Measures for Electric and Magnetic Quantities. 431 



for the reduction of electrostatic to electromagnetic measures, 

 namely Weber's critical velocity, could not yet be effected 

 with the same degree of precision that could be attained within 

 the sphere of electromagnetic measurements on the one hand 

 and electrostatic measurements on the other. It was on this 

 account more advantageous to employ in each experimental 

 investigation that system of measures to which the quantities 

 measured could be referred with the greater exactness. 



To this is to be added the consideration of avoiding excessively 

 large numbers, which will probably induce us to continue to 

 employ for electrostatic and galvanic phenomena two kinds of 

 measures, although reducible to one another. At present the 

 electromagnetic methods of measurement are the most perfect; 

 they are unmistakably the most important practically for an 

 art that advances with giant strides ; and I have therefore 

 considered that the International Congress that met in Paris 

 last year acted quite suitably in endeavouring to establish an 

 electromagnetic system of absolute measures. Had the aim 

 been purely scientific, I should have preferred the electrostatic 

 system hitherto employed, since this, I think, best represents 

 the essential analogies of the phenomena by analogous for- 

 mulae, and gives to them the clearest and most intelligible 

 expression. It was on this system, grounded on Gauss's 

 principles, that most of the physical-mathematical treatises in 

 this department of science have hitherto been based. 



Just on this account it would appear to me very undesirable 

 if this system should now entirely fall, and even its name give 

 place to a new one, as proposed by Clausius in his recently 

 published memoir*. I would not at all recommend the mul- 

 tiplication of systems of measures without very urgent reasons; 

 and certainly the transference of a name already in use and 

 frequently employed to a new system would inevitably pro- 

 duce needless and vexatious confusion in physical literature, 

 even apart from any estimate of the relinquished in compa- 

 rison with the new system. 



Any determination of a new absolute measure must be based 

 on the measuring observation of a natural process or behaviour, 

 just as already, among the three fundamental units, the 

 gramme has been reduced to the two others by means of the 

 density of pure water at 4° 0. The measure of magnetic 

 quanta which has hitherto been exclusively employed is 

 founded on the definition laid down by Gauss, according to 

 which the repellent force between two magnetic quanta, m x 

 and m 2 , which are situated at the distance r from one another, 



* Clausius, Verhandl. des naturh. Vereins d. preuss. Rheinl. u. Westfal. 

 March 6, 1882 ; Wied. Ann. xvi. p. 529 ; Phil. Mag. June 1882, xiii. p. 381. 



