Page:Popular Science Monthly Volume 48.djvu/759

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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 PSM V48 D759 Current induced magnetic force and iron filings.jpg 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