584 PHYSIOLOGY. 



equalize the shocks, Helmholtz used a modification consisting of a 

 "side wire." Helmholtz's side-wire and the modifications it intro- 

 duces into the induction apparatus should be used when induced cur- 

 rents are applied to nerves. By this contrivance we diminish two 

 possible mistakes: (1) the undesirable predominance of currents in 

 one direction, that is, in that of the break; (2) unipolar stimulation. 

 Helmholtz's side-wire acts by short-circuiting instead of completely 

 breaking the battery current. 



Unipolar Induction. If you remove one of the wires of the 

 electrodes of the secondary coil, so that only one electrode is con- 

 nected with that coil, and slide the coil towards the primary coil, 



Fig. 219. Principle of Simple Rheocord. 

 G, Galvanometer. R, Rheocord. P, Battery. 



then send a strong current through Neef's hammer and the primary 

 coil, shocks will be faintly felt by the tongue, though only one elec- 

 trode is attached to the secondary coil. It is on account of this pos- 

 sibility of stimulating through only one pole that a short-circuiting 

 key is always used in the secondary spiral current. In the primary 

 or battery current a simple key is used, for a short-circuiting key 

 would let the battery quickly run down. 



Rheocord. A rheocord is an apparatus for dividing a constant 

 current by offering a circuit of relatively small 1 resistance, which is 

 capable of being varied so that a variable part only of the current 

 shall pass through the experimental circuit. It usually consists of 

 a platinum wire of known resistance, to the ends of which the bat- 

 tery poles are connected. With one of these ends, another wire is 



