188 



PHYSIOLOGY 



the nerve however is about 100,000 ohms, whereas that of the bridge is not the thousandth 

 part of an ohm. When a current divides, the amount of current that goes along any 

 branch is inversely proportional to the resistance. Here the resistance in the nerve- 

 circuit is practically infinite compared with that in the brass bridge, and so all the 



FIG. 44. Du Bois key, closed. Du Bois key, open. 



current goes through the bridge and none through the nerve. We say then that the 

 current is short-circuited. 



It is often necessary to reverse the direction of a current through a nerve-muscle 

 preparation or a galvanometer in the course of an experiment. For this purpose 

 Pohl's reverser may be used. It consists of a slab of ebonite or paraffin or other in- 

 sulating material, in which are six small holes filled with mercury. A binding screw is 

 in connection with the mercury in each of these holes. Two cross-wires (not in contact 

 with one another) join two sets of pools together, as shown in Fig. 45. A cradle con- 

 sisting of two wires joined by an insulating handle carries two arcs of wire by which 

 the pools at a and b may ba put into connection with either x and y, or the corresponding 



pools on the opposite side. It will be seen 

 that with the cradle tipped to one side, as in 

 Fig. 45 A, the current from the battery enters 

 the reverser at a ; this proceeds up the wire 

 of the cradle, down towards the right, then 

 along the cross -wire to the pool at x. x is 

 therefore the anode, and y the cathode. In 

 Fig. 45 B the cradle has been swung over to 

 the other side. Here the cross-wires are not 

 used at all by the current, which passes from a 

 up the sides and down the curved wire to y. 

 In this case y is now the anode and x the 

 cathode, and the direction of the current 

 through the circuit connected with x and y is 

 reversed. By taking out the cross -wires, 

 Pohl's reverser may be used as a simple 

 switch, by which the current may be led into 

 two different circuits in turn. 



With this form of reverser difficulty is often 

 experienced owing to dirt accumulating on the 

 mercury and forming an insulating layer be- 

 tween it and the binding screw or copper 

 wire. Several improved forms of reverser are 



now made where the mercury poles are replaced by brass banks, and these are gene- 

 rally to be preferred in practice. 



(2) INDUCED CURRENTS. In using these the muscle or nerve is stimulated by the 

 current of momentary duration produced in the secondary circuit of an induction-coil 

 by the make or break of a constant current in the primary. 



The construction of the induction-coil or inductorium is founded on the fact that if 

 a coil of wire in connection with a galvanometer be placed close to (but insulated from) 



FIG. 45. Diagram of Pohl's reverser. 



