millionth of a second is no longer a short interval. But they would be of little use if we were not able to compare them at that distance of about ten metres which we have proposed to ourselves. The means employed for this purpose are very simple. We fix a conductor—for instance, a straight metallic wire, having a slight interruption at one point—at the place where we desire to perceive the signal. When the electrical field is rapidly varied, a spark appears in the conductor.
The means of observation could be pointed out only by experiment. Theoretically it was hard to imagine it. The sparks are, in fact, microscopic, being hardly a hundredth of a millimetre long, and they continue less than a millionth of a second. It is extremely hard to conceive them as visible. Yet they can be seen, in a dark room and by an eye at rest. On so light a thread is hung the success of our undertaking. We had in the beginning a number of questions to answer. Under what conditions are the vibrations strongest? We must try to secure those conditions. What form should the conductor have? The phenomena will vary as we use straight or bent wires, or conductors of other forms. The form being determined upon, of what size should our conductor be? This is not a matter of indifference, for we shall see that we can not study all the vibrations with the same conductor. There are relations between the two elements like the phenomenon of resonance in acoustics. Lastly, in how many different positions can we arrange this conductor? We shall see the sparks at times increase in intensity, or become weaker, or disappear, I can not enter into these details; they are simply accessory to the theory as a whole. They are of importance only to the operator, and are simply properties of his instrument.
What the experimenter will educe from his process will depend on his knowledge of his means of action. The study of the instrument and the answers to the questions I have just mentioned therefore formed the most considerable part of my labor. This task having been disposed of, the solution of the problem was before me.
A physicist, given a number of diapasons and resonators, will find no difficulty in demonstrating that sound is not propagated instantaneously, even in the restricted space of a room. Having set the diapason in vibration, he goes with his resonator to different parts of the room and observes the intensity of the sound. He perceives that it becomes weak in some places, and infers from this that each vibration is annulled by another of later origin, which has reached the spot by a shorter route. If less time is taken in traversing the shorter road, propagation is not instantaneous, and the question is answered. But our physicist
will then show us that the points of silence succeed one another
