ELEMENTAKY EXPERIMENTAL PHYSIOLOGY 11 



primary circuit remains closed by the spring-key. An induction shock 

 is produced only at the make or the break of the constant current. 



Closure of the Du Bois key short-circuits the electrodes, and no shock 

 will be felt on make or break of the constant current. By means of 

 this key the make- or break-induction shock, or both, can be shut off 

 from the electrodes. 



The secondary coil is now removed from the grooves of the carrier, 

 and is placed close to, but at right angles to, the primary coil : no 

 shock is produced when the primary circuit is closed or broken. The 

 secondary coil is gradually turned on its vertical axis, and the spring- 

 key is opened and closed from time to time. A shock will be felt first 

 at break, then at make, and these will increase until the maxima are 

 reached when the secondary coil is parallel to the primary. 



These simple experiments show that the make and break of a 

 galvanic current can act as weak stimuli; that on connecting the 

 Daniell cell with the induction-coil induced currents are produced 

 in the secondary coil only at make and break of the battery-current 

 and not when it is running with constant strength through the 

 primary coil; that the induced currents are very effective stimuli, 

 can be easily graduated in strength and short-circuited by a key. It 

 has further been shown that the break induction-shock is stronger 

 than the make. The cause of this difference lies in the primary coil, 

 and needs explanation. 



When the battery- current enters the primary coil, it induces a 

 current in it as well as in the secondary coil. This " self-induced " 

 or make extra current, like that induced in the secondary coil, is a 

 momentary current in the opposite direction to the battery-current; 

 hence it delays the rapidity with which the battery-current reaches 

 its maximal intensity in the primary coil and weakens the effect which 

 change in current in the primary coil will induce in the secondary coil. 

 On the other hand, when the battery-current is broken, the current 

 in the primary coil suddenly runs down to nothing ; and although a 

 break extra current, running momentarily in the same direction as 

 the battery-current, is induced in the primary coil, it cannot delay the 

 rapidity of the fall of the battery- current, because a primary circuit 

 no longer exists in which the extra current could run. 



Demonstration of the Break Extra Current. Connect a cell with 

 binding-screws 1 and 2 of the induction-coil, placing a spring-key in 

 the circuit. Fasten to the same binding-screws of the primary coil 

 two wires, the free ends of which are placed on the tongue. On closing 

 the spring-key no shock is felt, but, on opening it, the shock of the 

 break extra current. 



