Page:EB1911 - Volume 27.djvu/768

 In connexion with the numerical values in the above definitions much work has been done. The electrochemical equivalent of silver or the weight in grammes deposited per second by 1 C.G.S. electromagnetic unit of current has been the subject of much research. The following determinations of it have been given by various observers:—

The above-described practical system based on the C.G.S. double system of theoretical units labours under several very great disadvantages. The practical system is derived from and connected with an abnormally large unit of length (the earth quadrant) and an absurdly small unit of mass. Also in consequence of the manner in

which the unit electric quantity and magnetic pole strength are defined, a coefficient, 4, makes its appearance in many practical equations. For example, on the present system the magnetic force H in the interior of a long spiral wire of N turns per centimetre of length when a current of A amperes circulates in the wire is 4 AN/10. Again, the electric displacement or induction D through a unit of area is connected with the electric force E and the dielectric constant K by the equation- D=KE/4. In numerous electric and magnetic equations the constant 4 makes its appearance where it is apparently meaningless. A system of units in which this constant is put into its right place by appropriate definitions is called a rational system of electric units. Several physicists have proposed such systems. Amongst others that of Professor G. Giorgi especially deserves mention. We have seen that in expressing the dimensions of electric and magnetic qualities we cannot do so simply by reference to the units of length, mass and time, but must introduce a fourth fundamental quantity. This we may take to be the dielectric constant of the ether or its magnetic permeability, and thus we obtain two systems of