Chap. XL] 



ROUND COMPENSATOR. 



125 



"Vrl n 



Fig. 64. Eound Compensator. 



which rests in a groove on the circumference of a 

 circular disc of ebonite. The wire is one millimetre 

 thick, and is marked off by a scale round the circum- 

 ference into 1,000 millimetres. A little platinum 

 wheel r makes contact 

 with the -platinum wire, 

 against which it is kept 

 pressed by a spring pro- 

 jecting from the support 

 at the side. The disc 

 is movable on a vertical 

 axis, and is turned by 

 the small projections 

 on its under surface. 

 When it is turned the 

 wheel r revolves on 

 the wire. The beginning of the platinum wire is 

 connected with the screw I, the end with the screw 

 ii. The wheel r is in communication with the 

 screw in, and a very short distance from its ter- 

 mination the wire passes over a small sharp wedge 

 of platinum, which is connected with screw iv. The 

 connections are diagramatically represented in Fig. 

 65, which shows further how a Daniell is connected 

 with screws I and 11, and a galvanometer and muscle 

 are interposed in the circuit of in and iv. In the 

 circuit of the Daniell a commutator with cross should 

 be interposed to enable one to reverse the direction of 

 the current, and in the circuit of the galvanometer a 

 key should be intercalated. 



The pillar which supports the platinum wheel 

 (Fig. 64 ) supports also a simple microscope and a 

 vernier, which projects on to the millimetre scale, by 

 means of which can be ascertained the precise extent 

 of the turning of the disc. 



The round compensator is used precisely as the 

 long one. The element, galvanometer, and troughs 



