ADDRESS. 7 



after the man who has done so mucli to develop the dynamical theory 

 of heat. 



Professor Clausius urges the advantages of the statical system of 

 measurement for simplicity, and shows that the numerical values of the 

 two systems can readily be compared by the introduction of a factor, 

 which he proposes to call the critical velocity ; this, Weber has already 

 shown to be nearly the same as the velocity of light. It is not imme- 

 diately evident how by the introduction of a simple multiple, signifying 

 a velocity, the statical can be changed into dynamical values, and I am 

 indebted to my friend Sir William Thomson for an illustration which 

 struck me as remarkably happy and convincing. Imagine a ball of con- 

 ducting matter so constituted that it can at pleasure be caused to shrink. 

 Now let it first be electrified and left insulated with any quantity of 

 electricity on it. After that, let it be connected with the earth by an 

 excessively fine wire or a not perfectly dry silk fibre ; and let it shrink 

 just so rapidly as to keep its potential constant, till the whole charge is 

 carried ofi". The velocity with which its surface approaches its centre is 

 the electrostatic measure of the conducting power of the fibre. Thus we 

 see how 'conducting power' is, in electrostatic theory, properly measured 

 in terms of a velocity. Weber had shown how, in electromagnetic 

 theory, the resistance, or the reciprocal of the conducting power of a 

 conductor, is properly measured by a velocity. The critical velocity, 

 which measures the conducting power in electrostatic reckoning and the 

 resistance in electromagnetic, of one and the same conductor, measures 

 the number of electrostatic units in the electromagnetic unit of electric 

 quantity. 



Without waiting for the assembling of the International Committee 

 charged with the final determination of the Ohm, one of its most dis- 

 tinguished members, Lord Rayleigh, has, with his collaborateure, Mrs. 

 Sidgwick, continued his important investigation in this direction at the 

 Cavendish Laboratory, and has lately placed before the Royal Society a 

 result which will prolaably not be surpassed in accuracy. His redetermi- 

 nation brings him into close accord with Dr. Werner Siemens, their two 

 values of the mercury unit being 0-95418 and 0-9536 of the B.A. unit 

 respectively, or 1 mercury unit = 0-941.3 X 10^ C.G.S. units. 



Shortly after the publication of Lord Rayleigh's recent results, Messrs. 

 Glazebrook, Dodds, and Sargant, of Cambridge, communicated to the 

 Royal Society two determinations of the Ohm, by difierent methods; 

 and it is satisfactory to find that their final values differ only in the fourth 

 decimal, the figures being, according to 



T T -n 1 • 1 1 nr. r\.c\QaK^ Earth Quadrant 

 Lord Rayleigh , , 1 Ohm = 0-98651 ^-^^^ — 



Messrs. Glazebrook, etc. = 0*986271 „ 



Professor E. Wiedemann, of Leipzig, has lately called attention to the 



