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has been made by Trouton^[1] at the suggestion of FitzGerald, and in which it was tried to observe the existence of a sudden impulse acting on a condenser at the moment of charging or discharging; for this purpose the condenser was suspended by a torsion-balance, with its plates parallel to the Earth's motion. For forming an estimate of the effect that may be expected, it will suffice to consider a condenser with aether as dielectricum. Now, if the apparatus is charged, there will be (§ 1) an electromagnetic momentum

${\mathfrak {G}}={\frac {2U}{c^{2}}}{\mathfrak {w}}$.

(Terms of the third and higher orders are here neglected). This momentum being produced at the moment of charging, and disappearing at that of discharging, the condenser must experience in the first case an impulse $-{\mathfrak {G}}$ and in the second an impulse $+{\mathfrak {G}}$.

However Trouton has not been able to observe these jerks.

I believe it may be shown (though his calculations have led him to a different conclusion) that the sensibility of the apparatus was far from sufficient for the object Trouton had in view.

Representing, as before, by U the energy of the charged condenser

1. Trouton, Dublin Roy. Soc. Trans. (2) 7 (1902), p. 379 (This paper may also be found in The scientific writings of FitzGerald, edited by Larmor, Dublin and London 1902, p. 557).