I 



ON DYNAMICAL AND ELECTRICAL UNITS. 223 



Up to the present time it has been necessary for every person who wishes 

 to specify a magnitude in what is called " absolute " measure, to mention the 

 three fundamental units of mass, length, and time which he has chosen as 

 the basis of his sj'stcm. This necessity will be obviated if one definite selec- 

 tion of three fundamental units be made once for all, and accepted by the 

 general consent of scientific men. We are strongly of opinion that such a 

 selection ought at once to bo made, and to be so made that there will be no 

 subsequent necessity for amending it. 



We think that, in the selection of each kind of derived unit, all arbitrary 

 multiplications and divisions by i)owers of ten, or other factors, must be 

 rigorously avoided, and the whole system of fundamental units of force, work, 

 electrostatic, and electromagnetic elements must be fixed at one common 

 level — that level, namely, which is determined by direct derivation from the 

 three fundamental units once for all selected. 



The carrying out of this resolution involves the adoption of some units which 

 are excessively large or excessively small in comparison with the magnitudes 

 which occur in practice ; but a remedy for this inconvenience is provided 

 by a method of denoting decimal multiples and submultiples, which has 

 already been extensively adopted, and which we desire to recommend for 

 general use. 



On the initial question of the particular units of mass, length, and time to 

 be recommended as the basis of the whole system, a protracted discussion has 

 been carried on, the principal point discussed being the claims of the gramme, 

 the metre, and the second, as against the gramme, the centimetre, and the 

 second, — the former combination having an advantage as regards the simpli- 

 city of the name metre, while the latter combination has the advantage of 

 making the unit of mass practically identical with the mass of unit-volume 

 of water — in other words, of making the value of the density of water prac- 

 tically equal to unity. We are now all but unanimous in regarding this latter 

 clement of simplicity as the more important of the two ; and in support of 

 this view we desire to quote the authority of Sir W. Thomson, who has for a 

 long time insisted very strongly upon the necessity of employing units which 

 conform to this condition. 



We accordingly recommend the general adoption of the Centimetre, the 

 Gramme, and the Second as the three fundamental units ; and until such time 

 as special names shall be appropriated to the units of electrical and magnetic 

 magnitude hence derived, we recommend that they be distinguished from 

 " absolute " units otherwise derived, by the letters " C. G. S." prefixed, these 

 being the initial letters of the names of the three fundamental units. 



Special names, if short and suitable, would, in the opinion of a majoritj' of 

 \\s, be better than the provisional designations " C. G. S. unit of . . . ." 

 Sevei-al lists of names have already been suggested ; and attentive considera- 

 tion will be given to any further suggestions which we may receive from 

 persons interested in electrical nomenclature. 



Tlie " ohm,'' as represented by the original standard coil, is approximately 

 lU' C. G. S. units of resistance; the "volt" is approximately IC C. G. S. 



units of electromotive force ; and the " farad " is approximately |^^ of the 



C. G. S. unit of capacity. 



For the expression of high decimal multiples and submultiples, we recom- 

 mend the system introduced by Mr. Stoney, a system which has already 

 been extensively employed for electrical purposes. It consists in denoting 

 the exponent of the power of 10, which serves as multiplier, by an appended 



