plane. On sending the current through the coil, a magnetic field is produced in the centre, the lines of force of which run east-west, that is to say, at right angles to the lines of force representing the horizontal component of terrestrial magnetism. The needle is now under the influence of two forces, one due to the earth trying to keep it in a north-south position, and the other due to the current in the coil trying to place it east-west. The true position it will actually adopt will be a compromise between these two tendencies. If the influence of the coil is equally strong with that of the earth, the needle will point north-west-south-east; it will have a deflection of 45°. If the current is made weaker, the deflection will diminish; if made stronger, it will increase. In this way, by observing the deflection of the needle, we can determine what is the strength of the current flowing through the coil. An instrument of this kind is called a "tangent galvanometer," the term "tangent" arising from the fact that the numerical ratio of the two forces acting at right angles is equal to the geometrical tangent of the angle of deflection.
If we were quite certain of the value of the horizontal component H of terrestrial magnete-
