii'tss of a negative ion is about 18C0 mass of a positive ion it is easily seen thnt negative ions reach the critical ionizing velocity un^ler the action of a much smaller poten- tial gradient than that required by the positive ion. In this work, therefore, it is seen th.^t the conditions of the problem are such thit t^e study of the effect due to varying the pressure of the gas, the distance between the electrodes, and the potential difference applied to the electrodes can best be effected by making use of ionization curves, I^'Ioreover, it is known from the theory of ionization that this work will have to do with the r^art of the ionization curve between a and d. To recapitulate; the study is made by varying the pressure of the gas, P, the distance between the electrodes, D, the potential difference between the electrodes, the intensity of illumination, L, the area of metal illuminat ed. A, and reading the current in terms of the deflection, d, of a d'Arsonval galvanometer. Detjcripbion of Apparatus, Figure 5i8 a full si^.e drawing of the details of the glasa tubes used in making the photo-electric cells. A spherical bulb, 3,5 cm. in diameter, has two tubes 1.0 cm. in diameter, sealed horizontally and diametrically opposite each other. A vertical tube, 1.8 cm. in diaraet'^r, about 15 cm. long is sealed in the top of the bulb. At the top of this vertical ^ube a platinum '^rire is .-sealed and fused to an aluminium rod 0.4 cm. diameter and 10 cm. in Isngtli, To tlie lower end of the aluminium rod is attached a brass spiral spring to which is connected the platinum wire anode. The anode a, being sealed through the lower end of the gl ass t ub e which telescopes the aluminium rod. At the ur.per end of this glass tube is attached an iron ring which fits neatly inside the larger tube. By means of an electromagnet, using a current of 2 amperes, the inner tube carrying the anode a, can be held in any desired position relative to the cathode c, at the bottom of the l ulb. At the bottom of the bulb and diametrically opposite the anode, a, is sealed the cathode, c, the upper point of which does not extend beyond the surface of the inside of the bulb. In some of the cells the inside of the lower surface of the bulb was silvered, the metal distilled into it and deposited upon the mirror surface. In this way a good contact was insured between the pl-^tinum and the metal. In some of the tubes the metal was not disti'^led into the bulb but poured into it while in the molten state and allowed to solidify over the platinum electrode. The metal in all cases was used as the cathode of the cell. A beam of light could be passed through the walls of the bulb and thus be incident upon the active metal directly under the anode. FIG. 4. f 13 Tig. 4 i8 a diap;ram giiowing th" )ri<»Mio-l of oh.-^nging the diBtance between the elecToJes by means of tlis elec tronjfignet , The electro- magnet M , ia about 13 cm. mean diameter with its axis coinciding wit}i the axis of the brass scr-^w f>, v/iiich has a pitch of one milli- meter. M ia attached to a nut v\hich is threaded to fit the screw S, and it can be raised or lowered by turning the knob B. The brass frame ia secured to th^ side of the light-tight box shown in Fig. 5 . It required two amperes to move and to hold the electrode a, in the desired positions. The distance between the el^^c trodes was determined as follows:- Py means of a cathe tome tor focused on the upper edge of the telescop- ing tube the position of the electrode was determined.. The electrodei were connected in series with a dry battery and a telephone receiver, which gave a click when the anode and cathode were in contact. The cathetoraeter reading for the position when the electrodes were in contact was called the zero reading; and the diff'^rence bet'^een this reading and th'^t of any other position of the anode a, gives the dis- tance between the electrodes. Fig. 5 shows the entire arrangement of the apparatus for the in- vestigation. The photo-electric cell is enclosed in a light-tight box. Wires connected to the anode and cathode, and to the electro- magnet pass through insulators in the walls of the box. The cell is sealed to the system containing a "acleod gauge for measuring pres- sures up to 0,24 cm., a closed manometer for measuring the higher pressures, a regulator for obtaining small variations in pressure, a tube containing palladium m'^tal strips for supplying pure hydrogen gas, and an air pump. The palladium metal was charged with hydrogen gas by the electrolytic method. With an electrolyte of one part of HgSO^ and three parts of HgO, the anode being platinum, and the 13 palladium metal the cathode, hy>lrogen gas was abeorb^^d by the cathode when C.5 volts was connected across the ec "rodes . After charging the palladium, the tube containing it whr .sealr^d to the syfitera. When the tube is heated wi^h a