328 THE ELECTRIC TELEGRAPH. 



of Grove's element, measured by one of Daniell's, is, accord- 

 ing to 



Poggendorff 1-812 to 1-670 



Joule 1-870 



Buff 1-787 



Levy and Saweljew 1-920 



Beetz 1-708 



Regnault 1*732 



The differences are so slight as to be amply accounted for 

 by small inequalities in the degree of concentration of the 

 fluids employed. 



V. UNITS or RESISTANCE. 



64. Siemens' Mercury Unit. The best of all the arbitrary 

 units of electrical resistance is that of a prism of mercury 

 defined and determined by Dr. Werner Siemens.* The want 

 of agreement between the resistance etalons distributed by 

 Jacobi, induced Dr. Siemens to direct his attention to the in- 

 troduction of some method of constructing an unit whose 

 reproduction, with small chance of error, would be a matter 

 of comparative facility. 



The metal, mercury, adapts itself best to this purpose ; 

 its conducting power, as we have already said, is less than 

 those of the other pure metals, while its molecular structure 

 at the same temperature is always the same. 



The difficulty first experienced was to get a vessel in 

 which the mercury could be contained, and glass tubes were 

 selected as giving an unalterable form to the body. 



The unit of resistance which Dr. Siemens defined is that of 

 a prism of pure mercury 1 millimetre section and 1 metre 

 long, at C. Glass tubes are generally irregularly conical, 

 very seldom, for any length, cylindrical, so that, in the 

 selection of uniformly conical tubes consisted the principal 

 difficulty. From a great number of glass tubes of different 

 calibres, cut to the length of a meter, a few were selected as 

 being most uniform, and the amount of their conicalness 

 * Fogg. Ann., Bd. ex., Seite 1. 



