ADDBESS. 6 



most accurate in physical science. Nor could any branch of science or 

 industry be named in which electrical phenomena do not occur, to exer- 

 cise their direct and important influence. 



If, then, electricity stands foremost amongst the exact sciences, it 

 follows that its unit measures should be determined with the utmost 

 accuracy. Yet, twenty years ago, very little advance had been made 

 towards the adoption of a rational system. Ohm had, it is true, given 

 us the fixed relations existing between electromotive force, resistance and 

 quantity of cnrrent ; Joule had established the dynamical equivalent of 

 heat and electricity, and Gauss and Weber had proposed their elaborate 

 system of absolute magnetic measurement. But these invaluable re- 

 searches appeared only as isolated efforts, when, in 1862, the Electric 

 Unit Committee was appointed by the British Association, at the instance 

 of Sir William Thomson, and it is to the long-continued activity of this 

 Committee that the world is indebted for a consistent and practical 

 system of measurement, which, after being modified in details, received 

 universal sanction last year by the International Electrical Congress 

 assembled at Paris. 



At this Congress, which was attended officially by the leading 

 physicists of all civilised countries, the attempt was successfully made to 

 bring about a union between the statical system of measurement that had 

 been followed in Germany and some other countries, and the magnetic or 

 dynamical system developed by the British Association, also between the 

 geometrical measure of resistance, the (Werner) Siemens unit, that had 

 been generally adopted abroad, and the British Association unit in- 

 tended as a multiple of Weber's absolute unit, though not entirely fulfil- 

 ling that condition. The Congress, while adopting the absolute system 

 of the British Association, referred the final determination of the unit 

 measure of resistance to an International Committee, to be appointed by 

 the representatives of the several Governments ; they decided to retain 

 the mercury standard for reproduction and comparison, by which means 

 the advantages of both systems are happily combined, and much valuable 

 labour is utilised ; only, instead of expressing electrical quantities directly 

 in absolute measure, the Congress has embodied a consistent system, 

 based on the Ohm, the Centimetre, the Gramme, and the Second, in which 

 the units are of a value convenient for practical measurements. In this, 

 which we must hereafter know as the ' practical system,' as distinguished 

 from the ' absolute system,' the units are named after leading physicists, 

 the Ohm, Ampere, Volt, Coulomb, and Farad. 



I would venture to suggest that two further units might, with advan- 

 tage, be added to the system decided on by the International Congress 

 at Paris. The first of these is the unit of magnetic quantity or pole. 

 It is of much importance, and few will regard otherwise than with 

 satisfaction the suggestion of Clauslus that the unit should be called a 

 ' Weber,' thus retaining a name most closely connected with electrical 

 measurements, and only omitted by the Congress in order to avoid the 