small bunsen flame, the metal '-jives off pure hydrogen gas. The whole glass system, when the pump was cut off, could be filled to a nreBsure of about S5 era. when the palladium was heated. Small variations of pressure of the gas could be produced by raising or lowering the m'^rcMry in the pressure regulator. The elec-t tromajnet was connected in series wi-^h five storage cells, requiring about two amperes to hold the anode at any desired position. The galvanometer used is a Leeds and Tv^orthrup type HS, , the sensibility being 3.78 x 10"^*^ amp. per mm. deflection for 3 meters scale distance. The anode of the cell was connected to the earth through the galvanometer and a megohm resistance. The cathode of the cell was connected to a variable point in a water rheostat which is in series v/i th about 840 volts from a storage battery. The voltage ay-plied to cathode could be ''■aried by means o"^ the water rheostat and it was measured by a Kelvin electrostatic voltmeter reading 0-600 ■'^olts. The variation o^ the area o^ the metal illuminated was obtained by varying the opening o^ the iris diaphram A, which is placed at the lower end of a brass tube. This tube was blackened on the inside to prevent reflection of light. The intensity of illumination was varied by changing the posi- tion of the source of light L on the guide. The source of light was a 4 candle power, 110 volt incandescent lamp, the entire bulb of which was frosted. The candle power 0^ hhe lamp after frosting was 2.47 at 107 volts. The current was supplied by a 110 volt storage 14 Fin. 5. 15 Py mv-)^^ino; th= lamp or. the guide the intensity of illumination on the metal in the pho lo-elec trio cell could be ^railed, and this varia- tion calculated directly by means of the inverse square law. Method. Hince the moat 8-'=n3itive conUtions for the photo-electric effect are being sought, it is necessary to study the effect due to varying all the poasible conditions in order to find the most effective set of conditions. The variables in this work are the following:- P the pres sure of the gas, D the distance between the electrodes, V the poten- tial difference applied to electrodes, A the area of metal illuminated L the intensity of illumination, t the temperature o^ the cell, and d the galvanometer deflection which is proportional to the current. Two sets of readings are possible for each cell, namely:- before forming and after forming the hydride surface. In this paper this process will be called forming. Four cells - ere studied: one v;i th caesium and three with potas- sium metal. The readings were taken in the order as follows :- with t, A J P and D constant the values of tlie deflections d, of the galvanometer were read for increasing values of Thus values of current and voltage were obtained for an ionization curve. This was repeated for three and in some cases four distances of D. From the above data four ionization curves are obtained which show the e-^'fect of varying the distance betv;een the electrodes for constant values of t, L, A, and P. If the above three or four ioni- zation curves be called a set, then it is possible to get as many sets as there are values of P, the gas pressure. From three tv- five different values of P were aelocted for e.ich cell and in this way Ihe effect due to change of r,rea>3ure was e tallied. Ionization curves were also obtained in '-rhich A, L, P ani D are constant for Mrree and four different t empera-^ur ea , Aft-^r the form- ing process similar sets of ionization curves were taken except those for temrerature changes. In adiUtion to the ionization corves taken after forming the cell !:o. 4, sets of data were taken in which h, V, P, t and D are constant while A and the current varied. Also reading were taken for A, 7, P, t and D constant while L and the current var i ed . t)ata and Curves A list of the tables of data and curves is given below, A in cm*^. represents the area of metal illuminated, L in candle feet represents the intensity of illumination, t in °C the temperature of the cell. P in ram, n-.ercury represents the preaoure of the hydrogen gas, D in cm, represents the distance between the electrodes, V in volts represents the potential difference between electrodes, d in mm. represents the galvanometer deflections •J 17 Cell No. 1. Caesium IJetal. The curves are all ionization curves, I, 8how8 effec^ of variation of D, before and after forming, r = 1.0 mm. A = 4.35 cm^. L = 0.186 C.f. t = 17^0 . IT, 8how3 effect of variation of ?, be-^ore and after forming D = 0.5 cm. A ^ 4.35 cm^. L = 0.186 C.f. t = 17oC. Ill, shows effect of variation of t, after forming, A - 4.35 cm2. L = 0.186 C.f. D = 0.5 cm. P = 1.0 mm. 21 Plate I. Tlie lower curvse wers obtairi«»(l te^oie tli^ cell was formed while the uoper cm"'^-. were obtained '"vrter rorrniiig; , The oorr eBporiding curves for before and a-^ter forming t^how