NRLF EE7 LIBRARY UNIVERSITY OF CALIFORNIA. RECEIVED BY EXCHANGE Class Ube TUntversits of Gbtcago FOUNDED BY JOHN D. ROCKEFELLER THE EFFECT OF TEMPERATURE ON THE IONIZATION OF A GAS A DISSERTATION SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL OF SCIENCE IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY (DEPARTMENT OF PHYSICS) BY J. HARRY CLO IQII ITbe THnfx>ersit of Cbtcaoo THE EFFECT OF TEMPERATURE ON THE IONIZATION OF A GAS A DISSERTATION SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL OF SCIENCE IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY (DEPARTMENT OF PHYSICS) BY J. HARRY CLO IQII THE EFFECT OF TEMPERATURE ON THE IONIZATION OF A GAS BY J. HARRY CLO The ultimate purpose of this experiment was to determine whether it is possible to change the kinetic energy of the molecule sufficiently to affect the stability of the atom. The ionization of gases was chosen as a type of the phenomena which involve the separation of the electron from the atom and which therefore depend on its stability. The experiments made heretofore upon these phenomena may be divided into three classes, namely, those on the ionization of gases, those on the photo-electric effect of ultra-violet light, and those on the emission of electrons from radioactive substances. H. L. Bronson 1 has shown that upon heating radium salts under conditions which eliminate radioactive transformations, volatiliza- tion of products, etc., the ionization of a gas by gamma rays is independent of the temperature of the radium. But the intensity of the gamma rays, and therefore their ionizing power, is generally believed to depend on the number of electrons given off per second from the radium. Hence it would seem to follow that the rate of emission of the electrons is not affected by the temperature. His observations extended from 180 C. to 1600 C. and showed practically no variation greater than i per cent. The expulsion of negative electrons from metals under the in- fluence of ultra-violet light was shown to be independent of the tem- perature of the metal by Millikan and Winchester, 2 Ladenburg 3 and others. Millikan and Winchester made observations on different metals up to temperatures about 350 C. Ladenburg experimented on platinum, gold, and iridium, varying the tempera- tures from 20 C. to as high as 860 C. With platinum his results 1 Proc. Roy. Soc., A, 78, 494, 1907. 2 Phil. Mag. (6), 14, 188, 1907. 3 Verh. der Deutschen Phys. GeselL, 9, 165, 1907. n6 /. HARRY CLO show no variation greater than about 5 . 5 per cent from the mean. For gold and iridium his results are even better. The effect of temperature on the ionization of a gas has been investigated by J. Perrin, 1 McClung, 2 . A. Gallarotti, 3 Herweg, 4 and Crowther. 5 With the exception of Perrin none of these ex- perimenters found any systematic variation of the ionization. Perrin measured the ionization produced in air by Roentgen rays. Correcting for the variation of the ionization with density, he found it to be proportional to the absolute temperature. McClung in- vestigated the phenomenon very thoroughly, using Roentgen rays as an ionizing agent. He measured the ionization in air, hydrogen, and carbon dioxide, working both at constant pressure and at con- stant density. His method allowed him to correct for the varia- tion in the ionizing power of the rays. For air at constant pressure and at temperatures up to 272 C., he found the ionization to be constant to within about 6 . 5 per cent of the mean value. At con- stant density he found no greater variation in readings up to 201 C. For hydrogen at constant density his results show no variation greater than about 15 per cent up to 226 and for carbon dioxide no variation greater than 4 . 9 per cent up to about the same tem- perature. Gallarotti investigated the effect of temperature on the ionization of air at low temperatures. With X-rays his results show the ionization to be constant to within 2 . 5 per cent of the mean for temperatures down to 187 C. With radium he obtained measurements of the ionization at 10, 60, and 187 C., which did not vary more than i . 2 per cent from the mean. These results show that there is no variation of the phenomena with temperature such as might have been expected from the re- sults of Perrin on the ionization of gases. They show that for the temperatures considered, the rate at which the electrons are sepa- rated from the atom does not vary more than about 10 per cent in the case of solids and about 5 per cent in the case of gases. 1 Annales de Chimie el de Physique (7), n, 496, 1897. 2 Phil. Mag. (6), 7, 81, 1904. 3 Atti delta, R. Accad. dei Lincei, 16, 297, 1907. 