Popular Science Monthly/Volume 48/March 1896/The Velocity of Electricity
|THE VELOCITY OF ELECTRICITY.|
THE determination of the velocity of electricity has been the ambition of many physicists; yet at present it is generally conceded that the velocity may be anywhere from the fraction of an inch per hour to millions of miles per second.
By the popular use of the words "current of electricity" we have grown to think of a fluid flowing through a wire, yet we do not know that there is any such fluid, and consequently we can hardly say that it has a velocity. However, the attempt was made, some years ago, to find the velocity of electricity, considering it as a fluid, by finding the time taken for a signal sent from the Harvard Observatory, Cambridge, to reach St. Louis. The distance between the two places was known, and the gentlemen who conducted the experiment easily found what they supposed was the velocity of electricity by dividing the distance by the time. To understand why this velocity is not really the velocity of electricity, as well as to understand the direction in which physical research is now directed, we must consider what we really do know about electricity.
When the two poles of a battery are connected by a wire we say a current of electricity is flowing through the wire. The evidences of the so-called current are two: in the first place, the wire is heated; and in the second, a magnetic force is set up in the neighborhood of the wire. It is this magnetic force that interests us, and we must get as clear an idea of it as possible. We find by experiment that in the neighborhood of the wire a compass needle is turned from its customary north-and-south position. The force which so turns the needle we call the magnetic force, and the direction in which the north end of the needle is pulled we call the direction of the magnetic force.
The adjacent figure is from a photograph of iron filings spread over a plate through which a wire is thrust, perpendicular to the plate. A current is passing through the wire whose cross-section 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 than before, because the magnetic force is stronger the nearer we get to a. Finally, let b touch a. We have then really only one wire, since the wires touch and form one conductor. Of course, now the magnetic force can not send a current through our wire, as it did through b, in the opposite direction to the current from the battery; but it tries to do so, opposing the current from the battery. Consequently, the current that the battery gives is very weak for a short time, but only for a short time, because this opposing current lasts only while this magnetic force is growing. This phenomenon evidently holds for every wire through which we try to start a current.
The instruments used in the experiments between Cambridge and St. Louis could not work unless the current from the battery had reached its full strength, so that the time the experimenters found between the sending of the signal and its receipt was not the time it took for electricity to pass from Cambridge to St. Louis, but was the time it took for the current they used to grow to its full strength.
We know that the strength of the magnetic force around a wire depends upon the size and form of the figures into which the wire is bent, and the time it takes for a current through the wire to reach its full strength depends upon the strength of the magnetic force. Therefore we should expect that, by using different instruments on which wire is wound in different forms and sizes, we ought to find that it takes different times to send a signal from one place to another. This has been tried and found true. In fact, it was in this way that it was first proved that the velocity found between Cambridge and St. Louis was not really the velocity of electricity.
It would seem, then, that in our search for connecting links between electricity and light we had better turn our attention to what goes on in the space around a wire carrying a "current" rather than to confine ourselves to what takes place in the wire.