that the negative ions have velocities high enough to cause ionization by collision n t -ibout the same "oltage. "he voltage required for illumination is much Isss before forming, Plate IT. All th^.t is said above Tor plate I is true for this plate. The voltage reqiured for illumination is much: greater for the pressure cf l,C mm. than for the other curves. These curv<*3 seem to show that ther'^ is niinirnim il lurriinating voltage for a certain critical pressure, "^he '^urvt for 1.0 mm. pressure show the best photo-electric condition for sensitiveness, Plate III. The curves for different temperatures indicate that the voltage at which negative ions attain ionizing velocity is least in those for higher temperature. The illumin- ating voltage is much less for higher teii^eratures than for higher t'^^mper atures for the lower. The curves show conditions more sensitive to photo-electric effect. P.2 Cell No. 3. Potaasium Metcal. The curves me all ionization curves, Plate, IV, shows effect of variation of D. A = 3.17 cm^. L = 0.22 C.f, t = 36°C. P = 5.5 mm. Cell not formed. Plate, V, shows effect of variation of D, A = 3.17 crr.'^. L = 0.22 C,f t = 25°C, P = 4.0 mm. Cell not formed. Plate, YI, sh07/8 effect of variation of D. A = 2,17 cm^. L = C.22 C.f t 24°C. P = 1.8 ram. Cell not formed. Also for P = 0.84. t = 24<^C. Plate, VII, shows effect of variation of P. r = 1,0 cm. Cell not formed. A = 2.17 cmS. 1, ~ 0.22 C.f ?4 Plate IV. These curves show thn for this pressure the order in o c (J ur vfhich the voltages for il lumination^i a the saine ae that for the values of D, Plate V, These curves ohow the same results as do those curves in plate TV. Pl-^te VI. The curves for P = 1,8 nun. show the same results as those in Plate IV. The curves for P = 0.84 mm. th'^ order of the curves is reversed indi Gating th'^ t the pressure is less than the critical pressure for minimum illuminating volt- age. The curve for D = 1,0 cm, is practically the same as that for C = CO cm, and D = 3,0 cm, and for this reason was not plotted, Pl^te VII, These curves indicate that ':h9 critical pressure for the minimum i luminat ing voltage is in the neighborhood of 1.8 mm. All of the curves in the plates IV, VI, and VII do not indicate very good conditions of sensitive- ness since the ordinates are not very large until the illuminating voltage is a-proached. 28 Cell No. 3. Potassium Metal. The curves are all ionization curves. Plate, VIII, shows the e'^fect of variation of D, before and after forming. A 0.17 cm^. L = 0.22 C. f . t ~ 26° C. P = 1,0 ram. Plate, IX, shows the e-^fect of variation of P, before and after forming, A ~ 2.17 cm^. h = 0.22 C.f, t - 26°C. D =■ 0.5 cm. 70 II 5! So tf5 5: 5: mm S5±r — o- m 5: o .t±t "So Si U- OF P. S. S. FORM 31 Plate VIII. The curves marked A, taken after forming, show thn t the illuminating ^''oltnge is in the order of their values for D. The curves marked B, taken before ■forming, ahoT^: that the illuminating voltages are not in the saiije order as those marked A, and that curve for D = 0.5 cm. shows an illum- inating voltage much greater than that for D - l.C cm. and D = S.O cm. These curves do not show very p:,ood conditions for sensi- tiveness since the ordinates are very small until illum- inating voltage is reached. Plate IX. The curves marked A show that the critical pressures for minimum illuminating potential is m.uch greater than that for 3.0 ma. CpII No. 4. PotasRium Metal. Data and Ionization curvea before forming. Plate X shows pffect of var i'ition of D. A — 4.36 cm^. L = D . 22 C.f. t = 24°G P = 60 rnrji Plate, XI ahowfl effect of variation of 13 . A = 4.36 cm^ . L = 0.22 C. f t = o4°G P = 5 mm. Plate XII shows e''^f'=ct of variation of P A = 4.36 cm^. L = 23 C f t = 25*^0 P - 3.0 mm. Plate , XIII shows eff«=>Gt of variation of B. A = 4.36 cm^. L=0 22Cf t- 26° C P = 2.0 mm. Plate, XIV shows off-^^nf o-f variation of D A = 4-36 cm^. Ii = 0.22 C f t = 26° C P = 1 mm Plate, XV shows effect of variation of D A = 4.36 cm^. L = 0.22 C . f t = 20° P = 3 rrm Plate XVI shows eff'='ot of variation of D A = 4.36 cm^. L = . 22 C • f. t = 0°C P = 3-0 mm Plate, XVII shows eff'=ct of V'^riation o"^ P A = 4 36 cm^. L = 0.22 C.f. t = 25° G P = 3.0 mm. Plate, XVIII shows effect of variation of D. A = 4.36 cm^ J\ 'I ^ JL ^ ^ 9^ XX Vy tV V X X Vv Vy w \y<^ V LX X X LjL >J X yj AX W X JLy 9 'A ^ ^ *^ V V> L = 0.22 C.f. t = 36°C P = 3 mm Plate, XIX shows effect of variation of P A =^ 4.36 cm*^. Ii y \ X y \ J u X X vy T> u ^ x x \^ ^y w ^y X .* la j» x la u x \y x x \y x « * 4 \^ w lii • = 0.22c. f. D =■ 1.0 cm. Ak/ X % w Vy ii i 9 Plate, XX shows effect of variation of P A = 4 36 cm^ L = 0.22 C. f. T) = 0-5 cm Ay w 4 «.y w i>ii • Plate, XXI. shows '=»ffeGt of formine^ rnet.