OF VITAL PHENOMENA 



4i 



(IHEOSrAT 



,v«£tjj- 



e -"fe fCA 



Fig. 11. Potentiometer and standard cell combined (L and N) from 

 MRR. 



The principle of the potentiometer is as follows : Suppose we 

 have a wire stretched over a scale divided into equal divisions, 

 and an electric current flowing through the wire so that each 

 division has a fall of potential of one millivolt. If we lead off 

 a shunt circuit from two points on the wire 10 divisions apart 

 through an electrometer, a current of 10 millivolts will flow 

 through the electrometer. Interpose in the shunt circuit an 

 emf of 10 millivolts in a direction opposite to the current in 

 the shunt, no current will flow through the shunt, and there will 

 be no deflection in the electrometer. Interpose an unknown 

 emf in the shunt and move the contacts on the potentiometer 

 wire until no deflection is seen in the electrometer, and the voltage 

 of the unknown is equal to the number of divisions between the 

 contacts of the shunt leads, in millivolts. In order to standardize 

 our current, we reverse this process by placing a known emf 

 (Weston cell) in the shunt and then changing the total length 

 of the wire until each division equals a millivolt. 



The hydrogen electrode is essentially a metal electrode satu- 

 rated with hydrogen. Platinum absorbs hydrogen greedily 

 especially when in the finely divided state known as platinum 

 black, and is excellent for the surface of the electrode. But it 

 is better to make the interior of gold so that it will not absorb 

 so much hydrogen and take it away from the surface. The gold 

 is covered with platinum black as in platinizing conductivity 

 electrodes, but it is not necessary to remove Cl 2 by H 2 S0 4 and 

 electrolysis, as the electrode is never used as anode and no Cl 2 

 is formed. In order to keep the electrode saturated with hydro- 



