Chap. xi.] COMPENSATION METHOD. 123 



commutator over, so that a current from K would 

 deflect the needle in the opposite direction. Then 

 slowly push c away from a. Step by step, with the 

 pushing away of c, the needle will swing back towards 

 zero, till a point is reached when it is exactly at zero. 

 In this condition of affairs, a current from, the muscle 

 passes round B in one direction, a portion of a Daniell 

 current passes round B in an opposite direction, and, 

 since the needle is at zero, both these currents are 

 equal, that is, they neutralise one another. Thus 

 the amount of Daniell sent is the measure of the 

 amount of muscle current, and this is proportional to 

 the distance between a and c. To put it more accu- 

 rately, the difference of potential between a and c is 

 equal to the difference of potential between the two 

 points of the muscle in contact with the clay guards 

 of the electrodes. Putting it that the difference of 

 potential between a and c is directly proportional to 

 the electromotive force of the muscle current, it will 

 be understood that if the compensator wire were pre- 

 viously graduated, it would be possible to arrive at an 

 accurate estimate of the amount of that force without 

 any delay. This previous graduation is, however, 

 necessary. That is to say, ab being a uniform wire, 

 having a millimetre scale pasted beneath it, and the 

 current through ab being constant, it is possible to so 

 graduate the compensator that every millimetre of the 

 wire through which the slider c is moved is equal to 

 a determined amount of current. It is then only 

 necessary to read off the distance between a and c, in 

 order to learn the amount of the constant current 

 which has been required to compensate for the muscle 

 current. 



To put it in another way, suppose the resistance of 

 the rheocord wire to be infinitely great in comparison 

 with the internal resistance of the Daniell, then the 

 resistance of ac will have the same ratio to the whole 