^l. A = 4.36 cm^. L = 0.22 C.f D = 5 cm. P - 3.0 mm Also 'for A - 4.36 cm2. L= 0.22 C.f. D=1.C cm. P = 3.0 iT.m . Tahl '^R X \-K. krf X O uaud. rix e Jiven ueio-. x or px 11- . U V V a V -1 U V i; . D , • •- T r. J. , L' "i.! P. 1 . u i . U / A K T A 44o 0. ^ o . u IDA i . o lO , U f 4 O , 'J -i . U o , O /IT!-" 4: OD •7 A AQ'7 o . o 4. b A A n 443 D . O c;ac Dv.Jb t o , u Jo f op n ot^ O D. 44b Q A Dl O X 1 o . u 'iD , u b T A A /I CIA i 1 , D 1 "7 r. 1 r O , L' i b. U T D A c; "7 A ■^A A 455 o O . U 28, 37. 34. D 540 55,0 4b0 38.0 541 70.0 461 48,0 545 58,0 545 65.0 550 78.0 551 "' 10. ^ .0 « ^ TABLE NO. XVIII. . „ "ell ;-o, 4 refor*? "or mi 11*3. 1 = 3.0 ram. t = 38^0. L = 0.33 f . A - 4.35 ifi 1,1, . in Tol ^■ , D = C . 5 in . n = 1 . cm. n = 2 . Id . 1} = 3.0 cm. • i u i o.c 0.5 334 0.5 363 0,5 579 2.3 318 1.0 345 1.0 401 1,8 435 6.0 331 3,2 370 3.0 423 3. 450 10.5 323 6.1 382 7.5 428 5,0 460 15.0 334 11.0 384 10.0 430 8,5 470 40.0 335 off 385 13,5 4 31 16,0 480 off 326 IS . 5 432 32.0 485 31.0 433 29 .0 49 34.0 434 41.0 49 5 39,0 435 56.0 5C0 436 76.0 505 70.0 437 oC . C ^ 7.5 3S2 7.5 383 11.0 431 3,0 . 439 39 APl.F m. ^ix. PotasGium Cell t:o. 4. Be-^ore Forming, D = 1,0 cm. t = 24°C. L ■ = . 32 C.I, A = 4.35 cm2. i?Crioical voltage a- 'it. d i n mm. V in volts. p - 1 . mm . P = 2. mm. P =^ 3,' J mm , P = 5 . mm. P = 8.0 mm. a V d V d V Cl V U • 2 3 3.0 G . 7 0,0 'Z T A ^34 Ot.' -± i. on A 2y 4 1.0 395 1 . 6 304 1 ,0 38 i i , U AAA o , U 335 <0 • ± 320 2,8 3o7 1 , b 408 i, O • D 345 4,4 331 4,0 , d 431 .t the illuminating voltage is less for the curve after forming. The conditions for sensibility are better as shown in curve taken after forming. The lower curves are for D = 0.5 cm, before and after forming. The same as above may be said of these curves. Cell No. 4. Data and Ionization curves after forming, Plate, rKII, shows the effect of ^'ari:ition of D for P = 10 ram. A = 4.3G cm2. L ^ 0.22 C.f. t ^ 24°C. Plate, XXIII, shows the effect of variation of D for P = 5.0 mm. A = 4.36 cm2. L = 0.22 C.f, t = 250C. Plate, XXIV, shows the effect of variation of D for P = 3,0 mm. A = 4.36 cm2. L = 0.22 C.f. t = 25«C. Plate, XXV, shows the effect of variation of D for P = 2.0 mm. A = 4.36 cm2, L = 0,22 C.f. t = 26°C. Plate, XXVI, shows the «rrcct of v^.riation of D for P = 1,0 ram, A = 4.36 cm2. L = 0,22 C.f. t = 2S'^C, Plate, XXVII, shows the effect of varintion of P -^or D = 1.0 cm, A = 4.36 cm2. L = 0.22 C.f. Plate, XXVIII, shows effect of variation of P for B = 0.5 cm. A = 4.36 cm2. L = 0.23 C.f. Tables of data are given below for the plates XXIII, XXVI, XXVII, and XXVIII, 1 :Iq, -t. AfLe L' 1 O i'llii = 5.0 Rirn. t = 25" 0. L = 0.22 G, f. A = 4.35 cra^ . for i 1 1 a 1 vol t "^.j-e 'iTjii owl V i n vcj 1 1 H . ■n ^ ^ p - - « • > > ■ J . , J •.i 1 , c 30 1 . r 1.0 .1 1 1.5 355 4 . 5 3 :i 3 . ■7 -7 O • J • J . J 7.4 4 30 2.5 410 11 . 350 5.0 363 17.0 472 7.5 494 30,0 351 11.0 327 29 , 5 482 10. 5 508 35.0 367 34.0 417 45. 5 490 17. 5 534 54,0 370 28. 5 422 70.0 495 28. 5 550 84.0 372 1 25 . 430 108. 500 41.0 560 13G. 374 175.0 431 188.0 505 57 . 5 570 370. 37 5 12 2.0 433 340. 506 72.0 575 80 .0 o SO 107.0 585 128.0 590 148,0 595 179 .0 600 205.0 605 238.0 610 # 370. r 'S' ■ • ^ ' 1 . . '1 . - — ^^ J 1 . . Potaesium Cell No. - 4. After "onuing. V = ] .0 him. t = 36" C, L = 0,22 . f . A - 4.35 c m"^ . ■ Vol t :\^Pt . J \ c ui I ?n t , d 'xl^ IIiliI , V i n ty 1 to . D = 0.5 cm. V - 1 . cm. D = 2 ,0 cm. D = 3.0 cm. 1 V 1.0 i '6 1 . i ' C 1 . c 153 l.C o 7 r, r ^ 55.0 303 351 TO O Potci3siura Coll Mo. 4. Ti = 1.0 cm. t = 2 5°C. L = 0,22 C.f, A = 4.35 cm2. ■^Critical vclt^.Pis ini curr'^ nt. d i n Yi\m . V in ' - 1 . jr. 111. ? - 2,0 in;:;. V ^- ::. ri.ir:. _ • .J • C , .... d V 1.0 160 1.0 IS I 1.0 221 1.0 243 1.0 360 3,0 239 3. 5 260 8.0 338 3,0 332 3.0 472 5,0 270 7.0 291 13,0 351 5.0 363 5,5 535 10.0 295 11.0 305 21.0 360 11,0 39 7 10.0 566 20.5 310 27.0 320 32,0 365 24,0 417 16.5 590 41.0 320 49.0 325 57,0 369 28.5 422 19.5 600 57.0 322 71.0 326 64,0 370 125.0 430 26.5 615 83.0 325 85.0 328 113,0 372 175.0 431 30.5 620 114.0 326 140.0 330 200.0 374 192,0 432 38.0 630 133.0 327 192.0 331 280.0 375 168.0 328 off 332 193.0 329 off 330 73.0 324 190.0 331 •-I ,^ '7, ^ 774 r TABLE NO. VXVIII. Potassium Cell Ko. 4. Af t- r Forming. D = 0.5 cm. t - 35<>C. L = 0,22 C.f , A = 4.35 cm^. #Criticnl volt'igs ind current. d in ram. V in ^'■ol ts. p = 1, mm . V - - 3, rcrn . P = 5. rirn. P = 10 mm . d V :1 '/ 1.0 150 1.0 165 1.0 19 6 1.0 200 0.5 350 3.0 320 4.0 250 3.5 265 4.5 319 1,0 312 7.5 370 8.0 371 5.5 289 11. C 350 2,0 361 13.5 286 13.0 380 10.0 305 20,0 361 3,0 388 21.0 3S5 31.0 385 16.0 315 35.0 367 5.5 420 37.0 300 31.0 390 22.0 320 54.0 370 9.0 440 55,0 303 55.0 294 32.0 322 84.0 372 12.0 451 70.0 305 . 