PHYSIOLOGIC APPARATUS. 



711 



moment it is broken, a current is induced in the secondary circuit, as shown 

 by a momentary deflection of the galvanometer needle. During the con- 

 tinuous flow of the current through the primary circuit there is no evidence 

 of a current in the secondary circuit. The induced current is but of momen- 

 tary duration. The current flowing through the primary circuit is termed 

 the inducing, the current flowing through the secondary circuit the induced 

 current. 



The induced current is opposite in direction to that of the inducing cur- 

 rent when the circuit is made or closed; it is the same direction, however, 

 when the circuit is broken or opened. 



If the circuits are arranged in the form of coils, it is found that, other 

 things being equal, the strength of the induced currents will be proportional 

 to the number of turns in the coils. 



If the coils are placed 

 at varying distances 

 from each other, the 

 strength of the induced 

 current varies, increas- 

 ing as the coils are ap- 

 proximated, decreasing 

 as they are separated. 



Approximation or 

 separation of the coils 

 while the current is flow- 

 ing through the primary 

 circuit develops an in- 

 duced current, which 

 disappears, however, the 

 moment the movement 

 of the coil ceases. A 

 sudden increase or de- 

 crease in the strength of the inducing current also develops an induced 

 current. 



When the coils are approximated or the primary current increased in 

 strength, the induced current is opposite in direction to that of the inducing 

 current; with the reverse conditions, the induced current has the same 

 direction. 



The induced currents have been termed, in honor of their discoverer, 

 Faradic currents. 



The du Bois-Reymond inductorium, based on the foregoing facts, 

 consists essentially of two coils of insulted copper wire, termed primary and 

 secondary (Fig. 362). 



The primary coil, R', consists of thick copper wire wound around a 

 wooden spool attached to a vertical support. The beginning of this coil is at 

 the binding post S", its termination either at binding post P" or S'". In the 

 course of this primary wire or circuit, there are placed two vertical bars of 

 soft iron, B', connected at their bases to form a horseshoe magnet, around the 

 ends of which the wire is coiled. The object of this device will be explained 

 later. 



FIG. 362. INDUCTORIUM OF DU BOIS-REYMOND. R', Pri- 

 mary, R", secondary spiral. B. Board on which R" moves, 

 i. Scale. + . Wires from battery. P', P". Pillars. H. 

 Neef's hammer. B'. Electro-magnet. S'. Binding screw 

 touching the steel spring (H). S" and S'". Binding screws to 

 which to attach wires where Neef's hammer is not required. 



