54 Profs. Ayrton and Perry and Dr. Sumpner. [June 4, 



action was not caused by an increased tension of the fibres, and that 

 therefore some other cause must be looked for to explain the observed 

 results. 



An investigation, which turned out to be both lengthy and very 

 laborious, was therefore undertaken to ascertain the cause of this 

 curious behaviour of our White electrometer. At first we thought 

 that it might be due to some capillary action between the platinum 

 weight and the sulphuric acid, which varied with the potential of 

 the acid, but, experiment having shown that this was not the explana- 

 tion, we decided to make an exhaustive series of experiments for 

 determining the laws connecting the variation of sensibility of the 

 White quadrant electrometer with the potential of the needle, the 

 distance between the silk fibres, and the distance between the quad- 

 rants. The investigation has occupied us on and off for some years, 

 and in carrying it out the quadrant electrometer has had to be taken 

 to pieces many times. 



To facilitate the frequent removal of the interior of the Ley den jar, 

 rendered necessary for carrying out the various experiments, an im- 

 provement was introduced into the method of clamping the needle, 

 and to diminish leakage, an improvement was introduced into the 

 replenisher, both of which are described in detail in the paper. 



The P.D. between the needle and the outside case of the electro- 

 meter was measured by means of one of Sir William Thomson's 

 absolute electrometers, made especially sensitive by thinning the 

 coach springs supporting the attracted aluminium disc. 



In July, 1888, several large P.Ds. were measured by means of this 

 absolute electrometer (using the constants that we had determined 

 for this instrument), and by means of one of Sir William Thomson's 

 commercial " electrostatic voltmeters," reading to 20,000 volts, kindly 

 lent us by Messrs. Elliott Brothers. The result of these comparisons 

 led first to a correction in the constants that had been previously 

 sent out with the electrostatic voltmeters from Glasgow, and secondly, 

 to a new determination of the value of " v" For Sir William 

 Thomson had calibrated these voltmeters electromagnetically on the 

 basis of the value of the electrochemical equivalent of silver, as 

 determined by Lord Kayleigh and Mrs. Sidgwick, while we had 

 checked the calibration of the electrostatic voltmeter by comparing 

 this instrument with the absolute electrometer. The value of " v " 

 thus obtained was 298 million metres per second. 



From the experiments made on varying the distance between the 

 fibres supporting the needle of the quadrant electrometer, it was 

 found that, when the control due to the fibres was large, the sensi- 

 bility of the quadrant electrometer increased more rapidly than the 

 potential of the needle, whereas, when the control due to the fibres 

 was small, the sensibility increased with the potential of the needle 



