Electricity Measured.
As important as the measurements of the astronomer are those of the electrician. It was as recently as 1819 that Oersted, a Danish physicist, published a discovery which became a foundation stone of electrical engineering, and upon which rises the art of electrical measurement. He observed that when an electric current is passing through a wire, a nearby magnetic needle tends to place itself at right angles to the wire, the deflection varying with the strength of the current. When instead of a wire, a coil, duly insulated, is employed to carry the current, effects much more decided are displayed. At first current-measurers, or galvanometers, employed simple compass needles; these proved to be unsatisfactory. They were affected by the variations which occur in the intensity of the earth’s magnetism; and no matter how carefully a needle was made, it varied in strength from week to week, from year to year; again, a current might be so strong as to create magnetism overwhelming in comparison with that of the earth, and quite beyond the measuring power of a compass needle. A galvanometer on a plan due to Professor James Clerk Maxwell, employs a permanent magnet, or an electro-magnet, which is stationary, between the poles of which may freely turn a coil bearing the current to be measured. This current in the case of an ocean cable is so weak that no other means of indication will serve. Lord Kelvin’s recording apparatus for such a cable is a galvanometer on this principle. In order to concentrate the lines of magnetic force on the vertical sides of the coil, a piece of soft iron, D, is fixed between the poles of the magnet. This iron becomes magnetized by induction, so as to produce a very powerful field of force, in the minute spaces between it and the two magnetic poles, through which spaces the vertical sides of the coil are free to move. Instruments of this kind, developed by D’Arsonval, are known by his name.