444 



Mr. Latimer Clark on a 



[June 20, 



of double refraction within the limits of errors of observation. The error, 

 if any, could hardly exceed a unit in the fourth place of decimals of the 

 index or reciprocal of the wave-velocity, the velocity in air being taken as 

 unity. This result is sufficient absolutely to disprove the law resulting 

 from the theory which makes double refraction depend on a difference of 

 inertia in different directions. 



I intend to present to the Royal Society a detailed account of the obser- 

 vations ; but in the mean time the publication of this preliminary notice 

 of the result obtained may possibly be useful to those engaged in the 

 theory of double refraction. 



XIII. " On a Voltaic Standard of Electromotive Force." By Latimer 

 Clark, M.I.C.E. Communicated by Prof. Sir William 

 Thomson, F.R.S. Received May 30, 1872. 



(Abstract.) 



In the year 1861 a Committee was appointed by the British Association 

 for the Advancement of Science to report on standards of electrical re- 

 sistance, and subsequently on other standards of electrical measurements. 

 Reports were presented in 1862, 1863, 1864, 1865, and 1867. 



They recommended the adoption of a system of electromagnetic units 

 based on the metre and gramme, the relations of the units being such that 

 the unit of electromotive force acting through the unit resistance should 

 give the unit current, and that the unit current flowing for the unit time 

 should give the unit quantity. 



They issued standards of resistance (known as the B. A. unit or ohm) 

 and standards of electrostatic capacity, or condensers of such magnitude 

 that when charged with the unit electromotive force they contained a sub- 

 multiple of the unit quantity of electricity (known as the farad). 



No material standard of electromotive force has yet been issued. Much 

 difficulty has, in fact, been found in devising such a standard. Mechanical 

 means, such as the rotation of a conductor in a magnetic field of known 

 intensity, are too complicated for ordinary use ; thermoelectric couples are 

 extremely variable, and voltaic elements, which would constitute the most 

 convenient form of standard, have been hitherto found singularly incon- 

 stant, and therefore inapplicable. The Daniell's element, which has been 

 most frequently used for this purpose, commonly varies five per cent, or 

 more without apparent cause. 



From a conviction that if similar conditions could be ensured similar 

 combinations would always give the same electromotive force, the author was 

 led to institute a series of experiments, extending over four years, which led 

 to the discovery of a form of battery that is sensibly constant and uniform 

 in its electromotive force. 



The battery is composed of pure mercury as the negative element, the 

 mercury being covered by a paste made by boiling mercurous sulphate in 



