T h e experiments described in this paper were designed with the object of investigating the behaviour of a charged electrical condenser moving through space, and to discover, if possible, whether there be a relative motion between the earth and the aether. It was previously suggested by one of us in the ' Proceedings of the Eoyal Dublin Soc.' (Yol. V II., P art XIV., April, 1902), in connection with an experiment of a similar nature suggested by the late Professor F itzG era ld , and mentioned in his collected papers edited by Professor L armor.The idea underlying the experiments will be understood from the following considerations :-If a charged condenser be placed with its plane in the direction of the aether drift, then, on the assumption th at a moving charge develops a magnetic field, there will be associated with the condenser a magnetic field perpendicular to the lines of electric induction and to the direction of the motion. W hen the plane of the condenser is perpendicular to the aether drift, the effects of the opposite charges will neutralise each other. Thus there is magnetic energy stored in the medium when the plane of the condenser and the direction of the drift coincide; and accordingly it is to be expected that under these circumstances the condenser, when freely suspended, would tend to move so as to take up the position with its plane perpendicular to the drift, in order to transform this energy A To realise this, a condenser, with its plane vertical, was suspended by a fine wire and charged. The charges were let into the plates of the condenser by means of the suspending wire, and by a wire which hung from beneath, dipping into a liquid terminal. Observations were made a t different times in the day, when the plane of the condenser made various angles with the direction of the drift. If the condenser * [According to the theory given by Larmor (' F itzGerald's Scientific Papers,' p. 566) the longi tudinal direction, being that of maximum kinetic energy, is the direction of equilibrium; the forces due to displacement from it however remain as above, if the form of the system be assumed permanent. The negative electric experimental result reached in this paper is connected with the other negative secondorder results of optical experiments relating to motion through the aether, Avhich have been obtained by Michelson be hung with its plane north and south, then about 12 o'clock in the day there would be no couple tending to turn it, because the aether drift due to the earth's motion in its orbit round the sun is at right angles to the plane of the condenser; on the other hand, at any other hour, say 3 o'clock, there would be a couple making itself felt by a tendency to rotate the plane of the condenser into the position at right angles to the drift.The effect to be looked for was an extremely small one, being a second-order effect only. In the earlier forms of the apparatus the calculated deflection was found to be entirely obscured by a number of accidental disturbing causes, and, in fact, the...
