180 



ELECTROTONUS 



[CH. XV 



altering the position of the cradle, the direction of the current from 

 one electrode to the other is reversed. The numbers 1, 2, 3, etc., 

 indicate the path of the current in the two cases. 



Sometimes the reverser is used without the cross wires for a different purpose. 

 The battery wires are connected as before with the middle mercury pools. Each 

 lateral pair of pools is connected by wires to a pair of electrodes. The two pairs of 

 electrodes may be applied to two portions of a nerve, or to two different nerves, and 

 by tilting the cradle to right or left the current can be sent through one or the other 

 pair of electrodes. 



The rheochord is an instrument by means of which the strength of 

 a constant current passed through a nerve may be varied. It consists 

 of a long wire (r, r, r) of high resistance stretched on a board. This 



FIG. 194. Simple Rheochord. 



is placed as a bridge on the course of the battery current. (See fig. 

 194.) The current is thus divided into two parts : one part through 

 the bridge, the other through the nerve, which is laid across the two 

 non-polarisable electrodes at the ends of the wires. The resistance 

 through the bridge is varied by the position of the slider (s s). The 

 farther the slider is from the battery end of the instrument the 

 longer is the bridge, and the higher its resistance, so that less current 

 goes that way and more to the nerve. 



The next figure shows the more complicated form of rheochord 

 invented by Poggendorf. The number of turns of wire is greater, so 



FIG. 195. Poggendorf 's Rheochord. (M'Keudrirk.) 



that the resistance can be varied to a much greater extent than in 

 the simpler form of the instrument. 



