XXVI—ON M. DUTER’S EXPERIMENTS ON ELECTRIC AC- 
CUMULATORS, sy GEORGE F. FITZGERALD, m.a., F.7.c.p. 
[Read February 17, 1879.] 
M. Duter has shown by his experiments, as published in the 
Comptes rendu, November, 1878, Vol. 87, p. 828, that when 
an electric accumulator is charged there is an increase in volume 
of the dielectric. 
I propose to show that a portion, at least, of this effect must be 
due to the known laws of electric action, but a number of quanti- 
tative experiments would be required in order to prove that the 
whole effect was due to the cause I am about to mention. 
In order to simplify the treatment I will take the case of a 
spherical accumulator of internal radius a, and external 6. 
For the equilibrium of this shell the pressures inside and out 
must bear a certain proportion to one another if it is to remain 
undeformed. If we consider the equilibrium of the semi-shell 
supporting inside a pressure P, and outside p, while along the 
edge there is a tension T, we must have— 
Pa?= pb?4-T (6?—a?), 
showing that unless P and p are to one another inversely as the 
areas on which they act, there must be either enlargement or 
contraction of the shell, as its surface must be either in tension 
or compression. 
Now, the superficial pressures produced when the shell is made 
into an electric accumulator are by no means inversely pro- 
portional to the areas. If the external surface be at potential 
zero, we have the following expression for the potential at any 
point in the dielectric at a distance 7 from the centre when the 
quantity of electricity on the inside is Q :— 
| alas | 
v=@ (7-3): 
Hence the electric force F= 2, and this produces a pressure 
on the inside— 
See Q? 
te) tas 
