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Use of Ttiode Valve in registering Electrical Contacts. 871 



The resistances r x and v 2 in the circuit enable a fine 

 adjustment of tin points p { and p 2 to be made. These 

 points can be advanced by micrometer screws until both 

 just touch the platinum tongue. This contact can be ob- 

 served by temporarily inserting the galvanometer G in the 

 circuit. The E.M.F/s of the batteries bj and b 2 act in the 

 same direction in this local circuit, and with suitable re- 

 sistances 1\ and r 2 a small local current flows when p 1 and p 2 

 are both in contact. The points can then be separated by 

 any desired amount. The resistances i\ and r 2 also prevent 

 a short circuit if an accidental contact is made from p { to p 2 . 



A relay of this type has several advantages. The current 

 to be transmitted through the contacts is very small, being 

 only the grid current of the valve. Variation in the re- 

 sistance of the contact within wide limits has no effect on the 

 action, owing to the great resistance already in the circuit 

 between the grid and the filament. The amplification of 

 mechanical .power, which is the function of a relay, is pro- 

 vided for bj r the electrical amplifying properties of the valve. 

 A further advantage, that may be important in some cases, 

 is that this relay may be operated by much weaker forces 

 than could be used with an ordinary relay. In the latter 

 type the contact is placed directly in the circuit of the second 

 instrument, and a certain contact pressure is necessary to 

 ensure the passage of sufficient operating current. With 

 the valve relay an extremely light contact between clean 

 platinum surfaces is sufficient to charge the grid of the valve, 

 and very weak forces will therefore work the relay in a 

 satisfactory way. Thus it has been found that a contact 

 force of 0000001 grin, is quite sufficient to charge the grid 

 and produce the required change in the anode current. 

 Since there is practically no current transmitted by the 

 contacts, there is no objectionable coherence of the surfaces, 

 and the movement of the contact tongue can be reduced to 

 a very small amount if desired. For example, the relay has 

 been operated with the travel of the tongue only about 

 1/1000 mm. 



Certain modifications to meet different requirements may 

 be suggested. If the contact is for any reason intermittent, 

 a comparatively steady current may be obtained in the anode 

 circuit by connecting a suitable condenser across the grid 

 and filament. A high-resistance grid-leak may be used if 

 it is desirable for the anode current to assume its normal 

 value immediately the contact in the grid circuit is broken. 



The writer has also made some experiments, using an 

 electrical contact in the way described for quite a different 



