124 PHYSIOLOGICAL PHYSICS. [Chap. xi. 



resistance of the rheocord circuit as the fraction of the 

 Daniell current sent round the galvanometer has to 

 the whole current of the Daniell. But the fraction 

 sent round B is the measure of the muscle current, so 

 the resistance of ac will be to the resistance of the 

 circuit as the current of the muscle is to that of the 

 Daniell. Now the resistance of ab, the rheocord 

 wire, may be made very great in comparison to that 

 of the Daniell, by interposing a resistance box, offering 

 say 5,000 ohms resistance, between b and E, and this 

 box becomes part of the circuit K6K, and is as it 

 were a prolongation of ab. For simplicity this has 

 not been shown in the diagram. Let us represent the 

 result by a formula. Let V be the electromotive force 

 of the muscle current, and E the electromotive force 

 of the Daniell ; let R equal the resistance of the 

 rheocord wire between a and c, the length that permits 

 of compensation of the muscle current, and let R + R' 

 be the resistance of the whole circuit K&K ; then the 

 electromotive force of the muscle current is to the 

 electromotive force of the Daniell as the resistance be- 

 tween a and c is to the total resistance of the Daniell 

 and rheocord circuit. That is, 



Now E is known (it is the electromotive force of 

 the Daniell), previous graduation of the rheocord wire 

 gives R, and we have supposed R' to equal 5,000 ohms, 

 therefore it is only necessary to substitute for E, R and 

 R', their values, and the electromotive force of the 

 muscle current V is obtained. 



The round compensator of Du Bois-Reymond 

 is a much more convenient instrument with which to 

 make these compensation measurements than the 

 ordinary long compensator. The round compensator 

 is shown in Fig. 64. It consists of a platinum wire, 



