HELMHOLTZ IN BERLIN 



Neef's interrupter, by which the circuit in the 

 primary coil was opened and closed, and as the inter- 

 rupter works automatically, a rapid series of shocks 

 may be transmitted to the nerve or muscle. It was 

 soon found that, with this arrangement, the opening 

 shock acts more powerfully on the muscle than the 

 closing shock, in other words, the opening shock has 

 the greater intensity. This arises from the develop- 

 ment, at the moment of closing the primary current, 

 of an extra current in the primary coil, which, being 

 in the reverse direction to that of the main primary 

 current, so retards the development of the latter as to 

 prolong the time during which the secondary current 

 flows, and thus make it pass through a lower maxi- 

 mum intensity than is reached by the secondary 

 current at opening when the change of the primary is 

 more abrupt, and the secondary current lasts for a 

 correspondingly shorter time. The total quantity of 

 current coming from the secondary coil is the same 

 whether the primary circuit is being opened or 

 closed ; but the secondary current, excited at opening, 

 attains momentarily a greater intensity. Hence the 

 opening shock is the more stimulating. As in stimu- 

 lating a nerve or muscle by an induction coil it is 

 important to stimulate with rapid opening and closing 

 shocks of the same intensity^ Helmholtz so modified 

 the Neef's interrupter as to equalise the currents. 

 By his arrangement the current in the primary circuit 

 is not wholly cut off. It is merely short-circuited. 

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