1895.] The Measurement of High Potential Difference. 



155 



Although the probable error of this result barely reaches the milli- 

 grams, it will be most correct to assume that the true value lies 

 between the limits from 999'85 to 999*82 grams, because the method 

 of hydrostatical weighings hitherto used does not guarantee complete 

 absence of air bubbles, but the coincidence between the results of 

 Kupffei and Chaney gives assurance that the probable weight is 

 not larger than 999 85 grams, but possibly a little smaller, about 

 999-84. 



Accepting the above-mentioned value, 999 "847, we have : — 



Temperat ure on the 

 hydrogen thermometer 

 scale. 



Weight of water in vacuo* 



Of a cubic deci- 

 metre, in 

 grams. 



Of a cubic inch 

 in English 

 grains. 



Of a cubic inch 

 in Russian 

 dolias. 



C. 



F. 



0° 



32-0° 



999 -716 



252 -821 



368 -686 



4 



39 -2 



999 -847 



252-854 



3H8 -734 



15 



590 



99X -979 



252 635 



368 -414 



16| 



62 -0 



998-715 



252-568 



368-316 



20 



68-0 



998 -082 



252 -407 



368 -i,83 



Until newer and more accurate investigations upon the expansion 

 of water and its weight in a given volume shall have been carried out, 

 the results discussed must be considered as more accurate than those 

 hitherto published. 



IV. " The Measurement of High Potential Difference." By H. 

 C. Leake, R. Leventhorpe, M.A., and C. S. Whitehead, 

 M.A. Communicated by Professor W. E. Ayrton. Re- 

 ceived October 2i), 1895. 



(Abstract.) 



In Part I the question of the measurement of high potential differ- 

 ence in terms of the absolute electromagnetic unit is considered, more 

 especially with reference to the calibration of electrostatic voltmeters. 



* In this no account is taken of the compressibility of water, that is to say, it is 

 supposed that the water is under a pressure of 1 atmosphere ; but as from this at 

 t° C. (from 0° to 30°) the volume decreases approximately by 50 — '22t million ths 

 parts, -we have in real vacuo, at 0°, the weight of a cubic decimetre equal to 999 666 

 grams, at 20° about 998'036. Therefore, the weight of a cubic decimetre of water 

 reaches 1000 grams under a pressure of 4 atmospheres, buttw vacuo at all tempera- 

 tures the weight of water is less than a kilogram. 



M 2 



