110 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [dl. X. 



Fig. 137 represents the Du Bois Keymond coil, the one generally 

 employed in physiological experiments, c is the primary coil, and d 

 and d' its two ends, which are attached to the battery, a key being 

 interposed for making and breaking ; g is the secondary coil, the two 

 terminals of which are at its far end ; to these the electrodes to the 

 nerve are attached ; the distance between the two coils, and so the 

 strength of the induction currents, can be varied at will. It is only 

 when the primary current is made or broken, or its intensity increased 

 or diminished, that induction shocks occur in the secondary circuit 

 which stimulate the nerve. When one wishes to produce a rapid 

 succession of make and break shocks the automatic interrupter or 



FIG. 137. Du Bois Raymond's Induction Coil. 



Wagner's hammer seen at the right-hand end of the diagram is 

 included in the circuit. 



The next thing to be noticed is that the break effects are stronger 

 than the make effects ; this is easily felt by placing the electrodes 

 on the tongue. This is due to what is called Faraday's extra 

 current. This is a current produced in the primary coil by the 

 inductive influence of contiguous turns of that wire on each other ; 

 its direction is against that of the battery current at make, and so 

 the make shock is lessened. At the break the extra current is of 

 such short duration (because when the circuit is broken there can be 

 no current at all) that for all practical purposes it may be considered 

 as non-existent. 



The same difference of strength occurs alternately in the repeated 

 shocks produced by Wagner's hammer. Helmholtz, to obviate this, 

 introduced a modification now known after him. It consists in 

 bridging the current by a side wire, so that the current never 