91.0 295 54.0 326 136.0 374 21.5 455 102.0 306 125.0 296 71.0 327 270.0 375 30.0 470 117.0 307 140.0 29 7 90.0 338 38.0 472 145.0 308 194.0 298 115.0 329 62,0 475 168.0 309 off 299 185.0 330 73.0 47 6 off 310 off 331 99.0 477 155.0 478 280.0 479 55,0 12 3.0 ff 331 270.0 375 280.0 4-^9 ei Plate XXII. The order of illumir,at ing voltages ia ref^ular. The beot conditione for aensitivenefls are shown by curve for L ~ 0,5 cm, Plate XXIII, The order of illuniinat ing volt'^g'='s is regular. The best corditions for sensitiveness are shown by curve for D = 0,5 cm. Plate XXIT. The order of illuminating voltages is regulat. The best conditions for sensitiveness are shov;n b y curve for D = C.5 cm. Plate XXV. The order of illuminating voltages is regular. The best conditions for sensitiveness are shown by a curve y;hich lies between curves for D = 0.5 cm and D = 1,0 cm. Plate XXVI. The order of illuminating voltages is regular. The best conditions for sensitiveness are shown b^'' a curve which lies betweer curves for D = 0.5 cm and D =^ 1,0 cm. Plate XXVII. These curves show conditions of sensitiveness for differ ent pressures and D = 1,0 cm. The curve representing best conditions for sensitiveness lies near the curve for P = 3,0 mm. Plate XX^rill. The curves show the conditions of sensitiveness for different pressures and D = 0,5 cm. The curve representing best conditions for sensitiveness lies near the curve for P = 3,0 mm. Variation of Area Illuminated. After Forming. Plate, XXIX, shows the effect of variation of A for P = 10 mm. D = 0.5 cm. V = 570, 490, 475, L and t constant. Plate, XXX, shows the effect of variation of A for P = 5.0 ram. D = 0.5 cm. V = 366, 365, 364. L and t constant. Plate, XXXI, shows the effect of variation of A for P = 3.0 mm, D = 0.5 cm. V = 323, 331, 320. L and t constant. Plate, XXXII, shows the effect of variation of A for P 3.0 mrn. D = 0.5 cm. V = 395, 394, 393, L and t constant. Tables of data are given below for plates ' XXXI and XXXII. P = 3.0 mm. D ^ 0,5 cm. t = 25"n. Unit of A 0.0R8 omS. T, = 0.22 H.f, r] i n >']rr,. A A A 8 18 32 50 70 105 135 1 4 8 16 ,1 ■ J 1 18 2? 75 120 J. 2 4 8 16 32 64 5 11 17 30 47 63 r> 4 8 IG 32 64 r I 'A 3 1 U!U Cell ::o. . 4, Aft 3T rorming _ 1' — 2.0 mm. D = 0.5 era. t 2.^ Unit of A = 0.068 L ^- ^2 . f . d v - I ''•olts L d L . d T O.ll 0.11 . 16 17 0.16 9 0.16 0.25 0,25 14 0.25 . 33 3 '7 0.33 19 0.33 0.44 50 0.44 27 0.44 0.64 70 0.64 40 0.64 131 1.00 115 1.00 .60 1 , 00 U- OF I. 3. S. FORM 3 U. CF 1. S. S. FOH^' 3 80 Curvee showing the Effect of the Variation of Inteiiaity of Illumination, Plates XXXII to XXXVI. The points for the smaller voltages lie more nearly on a straighl line. The largest error in the intenaity "^or the point farthest from the line is 7> of ,15 C,f, or ,01 C.f, The points for smaller inten- sities show much smaller deviations than those for larger intensities. The higher voltages applied caused unsteadiness of the current, hence, the galvanometer deflections are liable to larger errors. The most sensitive conditions represented py a curve for various plates are tahulat^d in the form below. From this table the con- ditions of highest sensibility may easily be selected. Table to show best conditions for senei tivenesa, Y = critical ^'■oltage. I = critical current in galvanome f^r deflections, r = best "listance between electrodes in cms. P = best pre3?.ure in tmr. ■Pefore "forming n-Tefiil, I XI 460 to 527 16 to 28 1 to 2 5.0 XII 324 32 0.5 3.0 XIII 304 26 0.5 2.0 XIV 349 to 366 6 to 9 1 to 2 1.0 XV 454 8 2 3 XVI 335 to 39 6 10 to 45 0.5 to 1.0 3 XVI I 324 32 0.5 3 XVIII 383 to 431 7 to 11 1 to 2 3 XIX 387 to 460 28 to 29 1 3 to 5 A X 304 to 324 26 tc 32 2 to n After Forming Metal. n'll 4 79 2B0 0.5 IC XXIII 375 270 0.5 5 XXIV 331 off scale 0,5 3 XXV §9 6 to 331 12 3 tc . ISO 0,5 to 1.0 2 XXVI 303 to 324 55 tc 72 C.5 to 1,0 1 XXVII 374 203 1 3 XXVIII 331 off scale 0.5 3 Values of t. (1) about -20° C salt and ice. (2) QOC. (3) 25°C. (4) 38«C. 82 By inspection of the taMe above, it is seen that the beat con- ditions for sensitiveness before forming are about, V = 300 volts. D = 0,5 era, P = 2 to 3 mm. t — C And the best conditions for Sf?n8i tiveness after forming are about, V = 330 volts, D = 0,5 cm, P = 3 ram. t - 25«C. The cell is ibout 100 times more sensitive after forming than before forming. 83 Theoretical rieduction on M3aGuremerit of Intensity of Illumination. If th«? intoniUty o^ i llurrr n^.tion varies dir°ctly with the cur- rent for very smnill intensities, then it it? po^gible to calculate the intensity measvirable with in instrument of given sensibility. From the Fig, 6 belo;v it is seen th"t for the curve of 366 s yir;. 6. 130 volts, plate XXXIT, the tan ^ - ~ - 2qo cd = I current flov/ing c = 3.78 X 10~^^ amp, per ram. = 0.65, or S tan 9 0,65 d = ram. 0.65 X 3.78 X 10-10 but 3 ~ ~j-2 , and for particular values of S = 200,. and ^ - ( 100cm)? c = S/^ = 200 X (lOO)*^cra. = 2 x 10^ cm; 106 , or, /^J^:^ substitute value of S = 3,78 x 10-TU then. J = X 10^ x~d.65 X 3.78 x To"^ = / 1. 9 "x 10-^ I I By means of an electrometer a current I = IQ-l'^ can be measured. 