4 Annalen der Physik, 19, 333, 1906. sProc. Roy. Soc., A, 82, 351. EFFECT OF TEMPERATURE ON IONIZATION 117 The kinetic theory, however, leads to the conclusion that the variation of the stability of the atom with temperature must be very slight and may even be beyond the limits of experimental determination. While the above results are consistent with this conclusion, they throw no light on the question as to whether a smaller variation takes place. In the present experiment an attempt has been made, first, to re- duce the observational errors with the purpose of measuring smaller variations than would have been detected in previous experiments, and second, to work at higher temperatures than had been employed in previous experiments on the ionization of gases. The experiment was made upon gases for the following reasons: (i) According to the kinetic theory the molecular structure of a gas is simpler than that of solids and liquids. (2) The application of the fundamental concepts of the kinetic theory to gases has been more thoroughly demonstrated than in the case of solids or liquids. (3) The previous results on gases are not so accurate as some of the results on solids, and the experiments have not been made at as high temperatures as should be attainable. OUTLINE OF EXPERIMENT The observations consisted in the measurement of the ioniza- tion current in a gas within a closed vessel, by means of the rate of leak of a charge to an electrometer. An attempt was made to measure the ionization at constant pressure, but owing to varia- tions and disturbances due to the change in density, this method was abandoned and all observations were made with the gas at con- stant density. Air and hydrogen were the only gases studied. Radium was used as the ionizing agent. In all the observations recorded here the gamma rays were the only radiation entering the ionization chamber. The current was measured with a quadrant electrometer of the Dolezelek type, arranged to have a sensitiveness of from 150 to 200 scale divisions per volt at a distance of 150 cm. With this sensitive- ness the spot of light from the mirror moved one millimeter in from o.i to o. 2 seconds, a rate which varied in different series of obser- n8 /. HARRY CLO vations. No observations were made in which the light did not move over at least 400 scale divisions. The temperatures were measured by means of a gas manometer, whose minimum sensitiveness was about one-half millimeter per degree. DESCRIPTION OF APPARATUS The apparatus as shown in the accompanying figure is as follows : C is the vessel in which the ionization took place and whose tem- perature was varied. It was made from an iron cylinder of about FIG. i ii cm internal diameter, by reducing the walls to a thickness of a millimeter or two everywhere except at the ends. The side through which the rays passed was further reduced. Ends of heavy iron plates were brazed to this wall, forming a cylindrical chamber of about 1 8 cm height. EFFECT OF TEMPERATURE ON ION I Z AT ION 119 The tube c leads downward to the manometer. It is sur- rounded by a water-jacket W. By means of the amber plug p, and the cap which presses the plug into position, it supports the rod R of about 2 mm diameter. This rod forms one electrode. The amber plug gave satisfactory insulation throughout the ex- periment. The vessel C rested upon asbestos and an iron plate. Surround- ing it was the electric furnace F. The furnace was surrounded by asbestos and the whole inclosed in an iron box /, with walls about 2 cm thick. The manometer M was made of a graduated capillary tube. It was thoroughly cleaned and filled repeatedly with dry air before using. For convenience in placing it and in reducing the readings to degrees of absolute temperature, a bulb m was blown in the tube to form a reservoir for mercury, thereby keeping that arm of the mercury at very nearly a constant height. The whole air column was inclosed in a water jacket (not shown in the figure) to regulate and determine its temperature. A is an auxiliary chamber. It consists of a metallic box in which a plate a is held insulated from the box by means of an amber plug. At the top of the rod which holds this plate is a mercury cup which, with the movable rod K, constitutes the earth- ing key of the system. This insulated system includes the rod R, the plate a, one pair of the electrometer quadrants, and the connect- ing wires. U is a layer of uranium oxide used to ionize the air in A . S is a metallic shield for A . E is the electrometer. The radium was held in a lead block L, in such a position as would expose all parts of the gas chamber to the gamma rays. It was of sufficient strength to give, in this position, a measurable rate of deflection of the electrometer through 5 cm of lead. The electrometer and all connecting wires were surrounded by earthed conductors to prevent leaks and electrostatic disturbances. For the source of potential small storage cells were used. It was necessary that this potential be fairly constant. By letting the freshly charged cells discharge to that potential which remained constant for the longest time and using them in this condition, they were found to be all that was necessary. 