616 Miss P. M. Borthwick on Potential Measurements in 



a millimetre or so o£ the plate. Moreover, the drop at the 

 plate was found to occur for currents so small that the effect 

 o£ the free ions must have been negligible. 



(iii.) The well-known view that an exploring wire, in a 

 region where one sign of ion predominates, takes up too 

 high a potential, as pointed out by Sir J. J. Thomson. 



It was at first thought that this error, if present, might be 

 eliminated by a small leak on the electrometer. By comparing 

 the effects of different leaks it was thought that a value of the 

 leak might be found which would keep the exploring wire 

 at the true potential of the air in its neighbourhood. This 

 would be the case if, upon adding a small further leak, no 

 further fall in potential resulted. No such value of the leak 

 could be found ; in fact, for higher potentials the shape of 

 the curve was entirely altered by a small leak and indicated 

 that in this region the exploring wire was below the potential 

 of its surroundings. For the final experiments, therefore, 

 leak was carefully avoided. 



Another attempt was made to test whether the potential 

 of the exploring wire is always too high. E was replaced 

 by a loop of fine platinum wire, which could be made 

 red-hot by an electric current. Potentials of the loop, when 

 hot and when cold respectively, were measured at any given 

 position. If the potential of the cold loop is higher than 

 it ought to be, there should be a decrease in the electrometer 

 deflexion when the heating current is switched on. The 

 readings obtained were found to be irregular, probably 

 owing to the large fluctuations in the temperature of the loop 

 caused by the electric wind. On the whole, however, no great 

 change, and certainly no marked decrease, was produced in 

 the potential readings by heating the wire. This appears to 

 show that the explorer E was actually indicating the true 

 potential of the air at its surface. 



(iv.) Disturbance may be introduced by the presence of 

 the sheath of the exploring wire. This explanation is sup- 

 ported by the fact that the drop near the plate is not quite 

 sudden. In the early experiments a glass sheath was used, 

 but in order to ensure that the effect was not due to con- 

 duction at the surface of the sheath, quartz was substituted 

 for the glass. More consistent results were obtained, but 

 the sudden drop was not eliminated. 



Hovda (loc. cit.) suggested a second method for tracing 

 the slope of potential between point and plate. This consisted 

 in replacing any given equipotential surface by a metal 

 surface, earthing the latter, and measuring the difference 



