102 



PHYSIOLOGY OF MUSCLE AND NERVE 



passed through these conductors by way of the two copper wires, the surface 

 tension of the mercury is changed, forcing the meniscus to move either upward or 

 downward in accordance with the direction of the current. If its point of entrance 

 (anode) is below, the meniscus moves upward, and vice versa. 



The strength of this electrical current 

 may be determined by noting the extent of 

 the movement of the meniscus, because a 

 direct relationship exists between these two 

 factors. It may also be measured by in- 

 terposing a resistance in the circuit outside 

 the electrometer or galvanometer which is 

 just sufficiently powerful to force the menis- 

 cus or the galvanometric needle to assume 

 its normal position. At this very moment 

 the resistance neutraUzes the current, and 

 hence, the number of ohms necessary to ac- 

 complish this end must correspond precisely 

 to the difference in the electrical potential. 

 Most generally, however, we make use of 

 the so-called compensation method which re- 

 quires the use of an artificial current in a 

 direction opposite to that produced by the 

 muscle. This end may be attained most 

 easily with the help of a rheocord (Fig. 

 58), consisting of a certain length of Ger- 

 man silver wire. The two binding posts at 

 the ends of this wire (A and B) are brought 

 into connection with the poles of a battery 

 cell. The circuit of the electrometer or gal- 

 vanometer (C) with its muscle preparation 

 (M) is then brought into relation with the 

 resistance wire by a lead from one of its posts, 

 while the return lead is effected by means 

 of a post which may be pushed back and forth upon the wire. By moving this 

 sliding post (D) nearer to or farther away from the end post (B), a greater or 

 less amount of the current generated by the battery is allowed to oppose the 



Fig. 57. — Capillary Electrometer. 

 A, tube and J5, receptacle filled 

 with mercury; M, meniscus of mer- 

 cury; L, lens of microscope; P, tube 

 leading to small rubber bulb for ad- 

 justing meniscus. 



Fig. 68. — Thb Simple Rheocord. 

 AB, German silver wire; C, galvanometer; M, muscle; D, sliding post; K, key. 



muscle current until an equalization has finally been attained. Knowing the 

 atrength of the counter current, the strength of the muscle current may be deter- 

 mined from the resistance which has been interposed, i.e., from the position oc- 

 cupied by the sliding post. The value of the action current of an ordinary muscle 

 scarcely exceeds 75 millivolts; its usual strength is 0.06-0.08 volt.^ 



1 Samjloff, Pfliiger's Archiv, Ixxviii, 1899, 1. 