120 /. HARRY CLO METHOD OF OBSERVING, SOURCES OF ERROR In taking the observations the method of procedure was as follows : The vessel C was repeatedly filled with the gas by exhaust- ing and allowing the gas to flow in through drying agents. The temperature of the gas and the barometric reading were then taken. The reading of the manometer being taken at a known barometric pressure and for a known temperature of the gas in the manometer, the latter was sealed on to the chamber C. These readings were again taken. The values under these known conditions gave the constants of a reduction formula which in turn gave the absolute temperature in terms of the readings of the manometer and the temperature of the air in the manometer. The needle of the electrometer was charged to a potential that would give the desired sensitiveness and at the same time minimize any errors due to the variation of this potential. The quartz fiber suspending the needle was made conductive by coating with a solution of zinc chloride. Although it was necessary to moisten the fiber every week or two, this method proved to be more satis- factory than any other. The potential of the system was held at zero, while C was given a potential above or below this value. This potential was generally about 300 volts, a value well above that which would give a satura- tion current. A was held at a potential of opposite sign to that of C, and of sufficient value to give the saturation current caused by the presence of the uranium oxide in A . To take a reading the radium was removed from its position near C, the uranium oxide from its position in A, and the system, insulated by opening the key K. Under ordinary conditions there would be a perfect balance of any small leaks and no deflection of the electrometer would result. If a charge leaked into the system from C, the uranium oxide was inserted into A sufficiently to cause a charge of opposite sign to pass into the system from A . By adjusting the position of the oxide the system could be kept at zero potential indefinitely. The radium was now placed in position and the rate of the deflection of the electrometer needle was measured. In suitable weather no difficulty was encountered with electro- EFFECT OF TEMPERATURE ON IONIZATION 121 static disturbances. It was seldom necessary to use the balancing device on account of failure of insulation. The insulation was always tested by keeping the system at zero potential for a period much longer than the time necessary for taking a reading. The absence of variations due to disturbances was considered sufficiently demonstrated when successive readings at constant temperature were found to agree as closely as it was possible to measure the rate of deflection. The first difficulty that was encountered was one similar to the disturbance mentioned above as probably due to change in density. It was found that while the temperature of the gas was changing, except when that change took place very slowly, the rate of deflection of the electrometer was not constant but varied irregularly. Since this variation was absent when the temperature remained almost constant, it was considered due to some convective disturbance in the gas and was eliminated by slow and careful heating or by taking the observations at constant temperatures. A second difficulty was found in the expulsion of ions from the hot metallic electrodes. This could not be overcome completely in the apparatus used for these observations. It was, however, partly overcome. This heat leak 1 was found to begin ordinarily at about 350 C., but, by prolonged or repeated heating at tempera- tures above this value, the temperature at which the leak first appeared was changed to about 450 C. Above this temperature it was always present. It was to meet this difficulty that the auxiliary chamber A was introduced into the system. To measure the ionization under these conditions the tempera- ture of the chamber C was first brought to as nearly a constant value as possible. With the radium removed, the position of the uranium oxide was varied until the system when insulated would remain at zero potential for a period at least several times as long as that required for a reading. The radium was then placed in posi- tion, the reading taken, and the balance again tested. If the balance was now disturbed enough to affect the reading by as much as o. i per cent the reading was discarded. As is well known, a temperature is soon reached at which this 1 Richardson, Proc. Camb. Phil. Soc., n, 287, 1902. 