the Mercury Unit of Electrical Resistance. 173 



The copies of an arbitrarily chosen unit of resistance, subject 

 to the errors incidental to copying, must be less exact than 

 the resistances directly calculated according to the definition of 

 Dr. Siemens. In addition to this, we have no certainty that 

 coils of wire do not change their resistance in course of time. 



I believe it is generally the disposition of English physicists 

 to accept as common unit of resistance the absolute unit of the 

 beautiful system of Weber. The advantages of this unit are, 

 however, limited to facilitating the solution of some purely scien- 

 tific problems, and may be completely reached, without materially 

 increasing the difficulties, by a single careful determination of 

 the mercury unit in absolute measure ; and its general adop- 

 tion by electricians would only be serviceable in so far as it would 

 be the means of introducing a common measure. But, to its 

 disadvantage, the reproduction of the absolute unit is attended 

 with expense and time which few physicists would be in a position 

 to sacrifice to it. Then, again, its determination depends upon 

 the measurements of several forces, of which each is burthened 

 with comparatively great sources of error. Attending each sub- 

 ordinate measurement, in fact, the possible error is greater than 

 that of the simple comparison of two nearly equal resistances. 

 It is evident, therefore, that the determination of resistance in 

 absolute units is not adapted for the production of normal coils. 

 The differences between the values of the mercury unit, according 

 to the determinations of absolute measure by Weber and Thomson, 

 entirely confirm this opinion ; for we can look for the causes of 

 differences between the results of two of the most world-renowned 

 physicists only in the system, not for a moment in the manipu- 

 lation. 



Supposing, however, the absolute unit to be reproducible with 



certainty within 0*1 per cent., and supposing that the electrician 



overcame the inconvenience of the mass of figures required 



millimetre . , , . . 



to express a = — unit by employing as unit of resistance 



the same multiplied with 10 10 (which would bring it to about the 

 value of one mercury unit)*, recourse must still be had to the 

 arbitrary system as a ready means of reproduction. A second 

 definition of the 10 10 absolute unit, as "the resistance of a prism 

 of mercury a metre long and 10257 square millimetre section at 

 0° C," would inevitably sooner or later creep into general use. 

 The necessity of accepting mercury as unit of conducting- 

 * 111 computing resistances of insulation, a still more capacious unit is 



necessary; for example, 10 16 mi "netre ^ ^ kilo-kilometre (mercury) 



second ' 



unit. The mercury unit at 0° C, according to Weber's last determination 

 (ZurGalvanometrie,G'6ttmgen,\862,ip.58), is equal to about 10,257,000,000 

 absolute units. 



