On a new absolute galvanometer

Abstract

IN the ordinary form of galvanometer the current is measured by the ratio of the force it exerts on the needle to the directive force of the earth, the ratio being determined by a measurement of the angle of deflection. The moment of the force with which a unit current acts on the needle may be expressed in a series of the form G1 gl sin IJ + G,g, sin IJ Q,'(IJ) + etc. where G" G 2 are constants depending on the dimensions of the coil, and g" g 2 on those of the suspended apparatus, coil or magnet, as the case may be. Q/ ({}), Q/ ({}) are quantities varying with the deflection. Only in case all the terms after the first may be neglected are the values of the current proportional to the tangents of the deflections. With a single coil this is not the case. By increasing the number of coils and suitabl,V placing them, the magnetic field may be rendered more uniform. In reading the deflection ei ther a di vicled circle or a telescope and scale may be used. With the divided circle the deflection may be as great as 45°, but not more, or else the instrument would not be sensitive to changes in the current. The use of telescope and scale necessitates much smaller deflections. To regulate the strength of the current shunts of small resistance often have to be used; and the proportion of the current throngh the instrument is rendered doubtful. If, instead of placing the plane of the coils parallel with the magnetic meridian, they are placed perpendicular to it, the sum of the force of the current and of the directive force of the earth would influence the magnet.