562 Sir W. Tliomson on Electrostatic Measurement. [Feb. 8, 



2000 or 3000 volts ; and with not raiicli less accuracy, by aid of an 

 intermediate electrometer, up to 50.000 volts. 



He also explained, and illustrated by a drawing, an absolute elec- 

 trometer which he had constructed for the purpose of measuring " ?;," 

 the number of electrostatic units of potential or electromotive force 

 in the electro-magnetic unit of potential. This number " v " is 

 essentially a velocity, and experiments have proved it to be so nearly 

 equal to the velocity of light that from all the direct observations 

 hitherto made we cannot tell whether it is a little greater than, or a 

 little less than, or absolutely equal to, the velocity of light. 



The determination was made by comparing the electro-magnetic 

 with the electrostatic value, in C. G. S. units, as given by the balance, 

 for a potential of 10,000 volts : but hitherto he has not been able to 

 make sure of the absolute accuracy of the electrostatic balance to 

 closer than ^ per cent. 



The results of a great number of measurements which had been 

 made in the Physical Laboratory of the University of Glasgow during 

 the i)revious two months gave the required number, " r," within ^ per 

 cent, of 300,000 kilometres per second ; the velocity of light is 

 known to be within \ per cent, of 300,000 kilometres per second. 

 Eesnlts of previous observers for determining " v " had almost 

 absolutely proved at least as close an agreement with the 300,000,000 

 metres. He exj^ressed his obligations to his assistants and students 

 in the Physical Laboratory of Glasgow University, Messrs. Meikle, 

 Shields, Sutherland, and Carver, who worked with the greatest 

 perseverance and accuracy, in the laborious and often irksome 

 observations by which he had attempted to determine " v " by the 

 direct electrometer method, as exactly as, or more exactly than, it has 

 been determined by other observers and other methods. 



Note added March Uth, 1889. 



The measurement of " v " by Sir "William Thomson and Profs. 

 Ayrton and Perry, communicated to the British Association at Bath, 

 was too small (292) on account of the accidental omission of a 

 correction regarding the efiective area of the attracted disk in the 

 absolute electrometer. When this correction is apj^lied their result is 

 brought up to 298, which exactly agrees with Profs. Ayrton and 

 Perry's previous determination by another method, in Japan. Prof. 

 J. J. Thomson's result is 296-3. It is understood that Eowland has 

 found 299. The result of Sir William Thomson's latest observations, 

 founded wholly on the comparison of electrometric and electro- 

 magnetic determinations of potential in absolute measure, is 30*1 

 legal ohms, or 30-01 Piayleigh ohms. Assuming, as is now highly 

 probable, that the Eayleigh ohm is considerably nearer than the legal 

 ohm to the true ohm, the result for " v " is 300,400,000 metres per 

 second. Sir William does not consider that this result can be trusted 

 as demonstrating the truth within ^ per cent. 



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