278 Professors Ayrton and Perry on the Ratio of the 



of a given section, at a given temperature ; but as soon as it 

 had been ascertained that a comparatively slight trace of certain 

 impurities introduced into a conductor seriously affected its 

 specific resistance, it became clear that no one of the previously 

 proposed standards was sufficiently definite. Consequently 

 Jacobi, in 1848, felt it necessary to send to Poggendorff and 

 others a certain copper wire, since well known as " Jacobi's 

 standard," in order that electric copies of it might be taken, 

 to avoid the growing inconvenience of the multiplicity of 

 standards. 



But measurements of resistance can be conceived and carried 

 out entirely without reference to the special qualities of any 

 material whatever ; and in 1849 KirchhofT had effected a 

 measurement of this nature : it was not, however, until 1851 

 that Weber proposed a distinct system of measurement based 

 on the fundamental units of length, mass, and time, and such 

 that electrical resistance according to it would be expressed 

 by an absolute velocity. Previous to this, Gauss, desiring to 

 obtain precise measurements of terrestrial magnetism at 

 different parts of the earth's surface, found it necessary at the 

 outset to decide on a unit of force which was not, like the 

 weight of a pound, affected by the position of the place in 

 which the experiment was made. He therefore devised what 

 has since become well known as Gauss's " absolute " or relative 

 unit of force, based on the fundamental units of length, mass, 

 and time. In accordance with this nomenclature of Gauss, 

 "Weber called his method of electrical measurement the 

 " absolute electro-magnetic " system. As soon as the pro- 

 posed system of Weber appeared, Thomson accepted and ex- 

 tended it, showing that the unit of absolute work, the con- 

 necting link between all physical forces, formed part of the 

 same system ; consequently the units of resistance of Weber 

 and Thomson were not based on the physical properties of 

 any special substance, but merely on the fundamental units of 

 length, mass, and time. 



Mention must not be omitted of the mercury unit of Siemens, 

 since, although not an absolute one, the coils and apparatus 

 constructed by Dr. Siemens were made with such care that 

 his system has materially helped in obtaining the present 

 accuracy of the standards issued by the Committee of the 

 British Association. 



In addition to the vagueness introduced by selecting the 

 resistance of a special rod of some material as our standard, 

 there would be the consequent necessary introduction of 

 various numerical coefficients into the equations connecting 

 current, resistance, electromotive force, work, &c. It was 



