JOURNAL OF CHEMICAL EDUCATION
DEMONSTRATING IONIZATION POTENTIALS WITH MERCURY RECTIFIERS AND GAS THYRATRONS ALEXANDER P. MARION Queens College, Flushing, New York
readings are possible. If the projection is made in the S E V E R A L authors (1, 8) have made use of the ionization potential approach to the problem of compound manner indicated the curve approximates that of the formation in elementary chemistry. The inclusion of current-voltage characteristic of a typical gas-filled cold-. this material in a course of study introduces the desir- cathode diode (9). The shape of these curves is dependent to some extent ability of illustrating the iotlization effect. Three demonstrations using simple equipment and standard elec- upon the tube temperature; thelower the temperature the shorter the length of the "plateau." Therefore it is tron tubes are described below. Mercury vapor rectifiers and gas thyratrons are recommended that the tube be permitted to warm up for readily available and possess the prime requisites-of a ten minutes or more before readings are taken. A more direct proof of the formation of positive ions source of electrons, an anode to accelerate these electrons, and an ionizable gas. Quantitative results using is achicved by using hot-cathode gas-tetrode thyratrons these tubes were not to be anticipated, for the complete elimination of complicating factors seems impossibk, although the qualitative effect is easily attained. The circuit diagrammed in Figure 1 is suggested for student use in the laboratory since a conclusion can be reached only after the variation of anode potential with plate current is plotted. The anode voltage is obtained from the potentiometer connected across a d.-c. source of approximately 12 v. and capablt? of furnishing a current of about 150 ma. A flament curremt of 3 amp. at 2.5 v. for the type 82 and a t 5 v. for the type 83 was su7plied by a transformer connected to the a.-c. line. The curves for a type 82 and a type 83 tube which appear in Figure 1were obtained by recording the voltage between one plate of the tube and one terminal of the filament for 10-ma. increments of current. I t will he noticed that the graphs rise sharply to approximately 10 v. and then level off. The sudden increase of current can be satisfactorily explained by the ionization of the mercury vapor which leads to positive ion bombardment of the filament surface and subsequent emission of secondary electrons. Deviation from the 10.38-v. first ionization potential of mercury may be attributed to an additional amount, of energy the electrons possess upon being evaporated from the filament surface, as well as to ~ 1 50 ~ 1100 the gas pressure within the tube. The dotted line in curmnt ( m 3 the 10 to 20-ma. section of the 82 curve indicates a region in which the tube oscillates and no steady state Figure 1
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JULY, 1949
373
of the 2050 and 2051 type in the circuit diagrammed in xenon-filled and the 2051 is argon-filled (5) and the Figure 2. Although the shield grid (terminal 6 in the ionization potentials of these gases is quite different tube drawing) i s represented in the conventional man- fro& the "firing" voltages recorded above. The point ner, this element is not located completely between the of the demonstration, however, is that for a certain, recontrol grid (pin 5) and the anode (pin 3) but includes a producible, potential diierence between the control large solid metal plate surrounding the heater, cathode, grid and cathode, electrons acquire sufficient energy to control grid, and anode. Consequently, if the shield ionize the gas and produce a flow of positive ions to the grid is maintained a t a negative potential with respect shield grid: The circuit pictured in Figure 3 employs a type to the cathode while the control grid and anode are positive, the formation of positive ions will result in a 2A4G grid-controlled gas triode and is best adapted for current flow through the milliammeter. If the anode of the 2050 is about 220 v. above cathode and the shield grid 11v. below, no appreciable current flows as the potential on the control grid is increased until abruptly a t 10.5 v. the tube "fires." Currents in this conducting state are approximately 9 ma. in the shield-grid circuit, 16 ma. in the anode leg, and 3.4 ma. between the control grid and cathode. The potentials listed should be taken as illustrative of the control grid potential necessary, for conduction is dependent upon anode voltage, shield-grid voltage, heater-supply voltage, and individual tube variations (4). Potentials as low as 40 v. on the anode and 2.3 v. on the shield grid serve satisfactorily, but the magnitude of the shieldgrid current drops. The 2051 tube shows the same general characteristics, and with an anode supply of 100 v. and a shield-grid potential of 13 v. conduction takes place when the conFigure 3 trol grid is approximately 12.7 v. About 25 ma-flows K = k e y , normally closed; L ism^, 3 watt. l l o v . , Mazdx No. 67: R,= through the plate circuit and 5 ma. through the shield380 ohm, 2 wstt: R*=14,500 ohm. 100 watt: R ~ = 7 5 , 0 0 0ohm, 1 watt. grid circuit. Heater supply for either the 2050 or2051 is 6.3 v. a t 0.6 amp. demonstration purposes. With about 200 v. on the The value of the control-grid voltage a t conduction plate, the lamp L will light abruptly to full brilliance on these tubes has no special significance,for the 2050 is when the mid ~otentialreaches-about 8 v. Considerable leeway existsin the choice of resistances 13 I and power supply. The values of R I , RP, and R3listed weriused witha 350-v. power pack. Any and all of the batteries indicated in the diagrams can be replaced by a well-atered d.-c. source and the correct potential obtained from a tap on the bleeder resistance. The key K is essential for once conduction starts the grid loses control, but by momentarily opening thecircuit a t K and reducing the grid potential below the conduction v value control can be returned to the grid. To prevent destruction of the tube it is vital to include sufficient resistance in the place circuit of the thyratrons to limit the current to a safe value. A minimum of 2000 ohms is suggested by the manufacturer.
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LITERATURE CITED
Figure 2
A=milliammeter 0-10 d, c.: K = k e y . normally closed; R 1 = 3 8 0 ohm. 2 watt: R1=14.C00 ohm, loo wstt: Rz=1570 ohm, 1s watt: V =voltmeter, 0-15 d. C.
( 1 ) W a r m n ~ R. ~ , M., "Rudiments of Chemistry," Ronald Press Co., New Yark, 1947. '?onimtion'~o(2) SISLER,H. H., AND C. A. VAADERWERF, tentids in the Teaching of Elementary Chemistry," J. CEEM.EDCC.,22, 390 (1945). (3) REICH,H. J., principle^ of Electron Tubes," McGrew-Hill Book Co., N e a York, 1941, p. 234. (41 . . Radio Cornomtion of America Data. Sheet on 2050. 1944. p. 3. (5) WITTENBERG, H. If., "Frequency Performance of Thyratrons," Publication No. ST-337, Radio Corporation of America, 1946.
