54 THE POPULAR SCIENCE MONTHLY. 



intensity of field produced by a Weber at a distance of one centimetre ; 

 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, will be one dyue ; 

 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 re- 

 quired to raise a pound of water at the freezing-point through 1 Fahr. 

 or Cent., or, again, the heat necessary to raise a kilogramme of water 

 1 Cent. The inconvenience of a unit so entirely arbitrary is suffi- 

 ciently apparent to justify the introduction of one based on the elec- 

 tro-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 10' C. G. S. units, and, assuming Joule's 

 equivalent as 42,000,000, it is the heat necessary to raise 0'238 gramme 

 of water 1 Cent., or, approximately, the 10 1 00 part of the arbitrary 

 unit of a pound of water raised 1 Fahr., and the i0 \ of the kilo- 

 gramme of water raised 1 Cent. Such a heat unit, if found accept- 

 able, might with great propriety, I think, be called the Joule, after 

 the man who has done so much 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 immediately evident how, by the introduction of a simple multi- 

 ple, 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 fiber ; 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 center is the electrostatic measure of the con- 

 ducting power of the fiber. Thus we see how " conducting power " 

 is, in electrostatic theory, properly measured in terms of a velocity. 

 Weber has 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 elec- 

 tromagnetic, 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 Commit- 



