10 ELECTROLYSIS. 



units must be sufficient to allow of the valuation of all the quan- 

 tities which characterise electric currents, and also to estimate in 

 dynamical work the energy to which they correspond. 



Physical phenomena being always dependent on the three 

 elements : matter, space, time, it is necessary to have a unit of 

 mass for estimating the quantity of matter, a unit of length for 

 comparing the spaces, and a unit of time for estimating the 

 duration of a phenomenon. 



These three units, which are called fundamental, and have 

 been finally determined at the International Congress of 1881, 



are: 



Unit of length : L = 1 centimetre. 



Unit of time : T = 1 second. 



Unit of matter : M = the mass of 1 gramme.* 



This system is known as the 0. G. S. system (centimetre, 



gramme-mass, second). 



The derived units, always in view of theoretical calculations, 



are: 



Resistance f .. R = L T~i 



Electromotive force E = M* L* T~i 



Intensity C = M* Li T~i 



Quantity Q = M* L* 



Capacity K = L-i T2 



Force F = ML T~2 



Work W= ML2 T-* 



* The decision of the Congress relating to the unit intended for estimating the 

 value of matter has been very much criticised. The two other fundamental units 

 having been selected amongst the usual practical systems, it was deemed advis- 

 able to select the third one from amongst the same. This amounts to saying that, 

 by adopting the weight of 1 gramme instead of its mass, not only would the 

 electrical calculations have been facilitated, but especially their comparison with 

 mechanical calculations. 



The Congress considered that the action of gravity varied with the altitude 

 and latitude, so that one standard gramme carried from Paris to London, Berlin, 

 or even to Versailles, would not exactly correspond to the weight of one cubic 

 centimetre of water, and that a particular standard of size would be required at 

 every point of the globe. The unit of mass is, on the contrary, invariable. The 

 weight varies, but the acceleration which the gravity impresses upon it varies in 



W (weight) 



the same ratio, so that ^ or the mass is invariable. This is the reason 



g (gravity) 



why the Congress, ratifying Gauss's notions, adopted, as a third fundamental 

 unit, the mass of 1 gramme and not that of 9 '81 grammes. 



t We are using here the most usual formulae, without positively insisting on 



