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314 PROTOPLASM 



The potentiometer is an instrument for comparing an unknown 

 potential with a known one, without drawing current from the 

 unknown, A known and an unknown electromotive force are 

 connected so that the known force E (Fig. 145) is joined to the 

 two ends A and 5 of a wire of uniform resistance per unit length, 

 and the unknown force E' connected so that one or both of its ter- 

 minals C and D are movable along the wire. A galvanometer G is 

 inserted in the circuit with the unknown e.m.f. E'. {R and S 

 in Fig. 145 are for the moment ignored.) The known e.m.f. E 



may be a voltaic cell of about 

 R E 2 volts; the unknown is the 



feeble potential set up by free 

 hydrogen ions in a solution. 

 The e.m.f. of each of these two 

 systems (the upper one M and 

 the lower one N) will oppose 

 each other if like poles of E 

 and E' are connected. If the 

 two currents (of M and N) 

 . oppose each other, then there 



Fig. 145. — Diagram illustrating the .,, . n j^i i at- • 



principle of the potentiometer. Will be nO floW through N in 



either direction when the poten- 

 tial drop between C and D is equal (and opposite) to the 

 potential of E'. At this point, there will be no deflection 

 of the galvanometer. Once the position of C and D is deter- 

 mined, at which point the galvanometer shows no deflection and 

 CD = E', then it is necessary only to ascertain what part CD is 

 of the total length (potential drop) of AB, that is to say, what 

 part E' is of the known potential E. If the unknown potential 

 E' is a part of E, then, naturally, E must always be larger than E'. 

 The calculation is made from the ratio E:E';;AB:CD, from which 



X,, CD 



E' = ^XE. 



Whatever we now add to our Poggendorff equipment has 

 nothing to do with the fundamental principle involved but adds 

 only to the refinement of the method. The source of current 

 E must be of a known and a constant potential, which is never 

 true of the usual electric cell. E must, therefore, be calibrated 

 and frequently checked against a very accurate standard cell 



