44 LABORATORY GUIDE IN PHYSIOLOGY. 



from S through D and back to B. The total cur- 

 rent is divided into two parts, C which passes 

 along the wire from S to B, and C' the derived cur- 

 rent which passes through the detector. Sup- 

 pose the resistance to the last named current is R' 

 and that to the direct current is R, the relative 

 strength of these two currents is expressed in the 

 following proportion: C' : C : : R : R'. 



But the resistance of the German silver wire may 

 be conveniently divided into 100 equal parts (100 r). 



If the slider be placed at any position along the 

 wire, say at x centimeters from the end, then the 

 formula would be C' : C : : lOOr xr : R'. 

 r , Cr (100 -x) 



~R ' 

 Suppose that R = 1 ohm (r = 0.01 ohm); R' = 



2 ohms and x = 0; i. e., suppose the slider to be 

 hard up to A, then C' = Cr( ^ 00 ~ x) = -f- ; or the 

 current which passes to the detector is one-half as 

 strong as the current through the rheocord. 



(2) What is the relative strength of the two currents 

 when x = 10? 



(3) What is the relative strength of the two currents 

 when x = 50? 



(4) What is the relation of C' to C when x = 99? 



(5) What is the relation of C' to C when x = 100? 

 From this course of reasoning it is evident that 



in the simple rheocord we have an instrument with 

 which we can vary a derived current from zero to a 

 maximum. Just what the value of this derived cur- 

 rsnt will be will depend upon the voltage of the cell 

 or battery and the total resistance to be overcome, 

 as well as upon the distribution of that resistance. 



(6) Verify the theory just developed, making out a 

 table of detector readings. 



