Chap, vi.j THE COMMUTATOR. 59 



intact for further experiments, and the control experi- 

 ment performed, by laying a thread moistened in salt 

 solution over the electrodes, and placing the nerve on 

 the part of the thread projecting beyond the electrode. 

 The thread will equally well conduct the electricity, 

 but of course will not occasion nervous stimulation. 



A commutator is an instrument for reversing 

 the direction of a current. It is also called GYROTROPE 

 or RHEOTROPE. Fig. 33 shows the form constructed 

 by Pohl. In a thick disc of 

 wood or vulcanite there are six 

 little cups for holding mercury. 

 Each cup has a binding screw 

 in connection. Attached to the 

 cups 1 and 2 are the upright thick 

 wires a and b, which are con- 

 nected to one another by a bridge 

 of glass tube filled with wax, so Fig> 33> _ Pohrs Com . 

 that, though connected, they are in- mutator. 



sulated from one another. Spring- 

 ing transversely from the upright wire on each side 

 are arcs of thick copper wire, of such a length that the 

 bridge may be so inclined that the free ends on one 

 side may dip into the cups 5 and 6, or, by reversing 

 the bridge, the ends of the other side may dip into the 

 cups 3 and 4, but the free ends cannot dip into both 

 sides at once. Copper wires are also supplied, one, c, 

 stretching between 3 and 6, the other, , between 4 and 

 5, and not touching one another. These two copper 

 wires form what is called "the cross." The cross may 

 be removed ; and, according as it is in or out, does the 

 instrument serve one or another purpose. (1) Let the 

 cross be in, bring the positive electrode of the battery 

 to 1, and the negative to 2. Let the bridge incline as 

 shown in the figure, and suppose a wire to pass from 3 

 to the same instrument, and a wire to come back from 

 it to 4. The current enters at 1, passes up , then 



