CH. XII.] 



THE RHEOTOME. 



The electromotive force is usually measured in terms of a standard Daniell 

 cell. The two surfaces of the muscle (M) are connected to a galvanometer 

 (B) ; the needle swings, and then a fraction of a Daniell cell is introduced 

 in the reverse direction so as to neutralise the muscle current, and bring 



Fig. 168. Human heart. Diphasic variation, EE. and simultaneous cardiogram, 

 cc. Time U is marked in -j^th second. The lead-offs to the capillary electrometer were 

 from the mouth to the sulphuric acid, and from the left foot to the mercury. 

 (Waller.) 



back the needle to rest. From the Daniell cell K, wires pass to the ends a, b 

 of a long platinum wire of high resistance, called the compensator ; c is a 

 slider on this wire : it and c are connected to the galvanometer, the com- 

 mutator C enabling the observer to ensure that the current from the Daniell 

 passes in the opposite direction to that produced by the muscle. If the 



69. Scheme of a Rheotome. (Waller.) 



slider / is placed at the end b of the compensator, the whole strength of 

 the Daniell will be sent through the galvanometer and will more than 

 neutralise the muscle current ; if c is half way between a and b. half the 

 Daniell's strength will be sent in ; but this is also too much ; ac will be found 

 to be only quite a small fraction of ab ; and this fraction will correspond 

 to a proportional fraction of the electromotive force of the Daniell cell. 

 K.P. L 



