272 
IOWA ACADEMY OF SCIENCE 
application can be most readily understood, after the method has been 
explained. 
;The electrical method of measuring the distance between two plane 
conductors is based on a measurement of the electrical capacity which 
the two planes possessed by virtue of their nearness. For small distances 
such as I used the distortion of the lines of force at the edge of the planes 
was so small that no appreciable error accrued therefrom, and the 
capacity of the connecting wires and the upper plane isolated was only 
from three to five units and could be corrected for or neglected. This 
method involves the elementary but fundamental and absolute formula, 
C=S/4*& (1) 
where C is the electrical capacity of the two neighboring surfaces of 
area S, and, d, is the distance between the electrical charges on opposite 
planes. 
Now if it should be revealed that the distance between the opposite 
electrical charges is the same as the distance between the mechanical 
surfaces, then obviously it should be concluded that within the accuracy 
of measurement the electrical and mechanical surfaces are identical. In 
other words the identity would establish that an electron atmosphere 
does not extend beyond the mechanical surface. The question as to 
whether both negative and positive atmospheres exist need not be con- 
sidered at this time. 
The capacity of two parallel plates can be measured readily to a 
satisfactory degree of precision by the method of mixtures. I used a 
Dolazaleck electrometer of 20 e.s.u. capacity for comparison capacity and 
also for measuring the necessary potentials. The procedure was first to 
charge up one pair of quadrants to a potential V ± , represented by a 
deflection of the needle, D x . The quantity of electricity on the needle 
was then allowed to distribute itself between the electrometer of capacity 
Ce, and the parallel plates of capacity C. The resulting potential V 2 , 
gave a deflection D 2 . As usual the capacity of the parallel surfaces is, 
C=Ce (D— D 2 ) (2) 
D 2 
Substituting the value of the capacity in equation (2), the distance 
between the conducting surfaces is obtained in the form 
d=S.D 2 /4 7r Ce (D x — D 2 ) (3) 
