308 COMPARISON OF RESISTANCES. 



Pouillet,* who observed these different causes of variation, referred 

 all measurements of conductivity from 1837 to that of distilled 

 mercury. He took as a standard for comparison the column of 

 mercury comprised within a cylindrical tube. The diameter was 

 determined by weighings of mercury, and the ends terminated in 

 two flasks of large aperture. 



Werner Siemens f has supplied for industrial purposes a great 

 number of standards which represent very approximately a column 

 of mercury at 0, a metre in length and a millimetre in cross 

 section. 



This is still an arbitrary unit. While retaining mercury as 

 standard metal, it is more rational to choose a column .the resist- 

 ance of which is in a determinate ratio with -the absolute unit. 

 The International Commission on Electrical Units, assembled at 

 Paris in 1884, adopted as practical unit, under the name of the 

 legal ohm, the resistance of a column of mercury a square milli- 

 metre in cross section and 106 cm. in length at the temperature 

 of melting ice. From numerous experiments made by different 

 methods, this unit only differs by a few thousandths from the 

 value, io 9 absolute C.G.S. units, which represents the theoretical 

 definition of the ohm (613). 



921. CONSTRUCTION OF THE STANDARD. The construction of 

 a standard which shall agree with its legal definition, is an operation 

 which involves calibrating a tube and weighing the mercury which 

 it contains at the temperature of zero. It is of little importance 

 that the shape of the tube is in exact conformity with the defi- 

 nition, provided we know the dimensions and the column of 

 mercury which fills it ; but, in order to facilitate calculations, 

 the resistance of the standard must not differ greatly from its 

 theoretical value. J 



The calibration should be made by the methods in use for 

 accurate thermometers. We shall assume that the tube, carefully 

 selected from among those with the most regular section, has first 

 been divided into parts of equal length. Let a be the number 

 of any given division, a + a the length of the cylindrical column 

 of section equal to the mean section of the tube, which has a 

 volume equal to that comprised between the division a and the 



* POUILLET. Elements de Physique, 3rd Edition, Vol. I., p. 586. 1837. 

 t SIEMENS. Pogg. Ann., Vol. ex., p. i. 1860. Works, p. 229. 

 % MASCART, DE NERVILLE, and BENO!T. Resumt d" 1 Experiences sur la 

 Determination de V Ohm* Paris, Gauthier-Villars 1884. 



