CAUSES AND PHENOMENA OF MOTION. 27 



battery are brought to the inner binding screws, and the bridge be brought 

 to connect them, the current passes across it and back to the battery. 

 Wires are connected with the outer binding screws, and the other ends 

 are approximated for about two inches, but, being covered except at their 

 points, are insulated, the uncovered points are about an eighth of an inch 

 apart. These wires are the electrodes, and the electrical stimulus is ap- 

 plied to the muscle, if they are placed behind its nerve and the connection 

 between the two brass plates of the key be broken by depressing the 

 handle of the bridge and so raising the connecting piece of metal. The 

 key is then said to be opened. (2) The induced current. An induced 

 current is developed by means of an apparatus called an induction coil, 

 and the one employed for physiological purposes is mostly the one (Fig. 

 278). 



Wires from a battery are brought to the two binding screws d' and d, 



FIG. 278. Du Bois Reymond's induction coil. 



a key intervening. These binding screws are the ends of a coil of coarse 

 covered wire c, called the primary coil. The ends of a coil of finer cov- 

 ered wire g, are attached to two binding screws to the left of the figure, 

 one only of which is visible. This is the secondary coil and is capable of 

 being moved nearer to c along a grooved and graduated scale. To the 

 binding screws to the left of g, the wires of electrodes used to stimulate 

 the muscle are attached. If the key in the circuit of wires from the bat- 

 tery to the primary coil (primary circuit) be closed, the current from the 

 battery passes through the primary coil and across the key to the battery 

 and continues to pass as long as the key continues closed. At the moment 

 of closure of the key, at the exact instant of the completion of the primary 

 circuit, an instantaneous current of electricity is induced in the secondary 

 coil, g, if it be sufficiently near, and the nearer it is to c, the stronger is 

 the current. The induced current is only momentary in duration and 



