GENERAL PHYSIOLOGY. 41 



current will probably not be sufficient to cause any 

 deviation of the detector needle; for the current varies 

 inversely as the resistance (C x -jjr), and if the re- 

 sistance of the long circuit (R) be incomparably 

 greater than the resistance of the short circuit (R')> 

 then the current of the long circuit (C) will be incom- 

 parably less than the current of the short circuit (C'), 

 i. e., C : C' :: - '- -~ } or C : C' :: R' : R; therefore if 

 R' = 0, C must equal 0. 



Suppose that the resistance of the detector circuit 

 be only 10 ohms, and suppose we remove from the 

 rheostat plug that represents 0.1 ohm resistance, then 

 one-hundredth of the current will pass through the 

 detector. If we make the resistance in the short cir- 

 cuit 0.2 ohms then one-fiftieth of the current will flow 

 through the long circuit. 



In this way we may increase the detector current 

 step by step until the maximum is reached. What 

 is the maximum current to be derived when the 

 resistance in the long circuit equals 10 ohms, the maxi- 

 mum resistance of the rheostat 100 ohms, external re- 

 sistance in circuit between cell and rheostat 1 ohm, 

 E. M. F. = 1 volt, internal resistance of cell four 

 ohms ? 

 b. The Du Bois=Reymond Rheocord. 



In the use of the rheostat the variation of the cur- 

 rent is step by step and not gradual. Experience has 

 shown that tor certain physiological experiments it is 

 necssary to cause a gradual variation of the current, 

 i. e., an increase by infinitessimal increments. The 

 Du Bois-Reymond rheocord is an instrument which 

 fulfills this condition by adding to the short circuit 

 millimeter by millimeter the resistance of a platinum 

 wire. The principle and use of the Du Bois-Reymond 



