JAMES CLERK MAXWELL 331 



along a wire. The number found differs little from the ratio of the 

 units ; that is, from the velocity of light, which is 300,ocx) kilometers 

 per second. Since the interference experiments made at Geneva by 

 Messrs. Sarasin and De la Rive have shown, as I said above, that in- 

 duction is propagated in air with the same velocity as an electric dis- 

 turbance which follows a conducting wire, we must conclude that the 

 velocity of the induction is the same as that of light, which is a con- 

 firmation of the ideas of Maxwell. 



M. Fizeau had formerly found for the velocity of electricity a num- 

 ber far smaller, about 180,000 kilometers. But there is no contra- 

 diction. The currents used by M. Fizeau, though intermittent, were 

 of small frequency and penetrated to the axis of the wire, while the 

 currents of M. Blondlot, oscillatory and of very short period, remained 

 superficial and were confined to a layer of less than a hundredth of a 

 millimeter in thickness. One may readily suppose the laws of propa- 

 gation are not the same in the two cases. 



Note IL — I have endeavored above to render the explanation of 

 the electrostatic attractions and of the phenomena of induction com- 

 prehensible by means of a simile. Now let us see what Maxwell's 

 idea is of the cause which produces the mutual attractions of currents. 



While the electrostatic attractions are taken to be due to a multi- 

 tude of little springs — that is to say, to the elasticity of the ether — it is 

 supposed to be the living force and inertia of the same fluid which pro- 

 duce the phenomena of induction and electrodynamical effects. 



The complete calculation is far too extended for these pages, and 

 I shall again content myself with a simile. I shall borrow it from a 

 well known instrument — the centrifugal governor. 



The living force of this apparatus is proportional to the square of 

 the angular velocity and to the square of the distance of the balls. 



According to the hypothesis of Maxwell, the ether is in motion in 

 galvanic currents, and its living force is proportional to the square of 

 the intensity of the current, which thus correspond, in the parallel I 

 am endeavoring to establish, to the angular velocity of rotation. 



If we consider two currents in the same direction, the living force, 

 with equal intensity, will be greater the nearer the currents are to one 

 another. If the currents have opposite directions, the living force will 

 be greater the farther they are apart. 



In order to increase the angular velocity of the regulator and con- 



