120 PHYSIOLOGICAL PHYSICS. [Cha P .-xi. 



below the first. (See Fig. 61.) Negative variation can 

 also be produced in the current from nerves, though 

 the nerve current causes a much smaller deflection 

 than the muscle current. For this purpose one end of 

 the nerve should be laid over the platinum electrodes 

 arranged in connection with an induction coil, as 

 described for the negative variation of muscle. 



To measure the electromotive force of the 

 muscle or nerve current, Du Bois-Keymond made use 

 of a method devised by Poggendorff to measure the 

 electromotive force of inconstant cells. The principle 

 of the method may be compared to the principle of 

 weighing, which consists in placing the body to be 

 weighed on one side of a balance, and accurately 

 counterbalancing its effects by standard bodies, whose 

 amount can be varied at pleasure, placed on the 

 opposite side. Thus the muscle current is sent round 

 the galvanometer, and deflects the needle in a par- 

 ticular direction. A current, whose amount can be 

 varied at pleasure and always accurately estimated, is 

 sent round the galvanometer in the opposite direction, 

 of such a strength that it exactly neutralises the 

 muscle current. This is indicated by the return of 

 the needle to the zero point. The amount of the 

 COMPENSATING CURRENT, as it is called, is then read 

 off", and it is a measure of the muscle current. Fig. 

 62 is a representation of the scheme of compensation. 



A reference to the description of the rheocord 

 (page 78, Fig. 42) will show that in this instrument a 

 means is afforded of graduating to any extent the 

 strength of the compensating or measuring current. 

 A simpler arrangement than the rheocord is, however, 

 found to suit the purpose. It consists practically of 

 a single wire of the rheocord (Fig. 62, AB), a uniform 

 wire of brass 2 metres long and 1'75 mm. in 

 diameter. It is stretched on a piece of wood be- 

 tween two brass plates, fitted with binding screws 