122 /. HARRY CLO leak increases very rapidly with the rise of temperature. At a temperature of about 650 C., it became impossible to keep the temperature sufficiently constant to be able to balance the leak for a period great enough to measure the ionization. At temperatures above 600 C. the escape of the gas from the ionization chamber began to introduce another source of error. This difficulty alone was sufficient to limit the range of observations to temperatures around this value. DISCUSSION OF READINGS, DATA The results of some of the observations are shown in the accom- panying tables. The rate of ionization of the gas, as represented by the rate of movement of the electrometer needle, is here expressed in millimeters per second. The rate in any one table is not to be compared with that in another table, as the sensitiveness of the instrument was not the same for the different series even when the same gas was used. In Table I, columns I, II, and III show readings for air. Columns IV and V are for hydrogen. Columns I, II, and IV show individual readings only. It was impossible to work with hydrogen at as high temperatures as those reached in air because the convective disturbances were much greatei than in air, and because the vessel C would not hold the hydrogen under as great a pressure as the air. As one may see from the table, the individual readings for air are constant to within about 0.25 per cent from the mean for temperatures up to about 500 C. For hydrogen column IV shows about the same uniformity, but the readings were taken to about 425 C. only. While the individual readings of column III are not so nearly constant, the mean of the readings at each temperature shows a variation of only about o . 5 per cent from the mean up to about 615 C. In this series the readings at the highest temperature are subject to a slight correction on account of the leak of air from the vessel. Either this correction or the difficulty of keeping the temperature sufficiently constant would account for the irregularity in the readings at this temperature. In general the readings agree as closely as those taken under the same conditions and at the same time upon the gas at room tern- EFFECT OF TEMPERATURE ON IONIZATION 123 perature. Hence the readings agree as closely as the method of observation would warrant, upon the assumption that there should be no variation at all. TABLE I Arc HYDROGEN I 11 III IV V Temp. Rate Temp. Rate Temp. Rate Mean Temp. Rate Temp. Rate Mean 22 25 6? 7-93 7-93 7.90 7.92 7-93 7 Q3 20 '28 42 7-77 7-77 7.78 7-77 7.78 7 7% 21 5-66 5-68 5.65 5 68 5^7 10 6-35 6-35 13 6-39 6-39 6-37 6. 30 6.' 3 8 107 6.38 38 6.39 6 ^ 6-37 77 7-93 7.91 7.90 7.92 7-93 7.90 7.92 7.90 7.92 7.90 7.92 7.92 7.90 7.92 7.91 7.91 7-94 59 Si 94 1 06 197 228 251 273 302 3ii 323 332 359 398 424 441 466 497 515 7.78 7-77 7-77 7-75 7-78 7-75 7-77 7.78 7-75 7.78 7-75 7-79 7-77 7-75 7.78 7.78 7-75 7-75 7.62 197 2OO 270 283 298 3OQ . 396 442 5.65 5-66 5-66 5-65 5-64 5-64 116 6.38 144 6-35 6 38 .... 5-66 5-64 6-35 6-37 6 33 6.38 .... 217 198 6.38 6-37 6-39 6-35 318 326. . 486 513 &3 5-72 5-67 5-67 5-62 5-69 5.67 5-47 5-8o 314 6-35 6-37 6-37 6-37 6^7 334 367 5-69 5-66 366 6-37 438 467 473 40 1 402 6-37 6-37 6 17 343 6-37 6-39 6-37 5-63 429 6-35 413 6-47 6.27 6 27 6^33 TABLE II Mean Rate from I' ii' III' IV V 0-IOO...... 7 024 7 771 S 667 6 37"? 6.380 IOO -2OO 200 -300 3OO 400 . 7-9I5 7.913 7 OI4. 7.765 7.766 776^ s 6*6 6.380 6 17 6.360 6.365 6 37O 4OO 500 7 Q2O 7 76^ * 663 6 36< 6 334 5OO -6OO 5.660 In columns I' ', II', III', IV, and V (Table II), the mean rate for one range of temperature of 100 is compared with the mean for 124 J- HARRY CLO other equal ranges. It will be noticed that for the higher' tempera- tures the mean rate is in general slightly lower. This is doubtless due to the difficulty in keeping the temperature sufficiently con- stant or in varying it slowly enough to avoid the disturbances due to convection currents in the gas. SUMMARY The temperatures were varied from room temperature to about 615 C. The absolute temperature and therefore the mean kinetic energy of the molecules was increased to three times its value at room temperature. From the kinetic theory it may be shown that about i per cent of the molecules have a probable mean energy four times this mean. Hence the energy of agitation of i per cent of the molecules was probably about twelve times the mean energy at room temperature. Readings were taken nearly 300 C. above the temperature at which electrons are first driven from the metals by heating. Both hydrogen and air were experimented with, the latter furnish- ing a desirable mixture of gases of different molecular weights. The individual readings were in general constant to within o. 2 per cent of the mean. In columns I', II', and III' (Table II), which are mean readings for air, the greatest variation is a little over o.i per cent. The ionization of air by means of the gamma rays from radium is therefore independent of the temperature of the gas to within o . 2 per cent up to about 600 C. For hydrogen the same independence is shown for temperatures up to about 430 C. A variation of over 200 per cent in the absolute temperature of a gas does not affect the stability of the atom sufficiently to change the ionization by more than about o. i per cent. In conclusion the writer wishes to express his thanks for their assistance and encouragement to Professor Michelson and the staff of Ryerson Physical Laboratory, and especially to Professor Millikan, under whose direct supervision this experiment was undertaken. THE UNIVERSITY OF CHICAGO January 26, 1911 JJ3SSS5?"" OVERDUE. 5>tf 10 2Jwi .. ' LD 21-'