TRANSACTIONS OF SECTION A. 487 



3. A New Determination of the Number of Electrostatic Units in the Electro- 

 Magnetic Unit. By W. E. AyRton and J. Perry. Telegram and 

 Letter to Sir W. Thomson from Professor W. E. Ayrton. 



The Red Sea, August 3. 



My Dear Sir William,— From Singapore I sent you the following telegram 

 on the 13th July from Professor Perry and myself. 



" Kindly inform British Association that air-condenser measured magnetically 

 and statically gives mean value ratio of these units (29-80) twenty-nine point eight 

 nought ohms ; Foucault's velocity light." 



In the autumn of this year 1 propose communicating a full account of this 

 investigation to the Physical Society, or the Society of Telegraph Engineers, or 

 otherwise as you may advise ; but I thought that, as the British Association Com- 

 mittee had for so long busied itself with the determination of electrical units, you 

 might deem the result of this investigation of Mr. Perry and myself worthy of a 

 preliminary notice at the meeting of the Association to he held this year at Dublin. 

 Not being "sure that I should arrive in Europe in time to reach Dublin at the com- 

 mencement of the meeting, although I hope to be present during the last three or 

 four days, I took the liberty of sending you the telegram quoted above. 



The result we have obtained for "v" is the more interesting, inasmuch as, 

 without any bias being given to any one of our experiments, the mean value is 

 identically the same as that obtained by M. Foucault for the velocity of light, 

 whereas all previous determinations of the number of electrostatic units in an 

 electromagnetic unit have led to results differing considerably from Foucault's 

 velocitv. 



It appeared to Mr. Perry and myself that the method best suited for the accu- 

 rate determination of " v " consisted in measuring the capacity of an air-condenser — 



1. electromagnetically, by the swing of the needle of a ballistic galvanometer ; and 



2. electrostatically, by a measurement of the linear dimensions of the condenser, 

 since the equation connecting these capacities — 



$ = v*S, 

 s being the absolute electrostatic capacity, 

 S „ „ electromagnetic capacity, 



leads to an equation involving only the square root of a resistance. 



Two difficulties of course presented themselves in this investigation— difficulties 

 that it took us many months to overcome, labouring as we were under the disad- 

 vantage of experimenting in a country like Japan. They were — 



1. To obtain a large air condenser, of which the plates had sufficiently true 

 surfaces that the electrostatic capacity could be accurately measured, at any rate 

 when the plates were not nearer than half a centimetre to one another. 



2. To obtain a galvanoinetric arrangement of sufficient sensibility to measure the 

 small capacity of such an air condenser, and sufficiently ballistic that the air 

 damping should be almost inappreciable. 



A full description of the condenser we employed (and which had a guard ring, 

 and all the different arrangements we could think of for obtaining accurate results) 

 will accompany the account of the investigations to which I have referred. It is 

 sufficient here to mention that the errors arising from the surfaces of the condenser 

 plates not being true planes were practically eliminated by capacity experiments 

 being made with successive adjustments of the condenser-plates, a different set of 

 points in the upper plate being each time brought to the fixed distance from the 

 lower plate. 



The arrangement of a ballistic galvanometer to fulfil the two conditions men- 

 tioned in (2) was very troublesome. I made several astatic needles, none of which 

 satisfied us, and we were beginning to fear my departure from Japan would 

 necessitate the abandoning of the investigation. At last, however, an astatic 

 combination containing (40) forty small magnets (and of which a description will 

 accompany the paper) gave satisfactory results, and I obtained threo excellent sets 

 of observations on June 18, June 23, and June 25, when my departure put an 



