COMPARATIVE VALUES' OF THE PRINCIPAL UNITS. 597 



The microfarad is really the practical unit of capacity, for the 

 value of the farad is far too great. For instance, the electrostatic 

 capacity of the Earth is equal to its radius R ; its electromagnetic 

 capacity C is equal to the quotient of the radius by 2 , which gives 



708. lo-is CG.S. units, 



3.io 



that is to say, 708 microfarads. 



It is important to observe that the practical units themselves 

 constitute an absolute system, in which the fundamental units are 



[L] = io 7 metres, or the quarter of the terrestrial meridian, 

 [M] = io~ n of the mass of a gramme, 

 [T] = a second. 



Another remark, which is of some practical utility, is, that if 

 we divide the expression for electrical work by g expressed in 

 metres that is to say, practically by io we get its value in 

 kilogrammetres. 



The practical unit of work is obtained, for instance, if we multiply 

 a volt by an ampere, which gives io 7 C.G.S. units. But we 

 have seen (612) that a kilogrammetre is equivalent to io 8 C.G.S. 

 units that is to say, to ten times as much. The same quantity 

 of work in kilogrammetres will then be expressed by one-tenth 

 of that value. 



614. COMPARATIVE VALUES OF THE PRINCIPAL UNITS. We 

 shall collate in the following table the values of the practical units 

 in C.G.S. absolute units, and also as functions of the units of 

 Gauss and Weber, which have been employed in a certain number 

 of papers on electricity. 



FUNDAMENTAL PRACTICAL C.G.S. UNITS OF 



UNITS. UNITS. UNITS. GAUSS AND WEBER. 



Length, io 7 metres, Centimetre, Millimetre, 



Mass, IO~ U gramme, Gramme, Milligramme, 



Time. Second. Second. Second. 



Resistance Ohm io 9 io 10 



Electromotive force Volt io 8 io 11 



Current Ampere io 1 io 



Quantity Coulomb lo" 1 io 



Capacity Farad io~ 9 io~ 10 



