[372.4 
XLIX.—NOTE ON THE CONSTRUCTION OF GUARD-RING 
ELECTROMETERS, sy GEORGE FRANCIS FITZGERALD, 
M.A., F.T.C.D. 
[Read January 19th, 1880.] 
GUARD-RING electrometers have usually been constructed with an 
aluminium disk, for the sake of lightness, surrounded by a guard- 
ring of brass. It is essential for the accuracy of the calculation 
of the absolute values of capacity and quantity made with them 
that the electricity should be as uniformly distributed as possible 
on the surface of the disk and guard. It is for the sake of pro- 
ducing a uniform distribution on the disk that the guard is added 
Hence any arrangement which disturbs this uniformity of distri- 
bution is to be avoided. Now, whether the contact of dissimilar 
metals in itself produces an appreciable difference of potential 
between them, or whether it is the air near different metals that 
is at different potentials, there is no doubt that when the plates 
of an accumulator are of different metals there is an appreciable 
accummulation of electricity upon them. Consequently in the 
guard-ring electrometer the distribution of electricity on the 
aluminium disk cannot be the same as on the brass guard con- 
nected with it. It might seem as if the other plate should be of 
the same material, but as it is generally easy to apply the 
differential method of measurement, a constant even though un- 
known difference of potential between the plates is of no con- 
sequence. 
