246 HOWARD J. CURTIS 



It is sometimes convenient to make these bridges out of a potas- 

 sium chloride sokition in an agar gel. This is permissible except 

 where very exact measurements are required. There may be small 

 spurious potentials, probably introduced by impurities in the agar, 

 which cannot be predicted and in general will be different at the two 

 ends of the bridge. 



4. Insulation 



When making potential measurements in aqueous systems one 

 must exercise extreme care to make sure that there are no leakage 

 currents. Such currents often give rise to measured potentials much 

 larger than the potentials being measured. Many apparently 

 startling discoveries have been found to be due to this cause. Proper 

 insulation is not difficult, but certain precautions must be taken from 

 the start and constantly checked . 



In making such measurements it is necessary to so arrange things 

 that the resistance through the desired path is low relative to that 

 through any other path. In most cases the equivalent circuit for 

 such a system can be reduced to that shown in Figure 5. Here the 

 circuit under investigation is represented by RiEi and the object of 

 the measurement is to measure the magnitude of Ey. In general 

 there will be some leakage between the two electrodes. The leakage 

 resistance, R2, will in general have a potential associated with it, 

 represented here by E^. The potential measured at the measuring 

 instrument, E, will be : 



E = E,- ^'^^' - ^'^ (3) 



Ri + R2 



If R2 is very much larger than Ri, E = Ei and the measured value 

 will be correct. However, if R2 is the same size as Ri, as may well be 

 the case in an actual experiment : 



E = {Ei + Eo)/2 



and the spurious potential will play as large a part as the potential 

 being measured. Under such conditions the measured potential 

 would be meaningless. 



It is not always easy to make sure that the spurious potentials 

 are not significantly influencing the measured potential. A common 

 practice is to short-circuit the measured potential with a salt bridge 



