we see at A. The filings arrange themselves in the direction of the magnetic force, which we see to be in concentric circles around the wire. According to theory, this magnetic force extends to an indefinite distance. Near the wire the force is very strong, but grows weaker, losing strength with distance until it finally becomes imperceptible. Before we connected the wire to our battery this force did not exist. Where did it come into existence first—near the wire or far from the wire—or did it come suddenly everywhere at once? The late Dr. Hertz performed some wonderful experiments in this connection, in which he showed that the magnetic force comes into existence first near the wire and then makes its appearance a little farther off, and so on till all the surrounding space is filled with the force. Dr. Hertz's experiments seem to indicate, moreover, that the rate at which this magnetic force travels out from the wire is perfectly definite; in a word, that it travels with the velocity of light. You can picture this to yourself by imagining the wire suddenly to emit light just as we connect it to the battery; then the light and the magnetic force will both reach any point at which you may place your eye at exactly the same time.
This is the theory, and a very interesting one it is, but it does not stop here, for not only does this magnetic force travel with the velocity of light, but it has been proved by experiment that it can be reflected, refracted, and brought to a focus.
Many observers are now engaged in reproducing and extending Dr. Hertz's experiments, and many brilliant results are to be expected.
Now, to see why the velocity determined between Cambridge and St. Louis was not the velocity of electricity, we must go back to some fundamental principles which at first sight seem to have no connection with the question.
Just as a current of electricity produces magnetic force around the wire carrying the current, so does magnetic force around a wire produce a current of electricity, no matter how the magnetic force may be produced; but, whereas the current produces a magnetic force that lasts as long as the current flows, the magnetic force produces a current only while the force is growing, so to speak—while it is being made. If, now, we have a wire, a, so arranged that a current of electricity may be sent through it from a battery by pressing a key, and another wire, b, parallel to a, connected with an instrument for detecting a current of electricity, when we press the key we shall get magnetic force around a, extending as it grows to b. While this magnetic force is growing, we find there is a current through b in the opposite direction to the current through a. Now let us move b up closer to a. We get, of course, the same effect, only the current in b is stronger
