Pole in the Electrostatic System of Units. 359 



subject : the letter k is intended for the gravitation-constant 

 as determined by the Cavendish experiment. I am not aware 

 whether the question of the possible dependence of this con- 

 stant on the optical density of the medium surrounding the 

 attracting masses has ever been considered; but I feel sure 

 that a direct experimental attack on this question would not 

 be uninteresting, and it might lead to important results.) 



We now come to the (Ersted- Ampere discovery of the con- 

 nexion between m and e — the form of the connexion being 

 that an electric current flowing in a closed circuit can pro- 

 duce a magnetic potential, and therefore of course can act on 

 magnets, precisely as if it itself were a magnet of a certain 

 strength and form. The potential so caused at any point in 

 air is found to be simply proportional to the strength of the 

 current and to the solid angle which the circuit subtends as 

 seen from that point ; or, in other words, the moment of the 

 magnet which is equivalent to the current is simply pro- 

 portional to the strength of the current and to the area of the 

 contour round which it flows. 



The unit of current most simply and directly applicable to 

 these electromagnetic phenomena is not the old electrostatic 

 unit at all, but a new unit which may be defined in many 

 ways — as, for instance, these : — 



The electromagnetic unit of current is that which produces 

 unit magnetic potential at a point whence its circuit 

 subtends unit solid angle; 

 It is also that which produces unit magnetic intensity, in a 

 given direction, at a point whence the solid angle sub- 

 tended by its circuit is changing at unit rate, per unit 

 displacement, in that direction; 

 And, again, it is that current which when flowing round a 

 contour of unit area is equivalent to a magnet of unit 

 moment, — 

 all these statements being derived directly from the unit mag- 

 netic pole thus : — 



Unit magnetic potential is defined to exist wherever a soli- 

 tary and stationary unit pole would possess unit energy ; 

 Unit magnetic intensity exists wherever unit pole would 



experience unit force ; and 

 Unit magnetic moment is that possessed by two unit poles 

 of opposite sign rigidly connected by a bar of unit length. 

 (The connexion between the old electrostatic unit and this 

 new electrical unit thus magnetically defined may be ex- 

 pressed, if I am not mistaken, by saying that a ring charged 

 with the electrostatic unit of electricity would have to revolve 

 in its own plane with an angular velocity of about 3 x 10 10 



