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XIV. 



THE EESULTS OF AN ELECTRICAL EXPERIMENT, INVOLVING THE 

 RELATIVE MOTION OF THE EARTH AND ETHER, SUGGESTED BY 

 THE LATE PROFESSOR FITZ GERALD. By FRED. T. TROUTON, D. Sc, 

 F. R. S., University Lecturer in Experimental Physics, Trinity College, Dublin. 



(Read Xovembm 20, 1901.) 



I. — Professor Fitz Gerald's Arrangement. 



In the autumn of 1900 Professor Gr. F. Fitz Gerald proposed an electrical experi- 

 ment, with the object in view of detecting any relative velocity there might be 

 between the Earth and ether. 



The method has not up till now been published, except at a meeting of the 

 Dublin University Experimental Science Association last May. 



Professor Fitz Gerald asked me to carry out the proposed experiments. These 

 I began at once, but owing to delays in preparing the apparatus and getting it 

 into working order, only some preliminary determinations were made before his 

 illness and death. 



The fundamental idea of the experiment is that a charged electrical condenser, 

 when moving through the ether, with its plates edgeways to the direction of 

 motion, possesses a magnetic field between the plates in consequence of its motion, 

 in accordance with the generally held view that a moving charge is equivalent to 

 an electric current. 



The question then naturally arises as to the source supplying the energy 

 required to produce this magnetic field. If we attribute it to the electric gene- 

 rator, say a battery,* there is no difficulty indeed, as to there being energy 

 enough to do it, for, in general, the energy supplied by a battery when charging 

 a condenser is double that stored in electrostatic strains in the condenser — 



UQ = ^I^Q + energy lost as heat, etc. 



Fitz Gerald's view, however, was that it would be found to be supplied through 

 there being a mechanical drag on the condenser itself at the moment of charging, 

 very similar to that which would occur were the mass of any body situated on the 

 surface of the Earth to suddenly become greater. Again in discharging, the 

 condenser should experience an impulse of like amount, but now in the opposite 

 or forward direction. To estimate the extent of this blow, suppose a condenser 

 of capacity P to be moving edgeways through the ether with the velocity u, and 



* See Part II. of this Paper. 



TRAKS. EOr. DrB. SOC, N.S., VOL. VII. , PART XIT. 2 H 



