A detailed study of rectification phenomena in air following the earlier
work of Chiplonkar is given. The effect in hydrogen and CO is also discussed.
Recently Mr. Chiplonkar (1939) has made some quantitative investigations
of the well-known phenomenon of rectification in discharge tubes. His work
was mainly confined to the study of the precise role of the electrodes, their relative
sizes and the inter-electrode distance in determining the rectification effect.
This paper is an extension of Mr. Chiplonkar's work and deals with a detailed
study of rectification phenomena in air. The effect in hydrogen and CO has also
been partly studied

Chaudhari, K R

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82· RECTIFICATION IN DISCHARGE TUBES" By K. R. CHAUDHARI ABSTRACT. A detailed study of rectification phenomena in air following the earlier work of Chiplonkar is given. The effed in hydrogen and CO is also discussed. Recently Mr. Chiplonkar (1939) has made some quantitative investigations of the well-known phenomenon of rectificatiob in discharge tubes. His work was mainly confined to the study of the precise r(;le of thc electrodes, their relative sizes and the inter-electrode distance in detetbining the rectification effect. This paper is an extension of Mr. Chiplonkar's work and deals with a detailed study of rectification phenomcna in air. The efect in hydrogen and CO has also been partly studied. . E x p It RIM It N 'fA L The discharge tube had un internal diameter of 3.8 cm. and was provided with a cup-like cylindrical anode of aluminium (diameter 1.6 cm. and height 2 em.), the latter being fitted within a side tube. The cathode was made of an aluminium cylinder 9 1I11n. thick and with a diameter of 3 em., and the distance' between the anode and the cathode was 16 C111. A hole of 1 111m. diameter was bored coaxially at the centre and it was mounted coaxially by a brass rod of 3 mm. thickness. The brass rod was used only to facilitate the electrical connections. 'The window of the discharge tube was at a distance of 16 cm. from the cathode and the length of the tube from the window to the conical end was 42 cm. Thus the general form of the tube was sit~ilar to that suggested by Thomson (1933). The electrical circuit employed was the same as that used by Chiplonkar (loc. cit). In order to prevent the ascent of mercury vapour from the diffusion pumps and other measuring instruments liquid air traps were employed except in the case of CO, in the investigation of which traps of freezing ~nixture of ice andCaC12 were used. The total current passing through the discharge tube was, measured by an A.C. Weston milliammeter of the rectifier type, the rectified part of the current being measured by a D.C. Crompton milliammeter connected in series with the first. The current was measured every time immediately after starting the discharge. The ratio of the rectified to the total current is !ie~ignated here as the rectification ratio and denoted by the symbol p. The heating of the tube due to discharge was avoided, as far as possible, by interrupt-ing the .dlscharge for a suitable length of time, between two successive observa-tions. Evacuation of the tube was obtained by means of a set of mercury. piffusion pumps of the Waren type backed by a Cenco Hyvac oil pump. , The results {or air, hydrogen and CO are shown graphically in Figure I, .~here the values of the rectification ratio p are plotted against the correspond-iJJg·values of pressure p. Different sets of observations were taken and the :~~ of the rectification ratio obtained always gave the same type of curve. • Comm1UJicated by Dr. D. V. Gogate. 260 K. R. Chaudhari 1.2 1.0 0.8 .9 0.6 ... f! .~ 0·4 13 It:: ~ 0.2 .. ". 0 to" , .... -0.2 -0·4 (1) Air (2) Hydrogen (3) CarboD1l1uDoxitle FIG. I It can be easily sc:!n from the graphs that the rectification f!:ltio p is always positive in the case of air and H 2 • while in the case of cO the value of p becomes negative at a pres&ure of lIO x 10-4 cm. of Hg. It may be noted here that Chiplonkar (loco cit.) has recorded negative values of p in the case of air, nitrogen and oxygen at low pressures (about 50 x 10-4 CIII •. of Hg.). At these low pressures the fourth type of discharge begins and as Thomson remarks, the phenomena under these conditions become very complicated. In general the nature of the discharge is always different with different tubes. 'I'he heating of the tubes and the electrification of its walls modify the discharge considerably. The high speed cathode rays ionise the gas and cause changes in the current in the discharge tube. Again radiation is emitted from some parts of the discharge and especially from the negative glow, which, falling on the metal of the cathode, would by photo-electric effect cause it to emit electrons. There are also several sources of ionisation besides these, e.g., the impact of positive ions against the cathode, moving electrons in dark space, etc. All these factors contribute to the variations in the rectified current which may lead either to positive or negative values of p depending upon the nature of ihe gas, nature of the electrodes and so on. In Table I are given the values of the first maxima and minima of" the rectification ratio p against the corresponding pressures p for different gases. If we look to the densities of these gases, it becomes at once apparent that the first maxima-minima values of p appear at higher pressures for gases of