SIR WILLIAM SIEMENS, F.R.S. 417 



t ical electricians. The limit of resistance, or ohm, is represented 

 I'V :i column of pure mercury, of one square millimetre sectional 

 aiva, and of a length of 106 millimetres at zero Centigrade. This 

 lii-iiig fixed, it is only necessary to determine the relative conduc- 

 tivity of any metal or substance to that of pure mercury, in order 

 to introduce into our calculations the value in ohms of any con- 

 ductor or coil of wire intended for use. 



The perfect accuracy with which electrical quantities can be 

 determined, and used in calculation, makes it probable that in 

 dealing with other forms of energy, such as heat, light, and even 

 mechanical effects, the electric unit quantities will be substituted 

 for those in common use. Starting with this assumption, it 

 appears strange that the Board of Trade, in putting fonvard a 

 normal wire gauge for compulsory use, should have adopted a unit 

 inconsistent with the C.G-.S., or electrical unit system, and I hope 

 that it is not yet too late for the authorities of that department to 

 reconsider the measure,. before it is brought into compulsory use. 



The wire-gauge intended to be imposed for compulsory use, 

 from the 1st of March next, professes to be a compromise between 

 the old Birmingham wire gauge, the gradual and erratic growth 

 of rule of thumb, and the Whitworth gauge, based upon the 

 decimalised inch as the unit measure. The Whitworth gauge has 

 the advantage in its favour of being based upon a rational unit, 

 the tenth of an inch, of which each successive number represents a 

 multiple. It was introduced, together with a system of accurate 

 measurement, by means of the micrometer screw, its author having 

 thereby inaugurated a virtual revolution in the mechanical arts. 

 The Whitworth gauge would, in my opinion, leave nothing to be 

 desired, were it not for the unfortunate circumstance that its 

 author has not based it upon the decimal unit, the centimetre, 

 which, whatever may be urged against it on hypothetical grounds, 

 is undoubtedly making steady and continuous progress towards 

 universal adaptation throughout the world. It possesses this 

 great advantage over the decimalised inch, that it connects units 

 of linear with those of cubical measure and weight, that it has 

 been adopted by the European nations for common use, and that, 

 in this country, it is employed in all scientific investigations, 

 owing to the facilities just indicated. Legally speaking, its use 

 is authorised by Act of Parliament, but its practical employment 



VOL. III. E K 



