Il6 INSTRUMENTS. [CH. X. 



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 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 pur- 

 poses 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 entirely ceases in the primary coil, but is alter- 

 nately strengthened and weakened by the rise and fall of the 

 hammer; the strengthening corresponds to the ordinary make, 

 and is weakened by the make extra ciirrent, which occurs in 

 the opposite direction to the battery current ; the break is also 

 incomplete, and so it is weakened by the break extra current, 

 which being in the same direction as the battery current impedes 

 its disappearance. 



The two next diagrams show the way the interrupter acts. We 

 are supposed to be looking at the end of the primary coil ; the 

 battery wires are attached to the binding screws A and E (fig. 

 138). The current now passes to the primary coil by the pillar 

 on the left and the spring or handle of the hammer as far as the 

 screw (C) ; after going round the primary coil, one turn only of 

 which is seen, it twists round a pillar of soft iron on the right- 

 hand side, and then to the screw E and back to the battery ; 

 the result of a current going around a bar of soft iron is to 

 make it a magnet, so it attracts the hammer, and draws the 

 spring away from the top screw C, and thus breaks the current ; 

 the current ceases, the soft iron is no longer a magnet, so it 

 releases the hammer and contact is restored by the spring ; then 

 the same thing starts over again, and so a succession of break 

 and make shocks occurs alternately and automatically. 



