PULSATION AND TETANUS. 33 



to operate repeatedly in quick succession. An inductive 

 current is most suitable for this purpose, for it can 

 be produced at will by the closing and opening of an- 

 other current. Once more turning to the coils A and 

 B (fig. 10, p. 31), let A be connected with a chain, 

 B with the muscle. Within the circuit of the chain 

 which includes A, we can insert an apparatus capable 

 of repeatedly and rapidly shutting or opening the 

 current. For this purpose a so-called electric wheel 

 is used. The wheel Z is made of some conducting 

 substance, such as copper, 

 and its circumference is cut 

 into teeth like that of the 

 ratchet-wheel of a watch. 

 The copper wire rests on 

 this circumference. The 

 axis of the wheel and the 



7 T .,-, IMG. 11. ELECTRIC WHKEK. 



wire o are connected with 



the conducting wires by means of the screws d and/. 

 When the click rests on one tooth of the circumference 

 of the wheel, the current is enabled to pass through 

 the wheel, and thus also through coil A ; it is, how- 

 ever, interrupted during the interval which intervenes 

 while the click springs from one tooth to the other. 

 Therefore, by turning the wheel on its axis the current 

 in coil A is alternately closed and opened. Conse- 

 quently, inductive currents constantly occur in the 

 adjacent coil B, and these pass in rapid succession 

 through the muscle. Each of these currents irritates 

 the muscle ; and since they occur in such quick suc- 

 cession, the muscle has no time to relax in the intervals, 

 but continues permanently contracted. Enduring con- 

 3 



