14 EXPERIMENTAL PHYSIOLOGY 



whole length of the wire. Thus if the wire be 100 centimetres long 

 and r be placed at one centimetre from a, only T J F of the total 

 difference of potential will be operative and a proportional current 

 will be diverted into the experimental circuit. If r is in contact with 

 a no current is led through the preparation. 



When this kind of rheochord is used, the resistance of the experi- 

 mental circuit must always be relatively very great : as is invariably 

 the case in physiological experiments, where an animal tissue forms 

 part of the circuit. 



The wire of a rheochord may be stretched straight as in Fig. 19, or, 

 to economise space, it may be zigzagged upon a board (Fig. 20), or 

 arranged spirally round a vulcanite cylinder, or circularly round a disc 

 as in du Bois-Reymond's form of instrument. 



FIG. 21. Double-wire rheochord. 



In the shunt form of rheochord there are two wires, and a broad 

 metal rider (r) bridges across and forms a short circuit between the 

 two (Fig. 21). The cell circuit and the experimental circuit are both 

 connected with the one end of each wire. When the rider is brought 

 up to these ends the cell current is completely short-circuited, but 

 when the rider is moved away from them a gradually increasing re- 

 sistance is inserted into the short circuit formed by the rheochord 

 and its rider, and proportionally more of the current passes into the 

 experimental circuit. 



Induction coil. If the wires of two separate circuits are at any 

 point near to and parallel with one another and if, in the first or 

 primary circuit, the current of a cell is either made or broken by the 

 closing or opening of a key, an induced current is set up in the other 

 or secondary circuit at the instant of such closing or opening, but not 

 during the passage of the primary current. 



