PHYSIOLOGIC APPARATUS. 



709 



depend to some extent on its strength. This may be accurately regulated 

 by means of an apparatus known as 



The Rheocord. With this apparatus an electric current may be divided, 

 one portion continuing through a conductor back to the battery, the other 

 portion being sent off through the nerve. The strengths of these two currents 

 are inversely proportional to the resistances of their circuits. A simple form 

 of rheocord (Fig. 360) consists of a long wire arranged for convenience in 

 parallel lines on a small wooden base and connected at its two ends with 

 binding posts A and B. The resistance of this wire, 1.6 ohms, can be 

 increased by the introduction of small resistance coils, between D and B, 

 varying from 5 to 20 ohms. 



The two binding posts A and B are connected with the positive and 

 negative poles of an electric cell re- 

 spectively. A simple key is placed in 

 the circuit. 



From A, a wire passes to one of 

 the electrodes on which the muscle or 

 nerve rests. A second wire passes from 

 the second electrode to a clamp S, 



FIG. 359. A MERCURY KEY. 



FIG. 360. RHEOCORD. 



by way of the binding post C, which can be fastened to the long wire at any 

 given point. The current, on reaching A, will divide into two branches, one 

 of which will pass along the wire A, B, and thence back to the cell; the other 

 will pass through the nerve and back to S and thence also to the cell. The 

 amount of current passing through the nerve circuit will be inversely pro- 

 portional to the resistance of the nerve and directly proportional to the 

 difference of potential between A and S. If S is close to A, the difference of 

 potential is slight. If S is removed from A toward B, the difference of 

 potential is increased and the current sent through the nerve circuit is 

 increased. 



In many experiments it is necessary to reverse the direction of the current, 

 in other experiments to deflect it, without changing the position of the elec- 

 trodes. Both these results may be accomplished by the use of 



PohPs commutator. This is a round block of wood with six cups, 

 each of which is in connection with a binding post (Fig. 361). In each of the 

 two cups marked i and 2, + and , is inserted one end of a copper wire 