lower densities, e.g., the first maximum value of p for hydrogen appears at a pressure of 180 x 10-* cm. of Hg., while those for CO and air appear at 160 X·l0- f em. of Hg. and 125 x 10-4 em. of Hg.respectively. This conclusion is supported by the experimental data given by Chiplonkar (loc. cit.) also. Rectification in Discharge Tubes 261 It may be mentioned here that the hydrogen curve (Fig. I) has got subsidiary m~Xillla.mlD1ma in the beginning at pressures 39H x 10- 1 cm. of Hg. and 891 x 10-4 em. of Hg. respectively and they are marked with asterisks. - I No. I :I 3 4 5 6 7 Gas oxygen nih-ogen air co Density of gas gm. per litre 1·43 1.29 1.25 0.09 TABLE I First Maximu Pressure X. 10 -4 CIlI. oiHg. 125 180 125 160 180 p 0.85 0·90 LOu 0.80 0.83 First Minima Pressure X. w- 4 cm. of Hg. 80 100 95 JIu 170 (' O.uo 0.20 0.30 0·75 .. II Author .. VARIATION OF RECTIFICATION WITH TIME It was found that the rectified current always fell down rapidly within the first half minute and then after an interval of 5 to 15 minutes it became steady. During the study of this phenomcnon in the casc of air, it was particularly observed that there was a very marked fiuctualiolJ around the maxima and minima values of p (Fig. I). Therefore the points around these values were chosen as the most suitable ones for the study of variation of the rectified current. with time. Figure 2 gives a few typical curves showirg the fall' of tIle rectified part of the current with the time for which the discharge was passed. K. R. Chaudhari The :uppermost curve • a' clearly shows that at a pressure of I58)( IO"''' cm; of Hg. the variation in the rectified current is comparatively very small and tb~ rectification ratio p becomes steady at a value of 0.72 •. At this high value of pressure, the fluctuation is always very small but at low pressures the rectified current goes on falling continuously for about 5 minutes and then becomes fairly· steady at a very low value of p (see curves b, c, d, e-Fig. 2). It may also be interesting to study the variation of the steady value of p with pressure p. Figure 3 is a plot of the steady values of p (taken from Fig. 2), against the corresponding values of pressure p • . 8 .6 ·4 -·4 -.6 ; , -.8 1. Experimental. II Theoretical. FIG. 3 The form of experimental curve (Fig 3) suggests·that it can be represented by an equation of the type-Y=(X-l) (X-2) (X-3) where y denotes the value of p and x that of p. In Figure 3. side by side with the experimental curve is also given the theoretical curve satisfying the above equation. The point cor.responding to x=2 represents a pressure 102)( 10-4 cm. of Hg. Tha part of. tha experimeDtal curve on the right hand side of this point very nerely coincides with the theoretioal curve. The:odae:.,part .0Il. t~, left hand aide of the point x= 2 (Fig. 3) shows an increasing divergence from the theoretical curve. This divergence can be explained if we consider the heating of the tube due to tlte continuous discharge. The heating causes a rise- of pressure in fhe discbaTge tube and COD&equently the actual pressure existing in the discbarge tube for any value ofp is higher than that indicated on the pressure gauge. Thus the value of p obtauied experimentally fot the different values of pressure as recorded on tile pttIIata aauae really corresponds .to hig~ preseures actually existm, iD the Rectification in Discharge Tubes 263 heated discharge tube. This naturally places the p values on the experimental curve above those on the theoretical ones for the corresponding pressures. RFFECl' OF CARBON MONOXIDn ON ELECTRODRS While working with CO, it was observed that the curves obtained for different sets showed wide differences, especially when the interval between the sets was sufficiently long (about a fortnight), observations "vere therefore repeated ~ith air but here also a "vide departure from ,the original curve (Fig. I) was noticed. This definitely indicates that the electrodes were polluted by CO. 1'h is is in agreement with Oliphant's ('rhol11son-lpc. cit.) observations. SUMMARY This paper gives a detailed account of some, quantitative measurements of the rectification iu air in the discharge tubes. Th. variation of the rectification ratio p with pressure in H. and CO has also ,peen !>tlldied. It is found that the first maxima-minima values of p appear at higber pressures for gases of lower densities. While studying the variation of rectification with time, it is observed that the rectified current falls down rapidly at first and then becomes steady at a very low value of p. When steady values ()f f' are plotted against the corresponding pressures, a curve is obtained which can be represented by an equation of the form-y=(x-r) (X-2) (X-3). Lastly the effect of CO on the electrodes is discussed. ACKNOWLEDGMENTS I wish to express my gratitude to Dr. R. K. Asundi and to Dr. D. V. Gogate for their valuable help and discussion. My sincere thanks are also due to Dr. S. S. Jo~hi for giving m . every sort of facility for work in his~ labohtory, .and to Mr. V. T. Chiplonkar for the use of the apparatus. PHYSICS DI!PARTMIlNT, HARODACOIJ.ItGF., , BARODA.' REFERENCES Chiplonkar, 19.39. PT(lC. Ind. Acad. Sci., 10, 3Sr. Thomson, 193~, Conduction of Electricity Through Gases, 1'01. 2 (Ed. 3). p.320. 

Rectification in Discharge Tubes

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Rectification in Discharge Tubes

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