380 Trouton—The Results of an Electrical Experiment, involving the Relative 
that the density of its charge is o. Then the magnetic force between the plates 
which has to be established is H = 47o0u.* The energy which must be provided 
KE 
Aire 
where £ is the specific inductive capacity, £ the voltage between the plates, and 
to do this is 7= a x (volume of dielectric). Remembering that o = 
ce their distance apart, and also that the capacity P= = x (volume of dielec- 
7 
tric), we have 7= 4p KPH. 
We had at our disposal a capacity of about 8 mf., which I found was 
able to stand 1200 volts. When charged with this voltage, on the above 
supposition, it comes out that it requires about 1 erg to energize the magnetic 
field when moving edgewise through the ether at the velocity of the Earth 
round the Sun, that is at 19 miles per second; this, in centimetres per second, 
being w = 10° about. As regards the other quantities we may take p = 1, 
and for the specific inductive capacity of the paraffined paper of which the 
condenser was constructed, we may take 2 as a probable estimate, or in 
electromagnetic measure K = 2/v*, » being the velocity of light, and equal to 
3 x 10° cm. per second. 
Thus we see that, on charging such a condenser, when placed so as to have its 
plates edgewise to the direction of motion of the Earth round the Sun, it must 
get from some source or other 1 erg more energy supplied it than it requires 
when moving flatwise, unless indeed the ether be in some way dragged along 
with matter and there be no magnetic field. Fitz Gerald’s supposition was, as we 
have seen, that this came through the condenser, receiving a compensating 
forward jerk or impulse on being charged, transmitted from the Earth through 
the supports; and he proposed to detect it by suspending the condenser at the end 
of an arm with a balance-weight on the other side, by means of a wire, as shown 
diagrammatically in fig. 1 (see opposite page). 
It was originally intended to have two condensers, one at each end of the 
cross arm, the one to be charged at the moment the other was discharged, not 
only to double the effect, but also to secure a pure torque acting on the wire. 
This idea had to be abandoned in the final experiment, owing to all the con- 
densers available breaking down under the excessive voltage employed, save only 
one. A condenser similar to the one used for charging was employed as the 
balance-weight, so as to preserve symmetry as far as was possible. 
* The moving positive and negative charges on the two plates, that is, moving relatively to the ether, 
are equivalent to currents running tangentially in opposite directions in the two plates, so that 
we may take the field as existing only between the plates. If the plates move ‘ flatways,” the 
equivalent currents are in the normal directions, and neutralize each other’s magnetic action almost 
completely. 
