432 Prof. J. D. Everett on the Dimensions of a 



But 



Current = M*L$T _2 ; 

 therefore 



Pole=M*L* 3 



■which is Maxwell's result. This proof was supplied to me by 

 Professor Larmor. Clausius alludes to such a proof as having 

 been given by Maxwell, and objects to it on the ground that 

 the force which a current exerts upon a pole is not an electro- 

 statical but an electrodynamical force. But, inasmuch as this 

 objection applies equally to all definitions of " the unit pole 

 in the electrostatic system/' not excepting that offered by 

 Clausius himself, it cannot be admitted as valid when we are 

 discussing the merits of one as against another. 

 Again, we may employ 



II. The magneto-electric law. which determines the electro- 

 motive force produced by moving a conductor in a magnetic 

 field. This law, when stated without any assumption as to 

 units, is that the electromotive force is directly as the length 

 of the conductor, the velocity of its motion resolved in a cer- 

 tain direction, and the intensity of the field. Hence, bearing 

 in mind that the intensity of the field due to a single pole is 

 directly as the strength of the pole and inversely as the square 

 of the distance, we must have, in every system, 



E1 force m0the } =h X Length X YelocIt . v x Pole-r-(Distance) 2 (2 

 = k, x L x LT" 1 x Pole x L -2 

 = Polex/sT -1 . 

 If we detine our unit pole by the condition k 2 =l, we have 

 Pole = Electromotive force x T 

 = M'L^T-'x T 

 = M<IA 



This mode of obtaining Maxwell's result was in substance 

 supplied to me by Professor Fitzgerald. So far we have no 

 discrepancy. 



On the other hand, we may employ with Clausius, 



III. The law of (he magnetic (■hell, which asserts the equiva- 

 lence of a current to a magnet. Taking the simplest case — 

 that of a current in a plane circuit — the law of nature is that 

 the moment of the equivalent magnet is jointly proportional 

 to the strength of the current and the area of the circuit. 

 Hence, since the moment of the magnet is the product of the 



