PRACTICAL PHYSIOLOGY II 



completed through a wire attached to a rider (r) 

 which slides along the rheochord wire. 



When r is in contact with b the whole difference 

 of potential between a and b this difference de- 

 pending upon the E.M.F. of the battery and the 

 resistance of the rheochord wire is operative in 

 producing a current through the experimental cir- 

 cuit. 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 dis- 



FIG. 16. DOUBLE-WIRE RHEOCHORD. 



tance between a and r as compared with the whole 

 length of the wire. Thus if the wire be 100 centi- 

 metres long and r be placed at one centimetre 

 from a only T ^ of the total difference of potential 

 will be operative and a proportional current will 

 be diverted into the experimental circuit. When 

 this form of rheochord is to be used, the resistance 

 of the experimental circuit must always be very 

 great : this is invariably the case in physiological 

 experiments. 



In another form of rheochord (Poggendorff's) 

 the wire is doubled on itself, and a rider (r) bridges 

 across and forms a short circuit between the two 