84 Substituting this value of I in the eiuntion i"^ove, then X = v^-AAQ^^ = 2.21 X lO'^ cm. or 321 meters, the distance the 10-13 2.47 c.p, lamp could he removed from the cell and still he detected. By means of a tilted electroscope a current T = 10"1^ amperes can "be measured. Substitute this value in the equation, / = J ?^ ^ IQ"^ = 74.9 X IqH = 7 X 10^ cm. or 7 kilometers or 10-15 4,3 miles. The 2.47 c.p, lamp could be detected by means of an electroscope at a distance of 4,3 miles. To detect a candle instead of the 2.47 c,p. lamp at 4,3 miles distance by means of the tilted electroscope of 10""15 sensibility the distance could be as follows: Since the intensities of illumination of cell must be the same then, 3.47 _ 1 / _ ._4.3 _ ^ , Professor Joel ntebbins in his work on measuring the variation of intensity of illumination of the variable stars Algol 'and others used a selenium cell. It is possible to detect a candle at a dis- tance of 500 meters, or 0.3 miles, V7ith such a selenium cell. The equation for the distance at which this postassium cell is sensitive v,'hen the current is measured with a tilted electroscope is, and for some other distance it is ^ = v/ — •2 ^ = T y = 7 X 10^ cm. = 5 X 10^ cm, Iz, ^^1 ^ 49 X 10 1^ ? then -jT- = = 2 X 10^ aDoroximately 26 X 108 Thus it is seen that the potassium cell is a":out two hundred times m.ore sensitive than the selenium cell. Fr^r^jy required to produce an Ion, TO produce an ion a "ertain minimum amount o-^ energy is required This energy is thit required to draw an electron out of an isolated molecule against the force of attraction of the positive charge of the molecule. In Fig. 7 let "R-^ be the ra.iiua of the molecule, , the positive charge, G3 the nega'-ivc charge of electron. If the FIG. 7. electron be displaced a distance dR the work done will be dW = ~1 — 2_ dR, The total energy re:iuired to draw an electron out- R 2 1 side of the influence of the positive charge is 00 W ^ / ?L1A dR = "^X-fe unii-g Qf work. -^/f. r2 Ri 61=63 = 4.67 X 10~-0 y., S. U. R^ = 10"*^ cm. for hydrogen. (1) W = * ^Q«8~" ~ 2,18 X 10*"^^ ergs, the minimum amount of energy required to draw an electron from an isolated molecule of hydrogen or to produce a hydrogen ion. For an isolated molecule it is necessary to withdraw the' elec- tron to an infinite distance from the center. If the molecule how- ever, is in an electric field the force of attraction between the molecule ,?nd the electron is zero at a distance say from the center 86 The effect of the electric field tends to decrease the energy ic?- quired to withdraw the electron on one aide of the molecule (with references to the dirpotion of '^xt^rnal electric field) while that on the opposite side is increased. Let Fig. 8 represent the molecule of radius ; H is the direc- FTG. 8. tion of the external electric field in the direction of motion, Oa, of the electron. Let be the distance beyond which the electric attraction between the positive charge of the molecule and the elec- tron ie zero. The work required to withdraw the electron beyond the influence of the molecule is, W = / ilJ2 ^ ^ gl ^3 ^ gl H _ £lj2 4 h2 R Rl - Rs but.e^ = ^2 = 5 ; and it is reasonable to assume R3 = 4 , Rl 4RJ R3L ^ ' > 4 Substituting the values for R^ and S, then fPl TT - 3 (4,67)^ X IQ-^^Q _ . ^„ \d>) A' 1- = 1.63 X 10 ergs, 4 X 10-8 Thus the value for H in calculation (2) is three-fourths that in calculation (l). Hence 2.1-8 x lO^^^ ergs is' not the minimum value. The minimum amount of energy required to produce an ion b y collision cnn be det^riuined roughly from the -In^ri tf.V.on in this in- vestig.ition. For the conditions of this work t-hs minimum ener^^y is Tf = E C^, where T. is th;^ potential gradient, or electric force, e is the charge of a negative ion or electron, and ^ is the mean free path of an ion. The force E can be determined roughly as follows: - The voltage applied to the electrodes of the cell necessary to pro- duce velocities high enough to cause ionization by collision, is ob- tained from the ionization curves. This voltage is shown very. dis- tinctly at the point on the curve where the ordinate or current shows an increase after the saturation state has been reached. Let V be this voltage taken from the curve, and let D be the distance between the electrodes. Then, V ^ " 300 D ^* ^' a) First assumption regarding mean free path of electrons. Assuming that the negative ionb or 'Electrons and the molecules of the hydrogen gas act as a mixture of two gases, the equation of t ^fi mean ^lee path given by Max;"ell in the kinetic theory of gases is /= : s / ^ m-j^ for the electrons. for hydrogen molecules. For an electron the diameter of the sphere of action when two electrons collide is practically zero, Eut OJi the diameter of the sphere of action when an electron collides with a molecule of hydro- gen is assumed equal to the radius of the molecule or 10""^ cm, is the mass of an electron eq:;.al to 8.8 x 10"''^° grams. IvU is the mass of a hydrogen molecule equal to 1,6 x 10"^'^ grams. ^1 - _il?