I confess that in the course of my life since my Berlin days I afforded considerable amusement to my friends whenever I tried to explain to them the Hertzian experiments by appealing to what I considered a well-known action of the tuning-fork. Some of them objected on the ground that this action is just as difficult to understand as the action of the Hertzian oscillator. I met this objection by describing to them the action of the reed in Serbian bagpipes which I watched when I was a boy, and understood sufficiently well to recognize later in the action of the tuning-fork a performance similar to that of the reed in the Serbian bagpipes. I understood the tuning-fork because I understood the reed. An educated American, I claimed, should find no difficulty in understanding the action of a simple mechanism which an uneducated Serbian peasant boy understood. The Hertzian electrical oscillator, described above, acts like the tuning-fork. The process of pulling apart the two charges, the positive from the negative, and of forcing them to the surface of the spheres by the action of the electrical machine, is a parallel to the process of deflecting by the pressure of our fingers the prongs of the tuning-fork from their normal position. In one case the tuning-fork by its elastic stiffness reacts against the bending of the prongs. In the electrical case the electrical lines of force in the space surrounding the oscillator react against the action of the machine which crowds them into this space by stretching and compressing them. This is the picture of the action of the lines of force which Faraday gave me on the island of Arran, but I did not understand it. In the picture the dotted curves are the Faraday lines of force and the arrow-heads indicate the direction of the electrical force. The Hertzian oscillator, and what Helmholtz had told me before, made Faraday’s language and thoughts much more intelligible. The work done by the machine is all expended upon the stretching and compressing of the lines of force into the space outside of the spheres, that is, upon the _electrification of that space_.