352 



MESSRS. R. T. GLAZEBROOK AND T. C. FITZPATRICK 



TABLE I. 



Since the original standards of the British Association are at Cambridge, it is 

 possible to compare there the mercury unit and the B. A. unit directly ; and, on 

 hearing the results of Professor ROWLAND'S careful investigations, communicated to 

 the British Association at Manchester, it was thought advisable by several members 

 of the Electrical Standards Committee to repeat the experiments at the Cavendish 

 Laboratory. This seemed the more desirable, as the number expressing the B.A, unit 

 in terms of the ohm is given by Professor ROWLAND as '98644 ; and this agrees much 

 more closely with the results of the similar observations at Cambridge, viz., '98667, 

 than do the corresponding values of the mercury unit in terms of the B.A. unit, viz., 

 95349 (ROWLAND), and -95412 (RAYLEIGH). 



It may be useful to state clearly what is meant by the B.A. unit. In 1864 Messrs. 

 MATTHIESSEN and HOCKIN constructed a number of coils of various materials to 

 represent at certain specified temperatures resistances of 10 9 C.G.S. units of resistance 

 as determined by the Electrical Standards Committee. 



Eight of these coils (two being of platinum-iridium, two of platinum, three of 

 platinum-silver, and one of gold-silver) have been retained in the possession of the 

 Committee, while copies have been distributed to other electricians. 



The temperatures at which six of these coils are equal to each other, and to one 

 B.A. unit, are given in the B.A. Report for 1867. Since that date the coils have been 

 repeatedly compared among themselves, and also with others of the original copies ; 

 and, with one exception, the apparent changes in their relative values, if any have 

 occurred, are exceedingly small, and could be accounted for by the supposition that 

 the temperature of the coil at the time of observation was unceitain to about '1 C. 

 Since, then, it is exceedingly improbable that all these coils of such different materials 



" On the Specific Resistance of Mercury," ' Phil. Trans.,' 1883. 

 t ' Journal de Physique,' June, 1884. 

 J ' WIEDKMANN, Annalen,' vol. 25, 1885. 

 ' WIEDEMANN, Annalen,' vol. 25, 1885. 

 || Communicated to (he British Association, 1887. 



f ' Abhandl. der k. Bayer. Akad. d?r Wissenschaften,' II. ClaRse, vol. 16, Abth. iii. 

 ** ' Comptes Rendus,' June 4, 1888. 



