ELECTRICAL PROPERTIES OF MUSCLE. 



409 



provided with a millimetre scale, by means of which the distance of the 

 sliding contact from the block C can be measured. Between the block B 

 and a block A is a resistance coil. The resistance of the wire B C having 

 been determined, that of the coil must be so adjusted that the former is to 

 the latter as 1 to 13*56, for a reason which will appear immediately. 



The wire which connects the battery with C passes through a rheostat. 

 The terminals of a Latimer Clark's standard battery can be connected with 

 C and A, by closing a key. Between the key and A is a galvanoscope g, 

 by which it can be observed whether any current is flowing between A 

 and C or not. The branch of the Daniell battery which flows through g is 

 opposed to the current of the standard cell. Before making the experiment, 

 it is reduced with the aid of the rheostat, until g indicates no current. When 

 this is the case, the difference of potential between C and A is 1 "456 volt, i.e., 

 the electromotive force of the standard cell. Consequently, as the resistance 

 of C B is to that of the coil as 1 to 13*56, the difference of potential between 

 C and B is to that between B and A as 1 to 13*56 ; and hence to that between 



Srantfara ce// 



7V Gatvunc/'r.pter- 



Fig. 223. 



C and A as 1 to 14*56, or 0*1 to 1*456. That between C and A being 

 T456 volt, the difference between the two ends of the metre wire will be 

 0*1 volt, and each centimetre of it will correspond to a difference of 



T0V0 volt - 1 



The arrangement just described (the rheochord in connection with the 



"balanced" cell) is called a "Compensator." The use of an instrument of 

 this kind, for measuring physiological differences of potential, dates from 1861, 

 when du Bois-Reymond described the two forms of compensator which are 

 used in Germany, the "long" and the "round." Of these, and the method 

 of using them, a description will be found in Hermann's " Muskel-physik." The 

 arrangement described above has the advantage, as compared with these, that 

 so long as the standard cell for which the scale is graduated is used, no calcu- 

 lation is required. 



The ends of the wires at pr and d are connected with the surface of the 

 muscle by non-polarisable electrodes. Of such electrodes a suitable form 



1 The form of instrument mentioned above was described by the writer 

 in the "Practical Exercises" for the Laboratory of University College, London. 



in 



1873, 



