6 Heport— 1882. 



risk of confusion in the magnitude of the unit cilrtent with which his 

 name had been formerly associated. 



The other unit I would suggest adding to the list is that of power. 

 The power conveyed by a current of an Ampere through the diiFerence of 

 potential of a Volt is the unit consistent with the practical system. It 

 might be appropriately called a Watt, in honour of that master mind 

 in mechanical science, James Watt. He ifc was who first had a clear 

 physical conception of power, and gave a rational method of measuring it. 

 A Watt, then, expresses the rate of an Ampere multiplied by a Volt, 

 whilst a horse-power is 746 Watts, and a Cheval de Vapeur 735. 



The system of electro-magnetic units would then be : — 



(1) Weber, the unit of magnetic quantity = 10* C.G.S. Units. 



(2) Ohm ,, „ resistance = 10^ ,, 



(3) Volt 



(4) Ampere 



(5) Coulomb 



(6) Watt 



(7) Farad 



electromotive force = 10^ 

 current = 10"' 



quantity =10"' 



power =: 10'^ 



capacity := 10-^ 



Before the list can be looked ujion as complete two other units may 

 have to be added, the one expressing that of magnetic field, and the 

 other of heat in terms of the electro-magnetic system. Sir William 

 Thomson suggested the former at the Paris Congress, and pointed out 

 that it would be proper to attach to ifc the name of Gauss, who first 

 theoretically and practically reduced observations of terrestrial magnetism 

 to absolute measure. A Gauss will, then, be defined as the 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 dyne; 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° Fahren- 

 heit or Centigrade, or, again, the heat necessary to raise a kilogramme of 

 water 1° Centigrade. The inconvenience of a unit so entirely arbitrary 

 is sufficiently apparent to justify the introduction of one based on the 

 electro-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 grammes 

 of water 1° Centigrade, or, approximately, the riyoo^^ part of the arbitrary 

 unit of a pound of water raised 1° Fahrenheit and the ^Vij^h of the 

 kUogramme of water raised 1° Centigrade. Such a heat unit, if found 

 acceptable, might with great propriety, I think, be called the Joule, 



