L2 



EXPEE I M KNTA L PHYSIOLOGY 



lying 1 under a nerve or other structure to be stimulated. When the 

 key k is open any current in the secondary coil can travel through 

 the electrodes. If the key be closed, a current in the coil divides when 

 it reaches the key, passing either through the key or to one electrode, 

 thence through the nerve to the other electrode, and so back to the 

 key. As the resistance of the key is very low compared to the high 

 resistance of the piece of nerve, practically the whole of the current 

 passes that way, or, in other words, the secondary coil is short-cir- 

 cuited. A Du Bois key is always to be used in this manner when in 

 a secondary circuit. The second method of using the key is shown in 

 fig. 17, where it is used as a simple key. The electrode wires e, are 

 represented connected to one terminal of the coil and to one block of 

 the key k,. The other terminal of the coil is connected to the second 

 block of the key. When the key is closed any current in the coil can 

 pass through the electrodes, but when the key is opened the secondary 

 circuit is broken. The key can be used after this plan for making 

 and breaking any battery circuit, but should not be thus employed in 

 a secondary circuit. 



POHL'S COMMUTATOR (fig. 18) consists of a wooden or vulcanite 

 base in which are six mercury cups, to each of which a binding screw 

 is connected. A rocker made of a vulcanite axis h, to which two 



curved wires k and two ver- 

 tical ones l are joined so that 

 the vertical and curved wires 

 of the same side are connected 

 together, is so arranged that 

 the two straight wires are 

 supported in the cups a and 

 b, and the curved wire may 

 be made to dip into either 

 pair of the four remaining 

 cups. Two cross wires are 

 also provided which connect 

 c to f and d to e. Supposing now that the positive pole of a battery 

 is connected with a and the negative with b, and the key is turned over 

 so that the curved wires k dip into the cups c and d, and if c and 

 d are connected by wires to any circuit, then the current enters at a, 

 passes up l along k to c, thence through the circuit to d, and so to 

 b and back to the battery. If now the rocker be turned over so as to 

 rest in the cups e and p, as shown in the figure, then the current 

 enters at a, passes to e, thence by one cross wire to d, through the 

 external circuit to c, by the second cross wire to f, and so back to b. 

 In the first position of the rocker the current in the external circuit 



Fig. 18. — Pohl's Commutator. 