_Jl_i^IL, * 5.5 X 10"''^ grams. This" value is negligible in " 1.6 x"l0-3^ 88 comparison with unity or /^THH . The equation For the mean free path of the electrons is, / - 5 — cms. In this equation is th-^ number of molecules per cubic centimeter. From the kinetic theory of gases may be calculated, r V = R T the gas law which becomes p V = 2/3 ^ = 2/3 N 1/2 mO^. Where^is the average total kinetic energy. i 8 the number of molecules in ■''"olume V, is the square of average velocities of molecules, 1/2 mC^ - T, where o( is the universal constant, ? V = 2/3 i:oCT or, ? = 2/3 :ioC7 for unit volume, IT = 3/2 -JL, the number of molecules per unit volume, , 13,6 X 980 h . . . . = 3/2 — r — ) h IS m cm, 2 X 10**^° T Suppose T - 27 + 273 ^ 300 for this Tork, 15.6 X 980 2 X 10""!^ X 300 N = 3/2 i ^'^.^,-, h = 33 X 10^^ h in cm. mercury. Substitute this value for expression for mean free path, ^ ^ 1 _ 955 X 10-5 TT X 5:5 X lois h lo-is Applying the equation W = E t , e = 4.67 X 10-10 ij.^ c^^ Tj. cm. 300 D 965 X 10-5 ••— cm. For h = 0.5 cm. D = 1.0 cm. V - 330 volts, w ii-67 X 10-^Q 9 6 5 X 10-^ , ^ = "^0~0'~x 1,0 x"^,T = 1-^^ ^ .10 29 For an average of ten rou-hly det'='rrr,ined valuo3 1.05 x l O" ^^ er gs was o>?tai!:?>d for the ri.iwirriUia energy required to produce an ion in hydrogen gaa ly collision. b) Second assumption regarding mean. free path of elec':rons. If it be assumed that the negative iona or electrons occupy no space in the gas, or, thit the hydrogen gas -^cts as though the elec- trons '.vere not present, then the value of the mean free path is the same as th-.t of the molecules. That is, TT N (T^'f^ h - 685 X 10-5 cm. h For h = 0.5 cm." D = 1.0 cm. V = 330 volts, 4.67 X 10"^5 X 685 x 10*"^ x 330 ^ " " 300 x'oTs = •'^0 X lO'll ergs. About 40^ increase from 1,09 x 10""^^ ergs, c) Third assumption regarding mean free path of electrons. Ey using the assumption that the mean free path of the electrons is 47^_ times mean free path of ^he hydrogen molecules, as Bishop di4 the following results are obtained. TT 33 x lO^T^h X 4 xTo^ ~ ^ ^ ^ 4^Z^0z^xA36Q..jLJLai5 „ . ^ ..-15 V 300 x j3 - 21 x 10 ^ For P = 0,3 cm. D = 1,0 cm. V = 260 volts. T = 21 X 10-15 ^ ^^32 ^ ^Q^ll ^ 1x0.3 An average of 10 calculations gives 1.^7 x 10~'l ergs. To recapitulate, the values det^rmi^ed above and those by other inves tig?. tors are:- 1.- For an isolate molecule the theoretical value is 2,18 x 10"^^ ergs. 00 2. - For a molecule in an slectric field the theoretical value is 1.63 X 10""^^ erga. 3. - The average of ten vali^eo determined from the data in a-^cord- ance with the first assumption is 1,05 x 10""^^ erga. 4. - The vTlue determined in accordance vith the second assumption is 0.70 X 10"^^ ergs, 5. ~ The average of ten values deterrair.ed in accordance v;ith the third assumption is 1.77 x lO""-^-*- ergs» 6. - Bishops obtained a value, a method sirailar to that used in this work and in accordance 'A'ith the third assumption used in the '"ifth determination, 1.58 x 10""-^ ergs, 7. - Rutherford*^ determined the energy required to produce an ion by the alpha particle. His value is 2,7 x 10"" -^-^ ergs, 8. - Ceiger"^, and later Taylor , using the same method obtained about 5 X 10~^^ ©rgs, ani other investigators obtained values even as large as 10 x 10"^^ ergs. In the me*-hod, based on the ionizing poTver of alpha particles, it is assumed th-^t their kinetic energy is entirely'- transformed into energy of ionization, and that the decrease of the kinetic energy over a certain range divided by the total numbers of ions produced gives the energy required to produce one ion, flince a part of the kinetic energy of the alpha particle increases the average kinetic energy of the gas without producing ions, the ionizing energy is taken too large and th-? energy required to produce an ion is neces- sarily too large, 1. Phil. Rev. p,325, TTov, 1911. 2, Radio-Ac'-ivity, second edition, p. 552, 3, Proc. Royal "^.oc . "^xl. 82, p. 486, 1S09, 4. Phil,::ag, P.G7C, April, 1912, Value r!UirV»er (l) r <*pros'='r ts the rninimuru energy required to pro- duce an ion v...cii a molecuiLO is isolated. Thio value is niuch larger than that for a molecule in an electric field. Ru therf ord!s^ deterrrdn- ation is much nearer the vnlue nurr.ber (l) than any of the others. Pi shop's determination is very close to the value number (2), while my determination of the ionizing energy is slightly larger. Since the assumption regarding the mean free path in number 5 and 6 are the 3^-r..e, and these values differ very alic^htly from the theo- retical value number (2), it indicates that the assumptions made are not far from Lhe truth. Owing to lack of time a..... c^i^ce an exact determination of the minimum energy