380 Trouton — The Results of an Electrical Experiment, involving the Relative 



that the density of its charge is a. Then the magnetic force between the plates 

 which has to be established is H = 4:tt(tu.* The energy which must be provided 



K^E 



to do this is T=-^H^ x (volume of dielectric). Remembering that a = -. — 



where ^is the specific inductive capacity, E the voltage between the plates, and 

 c their distance apart, and also that the capacity P = j— j x (volume of dielec- 

 tric), we have T= ^ ixKPEhiK 



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 u = 10" about. As regards the other quantities Ave may take jx — \, 

 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-, 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 sujiposition 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, tliat 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. 



