(>4 Messrs. J. W. Swan and J. Rliodin. Absolute [May 24, 



more or less success, of many other forms of this combination, includ- 

 ing some in which very porous hollow carbon poles were used, and 

 through which the chlorine was forced, but the effects obtained were 

 less than those recorded. The research has proved that it is possible 

 to form pyro-batteries of the Upward type, although it is extremely 

 difficult to realise the conditions required for effective action. In a 

 future communication I hope to record the results of experiments 

 made, with a view to utilise oxygen as a depolariser in connexion 

 with cells with fused electrolytes. 



VII. " Measurements of the Absolute Specific Resistance of Pure 

 Electrolytic Copper." By J. W. SWAN and J. RHODIX. 

 Communicated by Lord RAYLEIGH, Sec. R.S. Received 

 February 28, 1894. 



At the beginning of 1893 it was resolved to make some very careful 

 measurements of the specific resistance of pure electrolytic copper, 

 drawn into wire without previous fusion. Researches made during 

 the latter end of 1892 had shown that the specific resistance of 

 electrolytic copper varies considerably. The resistance of about 

 thirty wires of the same length and diameter, made from specimens 

 of electrolytic copper, prepared in different ways in the laboratory, 

 showed differences of resistance amounting to a maximum of 1'4 per 

 cent., both when in a hard and when in a soft or annealed state, and 

 measured at the same temperature. 



These preliminary measurements were made by means of a Wheat- 

 stone's bridge, constructed for comparing the unknown resistances of 

 short well-conducting wires with the resistance of a standardised 

 platinoid wire, according to Thomson's method. The accuracy ob- 

 tainable by this method was 0'25 per cent. The best specimens of 

 wire were subjected to a further and still more accurate examination. 



The measurements of the specific resistance and temperature co- 

 efficient of one of these wires, and of some wire made from the same 

 copper, after undergoing a second electrolytic refining, form the 

 subject of this paper. It was resolved to make measurements giving 

 an ultimate accuracy of O'l per cent. As they were intended to be 

 absolute, the first problem was the determination of the exact 

 dimensions of the wires to be measured. The measurement of the 

 length was made by means of direct comparison with a standard 

 metre rule; that of the diameter was determined by the specific 

 gravity method, which consists in finding the absolute weight of a 

 known length of wire and its density or unit volume weight as 

 determined from its specific gravity, and then calculating its average 

 diameter. 



