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XVII. 



A SENSITIVE VALVE METHOD FOE THE MEASUEEMENT OF 

 CAPACITY, WITH SOME IMPOETANT APPLICATIONS. 



By JOHN J. BOWLING, M.A„ F.Inst.P. 



[Read 'December 21, 1920. Published February 21, 1921.] 



I. — Outline of Principles. 



When alternating potentials of increasing amounts are applied to the grid- 

 circuit of a three- electrode valve, the negative terminal of the filament heing 

 connected to the a. c. supply, the plate-current is found to vary, as shown in 

 curve II, fig. 1. This curve, of course, starts from the axis of zero-potential 

 at the point A, where the ordinary characteristic curve for steady grid- 

 potentials crosses this axis (curve i, fig. 1). We will call the curve ii the 

 a. c. characteristic of the valve. 



If an alternating e.m.f. E be applied to the terminals of a circuit com- 

 prising a high resistance R in series with a condenser of small capacity C, 

 there will be a drop of potential v across the terminals of It, given approxi- 

 mately by : — 



V = 2irnRCE, .... (I.) 



where n is the alternation frequency. 



Suppose this potential v be applied to the grid-circuit of a valve, as 

 shown in fig. 2. A deflection of the milammeter will be obtained which 

 depends on v, but will, in general, be somewhat less than that indicated by 

 the a. c. characteristic for an alternating potential i: There are two reasons 

 for this. Since the resistance R is large — perhaps half a megohm — the valve 

 will be operating with the grid at an average negative potential of a few 

 volts ; in the second place, the resistance of the grid- circuit, being in parallel 

 with R, lessens the effective value of v. 



It will be found, however, that the milammeter deflections increase 

 almost linearly for increasing values of the capacity C. The arrangement 

 may consequently be utilized for the measurement of capacity if we 

 construct a calibration curve by observing the milammeter readings due to 

 known values of capacity. In this form, however, the apparatus has a 

 limited range, and is neither very sensitive nor accurate. For zero capacity, 

 or, what comes to the same thing, with no alternating e.m.f. acting, there is 



SOIENT. PROO. K.D.S., VOL XVI, NO. XVII. 2 D 



