ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 13 



placed at one centimeter from a, only , ,',,-, 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 experimental 

 circuit must always be relatively very great : as is invariably the case in physio- 

 logical 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 zig-zagged 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. 



Via. -20. DIAGRAM OP THE OXFORD FORM OP RHEOCHORD. a, 6, RHECHORD WIRE ZIG-ZAOHF.H 

 UPON A MAHOGANY BOAKD, MARKED Br CROSS LINES INTO HUNDREDTHS ; r, RIDER. 



In another 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 battery 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 battery current is completely short- 

 circuited, but when the rider is moved away from them a gradually 

 increasing resistance is inserted into the short circuit formed by the 



