ii 4 THE POPULAR SCIENCE MONTHLY 



dred miles away, he might feel the enormous importance of the work 

 that he had accomplished to the world at large, but I much doubt 

 whether he would have felt a more lively satisfaction than wheD he first 

 saw the electric spark jump between the ends of his coil surrounding 

 the magnet. The chief question which interested Faraday during the 

 greater part of his life was the question of action at a distance. How 

 can the motion of a magnet or what amounts to the same thing, the 

 change of current in one coil, cause a current to flow in another coil in 

 a different place. This he explained by some change in the medium 

 surrounding the coils, but it was reserved for another to give the com- 

 plete explanation. This was Clerk-Maxwell, of whom I spoke at the 

 beginning, who was the chief expounder of Faraday's views, to which 

 he added and which he made precise by his wonderful ability to put 

 them into mathematical form. It was Maxwell's brilliant idea that the 

 medium which is affected by the presence of an electric current is noth- 

 ing else than the ether which is supposed to convey the waves of light, 

 and it was a result of his theory that the electric and magnetic actions 

 are transmitted through the ether in the form of waves. Not only this, 

 but he showed that the velocity of these electromagnetic waves would 

 be exactly that of light. He then made the startling generalization that 

 light waves possess all the characteristics of electromagnetic waves, and 

 in fact differ from them in no essential way. These ideas of Maxwell, 

 first put forward nearly fifty years ago, have now found universal ac- 

 ceptance, and the whole world believes that light is an electromagnetic 

 phenomenon. But it was a long time before Maxwell's ideas were ac- 

 cepted, especially on the continent of Europe. For Maxwell died in 

 1879 without ever having demonstrated experimentally that electric 

 and magnetic effects are propagated in waves. This was reserved for 

 another, the German Heinrich Hertz, who in 1887-88 was able to dem- 

 onstrate the propagation of such effects with a definite velocity, which 

 was found to be indeed the same as that of light. 



Hertz's first experiment by which this discovery was made was so 

 simple that it may be described. If we have two metal spheres near 

 enough together a spark will pass between them if they are electrified, 

 but only if the electrical potential or pressure is different for the two 

 balls. If the two balls form the ends of a circuit of wire, the whole may 

 be electrified as strongly as we please with never a sign of a spark pass- 

 ing between the balls, for the whole conductor has the same potential. 

 But Hertz found that if the wire, in the form of a rectangular circuit, 

 was connected with one of the ends of an induction coil producing 

 sparks, each time that a spark passed from the induction coil a spark 

 also passed between the balls of the rectangle. This was always sup- 

 posing that the connection was made to a point of the rectangle not 

 symmetrically placed with respect to the balls, and Hertz explained the 