required to produce an ion by collision is impossible; in the near future hov;ever, it is hoped that this can be done with the data already in hand. Design of a sensitive photo-electric cell. At the beginning of this investigation it was expected to de- sign a photo-electric cell which could be used in astro-photometric investigations. A tube has been constructed which has combined in its design as many of the most favorable conditions for sensitiveness as is practical, but owing to lack of tim.e it has not been tried out. The best angle of incidence is about 60°, the best hydrogen gas pres- and sure is about 2 mm.,^the best metal for practical use is potassium at about 25°cr . The diagram, Fig. 9 below shows the plan of the tube. The diameter of the tube, Fig, 9 is about 2 cm. and the end E is sealed as uniformly anJ as nearly plane as po^^sible. The incident light enters through E and is incident upon the cylindrical metal cathode C, ;:he end of which is turned to A G0° cone. Upon the 1. Radio-Activi ty, second edition, p, 552, I shaded portion of C is uiatilled a layer of potasaium metal -md the hydride surface formed. This is done bef'ore th- cathode is moved into its present position Ty means of an electrorcagnet. The anode A is a hollow cylinder with the upy-er sides making C0° with each oth^-^r so thnt the shortest distance between all pointfl on the surface of C are equidistant from the inner surface of A, the inner surface being highly polished or 8ilvf>red in order to reflect back to the cathode the rays ;vhich are first reflected from it. In this way multiple reflection is obtained with a large anode surface for collecting the ions pro^-uced by the action of the electrons emitted from the cathode Then light is incident upon it. It is hopexi that this tube with the most sensitive conditions now known, will be sensitive encugh to take the place of the erratic selenium cell used in astronomical work. E A FIO. 9 3 Summary and ConclueioriS, The follovring facts ar? established by this investigation for this type of photo-electric cell. 1. - Owing to the low melting temperature of caesium the use of this metal in photo-electric cells for photometric use is very im- prac tical . 2. - The temperature at ^vhioh it is best to operate a potassium c-^^ll is about c5°C. 3. - Cooling the potassium cell much below 25°G does not increase its 8(=»nsit iveness. 4. ~ The sensibility of a potassium cell can be increased more than 100 times by the process of forming the hydride surface. 5. -- The distance bef.veen the electrodes for best sensitiveness is about 0. 5 cm, 6. - The hydrogen gas pressure at V7hich the cell is most sensitive lies bet'.'-een 2 and 3 mm. of mercury, 7. - The potential difference applied to the electrodes for most sensitive conditions is about 330 volts, 8. - The minimum energy required to produce an ion by collision was calculated from the data and found to be of the order 1,77 x 1C~^1 ergs, while the theoretical value determined is 1.53 x 10-11 ergs. 9. - Assuming that the straight lines obtained which show the rela- tion between current and intensity of illumin-.tion hold for ex- ceedingly small intensities, th'^n by u«ing a tilted electro- scope of sensibility lO^lS amperes, a candle could be detected at a distance of 2.7 mdles. This indicates that it is highly I- robable th-'.t a photo-electric cell could be used in astro- photometric work. 04 The author takes great pleasure in acknowledging his indebted- nesa to rrofeoscr A. ?. Carman for the facilities for this investi- gation, and to TrofeGoor Jakob Kunz, both for his general supervision of the worTc and for many valuable sufygeations I Rcholaatic Record of Jacob Harrett Temp, The author was born Au.t^ust C6, 1877, in Haltiroore, Ud., .vhere he obtained his early education in the public schools. On October 15, 1900 he entered "The Deichmann college Preparatory r)Chool"of Balti- more, Md,, where he was graduated in 1903, During the years 1902-1006 he attended the University of Illi- nois, submitted a thesis on "Apparatus and Methods for Measuring Electric Wa-.^es", and received the degree A.^^. in flcience. During the years 1906-1908 he was assistant in Physicsf at Purd\ie University, Lafayette, Indiana. During the years 1908-1911 he was assistant in Physics at the University of Illinois, and at the same time completed graduate courses in Experimental and Theoretical Physics, and Mathe- n-.atics. In 1910 he submitted a thesis on "The Magnetic Properties of Certain Rare Eartiib", as partial fulfillment for the requirements for the degree of A.M. in Physics, For the year 1911-1912 he was awarded a fellowship in Physics, and during this time he completed the thesis on "The Conditions of Sensibility of Pho to-Elec tr ic Cells with Alkali Metals and Hydrogen". A paper in press, title,- "A Substitute for a Eronson Resist- ance," by Cornelius Kemp,