578 EEPORT — 1891. 



F = c^?^ 4,, for the repulsion 'bet-ween a linear current and a magnet ; a, b, 



and c depending on the speciBc inductive capacities of the 

 medium for electricity and magnetism. 

 The dimensions of the various units in electricity and magnetism, if written 

 out in full, would contain these coefficients, which are so related that able" \a a 

 velocity. Our knowledge of the significance of this standard velocity is chiefly 

 owino- to Clerk Maxwell, and the author suggested that it shall he called the 

 Maxwell. It appears from the electromagnetic theory of light that it is also the 

 velocity of light. 



The above relation may be written in either of the forms, 



a = v, ~, orb = v,~. 



Accordingly, if we choose to confine our attention to the cases in which c^ja has 

 its unit of value, 



b = ?' [multiplied by a coefficient of dimensions, c-/«, and of unit value] ; 



and if, on the other hand, we confine our attention to the cases in which c'-jb has 

 its unit of value, 



« = y [multiplied by a coefficient of dimensions, c"jb, and of unit value]. 



The first of these assumptions is that the specific inductive capacity for elec- 

 tricity of the medium (usually air) is taken as our unit of specific inductive 

 capacity for electricity, and the second assumption is that the specific inductive 

 capacity for magnetism of the medium is taken as the unit of specific inductive 

 capacity for magnetism. . ,, j i ^ 4. -.• 



Electrical units consistent with the first assumption are called electrostatic 

 units, those consistent with the second assumption are the electromagnetic units. 

 In the dimensional equations of the electrostatic system c'ja disappears aM as 

 replaced by unity, in those of the second system it is c'jb which disappears. This 

 makes the difference between the imperfect dimensional equations of the two 

 svstems, which is therefore only apparent ; and the ratios between the units of 

 each physical quantity, whether estimated electrostatically or electromagnetically, 

 are essentially numerical. ,■,,•,-<. ti 



Units consistent with both assumptions can only be obtained it we use tlie 

 IMaxwell velocity as our unit of velocitv, in which case a, b, and c can all have 

 unit values ; and the central column of the following table is based on this a^ump- 

 tion and is introduced to afibrd a common ground up to which it is sufficient 

 separately to trace from the right and left the numerical relations of the units of 

 the svstems in common use (by the help of the two systems of imperfect dimen- 

 sional equations), in order to arrive at the numerical relations the whole way 

 across. The Maxwell must be our unit of velocity in the central column ; but we are 

 at liberty to choose two other units arbitrarily, and thoy are so selected as to make 

 the unit of time and the unit of x/LM the same in the central column as in the 

 two adioinino- ohm columns. This reduces the numerical relations to their 

 simplest form. The table can easily be extended to include the units ot every 

 other electrical quantity. . n j j? xi. 



The name imtency is suggested for what is too often miscalled a force, or tne 

 intensity of the field, At every station in space there is potency over the magnet- 

 ism that is there present, over the electricity, over the mass, and over the volume 

 occupied (producing buoyancy), if the surrounding medium is excluded from it. 

 There are, therefore, four potencies at each point of space, each being one factor ot 

 a force, the other factor being a quantity of magnetism, of electricity, of mass, or 

 of volume, as the case may be. The author also expressed his hope that the 

 phrases electromotive force and pressure may be discontinued, for what is in fact 

 one factor of an energy, the other factor being a quantity of electricity. Voltage, 

 which has in some degree come into use, was recommended instead. 



