GENERAL PHYSIOLOGY. 47 



A slider makes contact with the platinum wire, but 

 slides along the standard by an ebonite arm. The 

 derived current passing along the wires A and B, and 

 the direct current from S to B along the platinum wire 

 sustain a relation similar to that of currents C and C' 

 in the rheocord. 



/. Appliances. Ludwig compensator; 2 wires; detector. 



2. Theory, experiments and observation. 



(1) Join the two poles, a and b, to the detector; place 

 the slider at cm., or hard up to the zinc plate, and 

 note the deviation of the needle. 



(2) Gradually move the slider from cm. to 50 cm. 

 (or 100) noting the effect upon the needle. 



(3) Suppose the detector circuit, from S through the 

 detector and back to B, has a resistance of 10 ohms 

 (R' 10). Let the resistance of the platinum wire 

 be 0.01 ohm per centimeter; for the instrument 

 figured, R = 0.5 ohm. Let C' be the detector 

 current, and C the direct current. Then C' : C : : 

 -JL : -jL., or C' : C : : R : R', or C' = ^ Let x be 

 the distance in centimeters from B to S, or the read- 

 ing of the position, of the slider; then the proportion 

 of R at any position of the slider would be . 



C'= ~j; substituting the assumed values, C'=-^-. 



(4) When x = how much current will flow through 

 the detector? 



(5) When the slider stands at 10 cm. what proportion 

 of the total current will flow through the detector ? 



(6) When the slider stands at 25 cm., how much 

 larger is C than C'? 



(7) When the value of x is 50 the ratio of the detector 

 current to the direct current? 



(8) Verify all of these theoretical results as far as 

 possible, by experiment. 



