324 THE ADDRESSES, LECTURES, ETC., OF 



Professor Clausius urges the advantages of the statical sysbem 

 of measurement for simplicity, and shows that the numerical 

 values of the two systems can readily be compared by the intro- 

 duction 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 immediately 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 

 conducting 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 off. The velocity with 

 which its surface approaches its centre is the electrostatic measure 

 of the conducting power of the fibre. Thus we see how " con- 

 ducting 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 assembly of the International Com- 

 mittee charged with the final determination of the Ohm, one of 

 its most distinguished members, Lord Eayleigh, has, with his 

 collaboratrice, Mrs. Sidgwick, continued his important investiga- 

 tion in this direction at the Cavendish Laboratory, and has lately 

 placed before the Eoyal Society a result which will probably not 

 be surpassed in accuracy. His redetermination 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 respec- 

 tively, or 1 mercury unit = 0'9413 x 10 9 C.G.S. units. 



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

 Messrs. Glazebrook, Dodds, and Sargant, of Cambridge, com- 

 municated to the Royal Society two determinations of the Ohm 



