170 



Professor J. A. Fleming 



[May 21, 



mode of usage in the case of valves with a certain degree of 

 exhaustion in the bulb gives very great sensitiveness in the detection 

 of radio-signals. It is commonly called the potentiometer method 

 because the extra steady voltage required in the plate circuit is 

 derived by employing a fraction of the voltage of the battery used 

 for incandescing the filament by means of a potentiometer resistance. 

 This is. perhaps, the place to refer to another view of the mode in 

 which my valve acts even when no additional E.M.F. is placed in 

 the plate circuit. On delineating, as above described, the charac- 

 teristic curve of a valve, it is found that this curve does not start 

 exactly from the point of zero voltage, but from a point on the 

 negative side about f to 1 volt (Fig. 5). This means that if the 



Fig. 5. — Chaeacteeistic Cueve neae Oeigix. 



{date is connected to the negative terminal of the filament battery by 

 a wire, there is found to be in it a small negative electric current flowing 

 from the plate through the external circuit to the negative terminal. 

 At first sight it seems to imply that the electrons return back on their 

 own origin, but the true reason probably is that the electrons are 

 shot out of the filament with a certain velocity and accumulate 

 round the plate. The result is a tendency for them to diffuse back 

 through the external circuit, creating a feeble electron current which 

 can only be stopped by introducing a small counter E.M.F. into that 

 circuit. 



Hence the characteristic curve starts from a negative point on 

 the voltage axis. At the place where it crosses the zero voltage point 

 that curve is concave upwards, and hence, for the reason just 

 explained, the introduction into the external thermionic circuit of a 

 feeble alternating high frequency electromotive force will result in 

 an increase in the mean or average thermionic current. Hence the 



