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LXXIII. The Use of a Triode Valve in registering Electrical 

 Contacts. By G. A. Tomlinson, B.Sc* 



A THREE electrode valve can be applied with advantage 

 to certain forms of apparatus in which use is made 

 of electrical contacts. A common case is that of a relay in 

 which it is usual to cause a feeble movement of one instrument 

 to make a contact and close the circuit of a second compa- 

 ratively powerful instrument supplied from an independent 

 source. An improvement in several respects can be made 

 if the first contact is placed in the grid circuit of a valve, 

 and the second instrument is connected in the anode circuit 

 and is operated by the anode current. 



An arrangement used by the writer is shown in fig. 1. 



IaAa aA 



Aj 



H'l"- 



^Aawawi 



5> 



FIG I 



A small tongue of platinum C is moved by the first in- 

 strument between two platinum points pi and p 2 , and on 

 making contact either raises or lowers the potential of the 

 grid by about 4 volts by introducing the batteries b L or b 2 . 



The reaction on the anode current, which is illustrated by 

 the diagram of the valve characteristic, operates the in- 

 strument D. Thus the points P x and P 2 show the anode 

 current when the tongue is in contact at/*! and p 2 , the dif- 

 ference being the range of current available, which is about 

 3 milliamperes. The actual values of the grid potential are 

 adjusted bv the battery b to vary between —2 and -f 6 volts, 

 to obtain the best range of anode current. 



In this particular case the relay is required only to detect 

 small angular movements of an instrument in either direction 

 from the zero position, and the instrument used in the anode 

 circuit is a pivoted moving-coil galvanometer. 



* Communicated by the Director of the National Physical Laboratory. 



