44 



Dr. J. A. Marker on a l)irect-reacVin<) 



amaloramated contacts throughout, rather than any form of 

 rubbing contact or phig. 



The interconnexions are made by means o£ a system of 

 copper bus-bars and a form of copper mercury-cup, designed 

 so as to be made vrithout undue waste of material on an 

 automatic lathe. 



A pair of these cups with a connector for short-circuiting 

 them are shown in full size in the diagram. It will be seen 

 that the excess of mercury squeezed out of the lower hole 

 may remain in the widened portion without overflowing. 



y 



u 



The slide-wire, which is of manganin. is protected from 

 oxidation by gilding, as in some of the Crompton-Fisher 

 potentiometers. Its ends are hard-soldered to massive copper 

 blocks, which are screwed to the transverse ends of the two 

 bus-bars above and below the row of '01 ohm balancing-coils. 



In order to make the bridge-wire exactly equivalent to the 

 two coils, it was deemed better not to attempt to alter the 

 cross-section by drawing or scrnping. but to make the whole 

 verv slightly above '02 ohm, and to reduce this to the desired 

 exact value by inserting in the interior of the ebonite handle, 

 holding together the two U-pieces forming the movable slide- 

 wire connexion, a small shunt of about 200 times the resistance 

 of the wire. In case of subsequent alteration of the relation 

 of the slide-wire resistance to that of the coils, this shunt could 



