PRACTICAL SYSTEM OF UNITS. 595 



nature of the phenomena. The velocity of light is very approxi- 

 mately 300,000 kilometres per second. This is the number which we 

 shall take for the ratio a, expressing it as a function of the length 

 which has been chosen as fundamental unit. 



611. CHOICE OF FUNDAMENTAL UNITS. The choice of units 

 adopted as units of time, of length, and of mass, is obviously arbitrary. 

 As unit of time the second, the sixtieth of the minute of the mean 

 time adopted by all civilised peoples, naturally suggests itself; as unit 

 of length we may take either the metre or a decimal of a metre. As 

 unit of mass it is advantageous to take the mass of unit volume 

 of water at its greatest density ; we thus retain the advantage that 

 the specific gravity of water is equal to unity, and that the weight 

 of a body is equal to the product of its volume by its density. 



The absolute system which would be least foreign to the 

 habitudes established in the use of the metrical system, would 

 consist in taking the decimetre and the mass of a kilogramme 

 as fundamental units. 



Gauss and Weber, who introduced into science the first absolute 

 system, had chosen the millimetre and the mass of a milligramme. 

 The British Association adopted the centimetre and the gramme on 

 the proposition of Sir W. Thomson. These latter units were defi- 

 nitely adopted for electrical and magnetic measurements by the 

 International Congress of Electricians, which met at Paris in 1881. 



612. ABSOLUTE C.G.S. SYSTEM. It has been agreed that the 

 units derived from the centimetre, from the mass of a gramme, and 

 from the second of mean time, shall form the absolute system, 

 properly so-called, and which will be denoted by the symbol C.G.S. 



These units have not received any special names. Thus the 

 unit of force C.G.S. is the force which, acting on the mass of a 

 gramme, imparts to it in a second the acceleration of a centimetre. 

 It follows from this that a gramme is g units of force C.G.S., and a 

 kilogramme g.io 3 C.G.S. units, g being expressed in centimetres. 

 In like manner, a kilogrammetre is g. io 5 , that is to say, 981. io 5 , 

 or about io 8 C.G.S. units of work. On this system the value of a 

 is 3.io 10 . 



613. PRACTICAL SYSTEM. The values of the absolute units of 

 the C.G.S. unfortunately do not stand in any convenient relation 

 to the magnitudes we have to measure in practice. Thus, the 

 absolute unit of the C.G.S. electromagnetic resistance is scarcely 

 the resistance of a twenty-millionth of a millimetre of copper 

 wire a millimetre in diameter, and the unit of electromotive force 

 would be the one-hundred-millionth part of that of a Daniell. 



Q Q 2 



