August 24, 1882] 



NA TURE 



39i 



construction, and tbat Telford, the founder of the Institution of 

 Civil Engineer?, must have had in his mind's eye, when he denned 

 civil engineering as "the art of directing the great sources of 

 power in nature." 



These considerations may serve to show that although we see 

 the men of both abstract and applied science group themselves in 

 minor bodies for the better prosecution of special objects the 

 points of contact between the different branches of knowledge 

 are ever multiplying, all tending to form part of a mighty tree — 

 the tree of modern science — under whose ample shadow its 

 cultivators will find it both profitable and pleasant to meet, at 

 least onca a year ; and considering that this tree is not the 

 growth of one country only, but spreads both its roots and 

 branches far and wide, it appears desirable that at these yearly 

 gatherings other nations should be more fully represented than 

 has hitherto been the ca-e. The subjects discussed at our meet- 

 ings are without exception of general interest, but many of tbem 

 bear an international character, such as the systematic collection 

 of magnetic, astronomical, meteorological, and geodetical obser- 

 vations, the formation of a universal code for signalling at sea, 

 and for distinguishing lighthouses, and especially the settlement 

 of scientific nomenclatures and units of measurement, regarding 

 all of which an international accord is a matter of the utmost 

 practical importance. 



As regards the measures of length and weight it is to be re- 

 gretted that this country still stands aloof from the movement 

 initiated in France towards the close of last century ; but, con- 

 sidering that in scientific work metrical measure is now almost 

 universally adopted, and that its use has been already legalised in 

 this country, I venture to hope that its universal adoption for 

 commercial purposes will soon follow as a matter of course. The 

 practical advantages of such a measure to the trade of this 

 country would, 1 am convinced, be very great, for English 

 goods, such as machinery or metal rolled to current sections, are 

 now almost excluded from the continental market, owing to the 

 unit measure employed in their production- The principal im- 

 pediment to the adoption of the metre consists in the strange 

 anomaly that although it is legal to use that measure in com- 

 merce, and although a copy of the standard metre is kept in the 

 Standards' Department of the Board of Trade, it is impossible 

 to procure legalised rods representing it, and to use a non- 

 legalised copy of a standard in commerce is deemed fraudulent. 

 Would it not be desirable that the British Association should 

 endeavour to bring about the use in this country of the metre 

 and kilogramme, and, as a preliminary step, petition the Govern- 

 ment to be represented on the International Metrical Commis- 

 sion, whose admirable establishment at Sevres possesses, inde- 

 pendently of its practical work, considerable scientific intere.-t, 

 as a well-found laboratory for developing methods of precise 

 measurement. 



Next in importance to accurate measures of length, weight, 

 and time, stand, for the purposes of modern science, those of 

 electricity. 



The remarkably clear lines separating conductors from non- 

 conductors of electricity, and magnetic from non-magnetic -ub- 

 stances, enable us to measure electrical quantities and effects with 

 almost mathematical precision ; and, although the ultimate 

 nature of this, the youngest scientifically investigated form of 

 energy, is yet wrapt in mystery, its laws are the most clearly 

 established, and its measuring instruments (galvanometers, 

 electrometers, and magnetometers), are amongst the most 

 accurate in physical science. Nor could any branch of science 

 or industry be named in which electrical phenomena do not 

 occur, to exercise their direct and important influence. 



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

 it follows that its unit measures should be determined w ith 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 current ; 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 researches 

 appealed only as isolated efforts, when, in 1862, the Electric 

 Unit Committee was appointed by the British As-ociation, 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, wh ch, 

 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 gene- 

 rally adopted abroad, and the British Association unit intended 

 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 determina- 

 tion of the unit meaure of resistance to an International Com- 

 mittee, 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, in which the units are of 

 a value convenient for practical measurements. In this, which 

 we must hereafter know as the "practical system," as distin- 

 guished from the " absolute system," the units are named after 

 leading physicists, the Ohm, Ampere, Vol', Coulomb, and 

 Farad . 



I w< uM venture to suggest that two further units might, with 

 advantage, be added to the system decided on by the Inter- 

 national 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 ■ uggestion of 

 Clausius that the unit should be called a " Weber," thus retain- 

 ing a name most closely connected with electrical measurements, 

 and only omitted by the Congress in order to avoid the risk of 

 confusion in the magnitude of the unit current with which his 

 name had been formerly associated. 



The other unit I should suggest adding to the list is that < f 

 pow er. The power conveyed by a current of an Ampere through 

 the difference of potential of a Volt is the unit consistent with 

 the practical system. It might be appropriately called a Watt, 

 in honour of that master mind in mechanical science, James 

 Watt. He it was who first had a clear physical conception of 

 power, and gave a rational method of measuring it. A Watt, 

 then, expresses the rate of an Ampere multiplied by a Volt, 

 whilst a horse-power is 746 Watts, and a Cbeval de Vapeur 735- 



The system of electro-magnetic units would then be : — 



(1) Weber, the unit of magnetic quantity =I0 8 C.G.S. Units. 



(2) Ohm ,, ,, resistance =lo 9 ,, 



(3) Volt ,, ,, electromotive force = io 8 ,, 



(4) Ampere ,, ,, current = IO- 1 „ 



(5) Coulomb „ ,, quantity =io- 1 ,, 



(6) Watt ,, ,, power =io 7 ,, 



(7) Farad ,, ,, capacity = 10- 9 ,. 

 Before the list can be looked upon as complete two other units 



may have to be added, the one expressing that of magnetic field, 

 and the other of heat in terms of the electro-magnetic system. Sir 

 William Thomson suggested the former at the Paris Congress, and 

 pointed out that it would be proper to attach to it the name of 

 Gauss, who first theoretically and practically reduced observa- 

 tions of terrestrial magnetism to absolute measure. A Gauss will, 

 then, be defined as the intensity of field produced by a Weber at 

 adi-tanceof oneceritiuitt.e ; and the Weber will be the absolute 

 C.G.S. unit strength of magnetic pole. Thus the mutual force 

 between two ideal point poles, each of one Weber strength held 

 at unit distance asunder, w ill be one dyne ; that is to say, the 

 force which, acting for a second of time on a gramme of matter, 

 generates a velocity of one centimetre per second. 



The unit of heat has hitherto been taken variously as the 

 heat required to raise a pound of water at the freezing- 

 point through 1° Fahrenheit or Centigrade, or, again, the heat 

 necessary to raise a kilogramme of water 1° Centigrade. The 

 inconvenience of a unit so entirely arbitrary is sufficiently 

 apparent to justify the introduction of one based on the electro- 

 magnetic system, viz., the heat generated in one second by the 

 current of an Ampere flowing through the resistance of an Ohm. 

 In absolute measure its value is io 7 C.G.S. units, and, assuming 

 Joule's equivalent as 42,000,000, it is the heat necessary to raise 

 o'23S grammes of water 1° Centigrade, or, approximately, the 

 -r-nrjth part of the arbitrary unit of a pound of water raised 1° 

 Fahrenheit and the^-j'-njth of the kilogramme of water raised 1° 

 Centigrade. Such a heat unit, if found acceptable, might with 



