ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 13 



duced at the ends of the wires. With one of these ends (a) another 

 wire is connected ; this forms part of the experimental circuit through 

 which a portion of the cell current is to be conducted ; this circuit 

 is completed through a wire attached to a rider (r) which slides along 

 the rheochord wire. 



FIG. 20. Diagram of the Oxford form of rheochord. a, b, Rheochord wire zig- 

 zagged upon a mahogany board, marked by cross lines into hundredths ; 

 r, rider. 



When r is in contact with b the whole difference of potential 

 between a and b which depends upon the E.M.F. of the cell or battery 

 and the resistance of the rheochord wire relative to that of the 

 experimental circuit is operative in producing a current through 

 the preparation. When r is at the middle of the rheochord wire only 

 one-half of this difference of potential comes into play, and so in 

 proportion to the distance between a and r as compared with the 



