IV 



ELECTROMOTIVE ACTION IX MUSCLE 



335 



current, it is of course easy to measure the intensity of the 

 muscle current ; yet, in view of the great and very variable 

 resistance of vegetable and animal tissues, such measurements are 

 on the whole of little value. Much greater importance attaches 

 on the other hand to exact measurements of electromotive force. 

 If two points of different potential are connected by a leading- 

 off circuit to a conductor which is the seat of electromotive force, 

 a branch of the current will flow through these, of intensity 

 directly proportional with the E.M.F. which may be conceived as 

 acting at the points of junction. The magnitude of the latter 

 may thus be measured from the difference in potential between 

 two points led off, and where 

 it is possible to determine this 

 exactly, it is also possible to 

 determine the magnitude of the 

 electromotive force. And the 

 E.M.F. of the longitudinal current 

 could be ascertained simply by 

 measuring the P.D. between 

 natural longitudinal surface and 

 artificial cross-section. The dif- 

 ference of potential between two a 

 points is easy to determine ex- 

 perimentally, by a method in- 

 vented by Poggendorff, and essen- 

 tially improved by du Bois- 

 Reymond (8). 



The principle of the method is to replace the magnet from 

 its deflected position to its original position of rest, by means of a 

 fraction of the current of a standard cell, opposing and cancel- 

 ling the original current. The known variable P.D. is thus 

 a measure for the magnitude of the unknown difference to be 

 determined. Such a " compensating " current can easily be 

 derived from a measuring circuit by means of a rheochord, termed 

 in this case a " compensator." If a constant current (K) is led 

 through a straight or circular wire (Fig. 106 a, H), a definite 

 " electrical fall " occurs in the circuit, since there are differences 

 of potential at different points. Now, if the longitudinal section 

 of a muscle (Jf), lying upon unpolarisable electrodes, is connected 

 by means of a reverser (O) with the end (a) of the compensator 



FIG. 106. Measurement of E.M.F. by com- 

 pensation. (Du Bois-Reymond.) 



