MUSCLE 



623 



ciently great to cause continuous electrolysis of the acid that is, so 

 long as it is below about 2 volts. The external current is therefore 

 at once compensated, and after the first moment no current passes 

 through the instrument, which is accordingly not a measurer of 

 current, but of electromotive force. It is very suitable for detecting 

 and measuring such small differences of potential as occur in animal 

 tissues. 



Induced Currents. When a coil of wire in which a current is 

 flowing is brought up suddenly to another coil, a momentary current 

 is developed in the stationary coil in the opposite direction to that 

 in the moving coil. Similarly, if instead of one of the coils being 

 moved a current is sent through it, while the other coil remains at 

 rest in its neighbourhood, a transient oppositely-directed current is 

 set up in the latter. When the current in the first coil is broken, a 

 current in the same direction is induced in the other coil. 



Du Bois-Reymond's Sledge Inductorium (Fig. 212). This consists 



FIG. 212. Du BOIS-REYMOND'S INDUCTORIUM. 



B, primary, B', secondary, coil ; H, guides in which B' slides, with scale ; 

 D, electro-magnet ; E, vibrating spring ; i, wire connecting wire of D to end of 

 primary ; v, screw with platinum point, connected with other end of primary ; 

 A, A', binding-screws, to which are attached the wires from battery. A' is 

 connected with the wire of the electro-magnet D, and through it and i with the 

 primary. 



of two coils, the primary and the secondary, the former having a 

 comparatively small number of turns of fairly thick copper wire, 

 the latter a large number of turns of thin wire. The object of 

 this is that the resistance of the primary, which is connected with 

 one or more voltaic cells, may not cut down the current too much ; 

 while the currents induced in the secondary, having a high electro- 

 motive force, can readily pass through a high resistance, and are 

 directly proportional in intensity to the number of turns of the 

 wire. 



By means of various binding-screws and the electro-magnetic 

 interrupter, or Neef's hammer, shown in the figure and explained 

 below it, the current can be made once in the primary or broken 

 once, or a constant alternation of make and break can be kept up. 

 We can thus get a single make or break shock in the secondary, or a 



