For experimental work and laboratory demon strations the tangent galvanometer is used. This instrument is shown in the illustration. It con the wires through which the current is sent are joined to the bending posts, which connect with the terminals of the coil. The number of degrees that the needles are deflected under the action of the current may then be read off, showing the strength of the current.
For most kinds of testing and measurement extremely sensitive galvanometers are required. Of these, the reflecting galvanometer, designed by Sir William Thomson, is one of the standard types. It is shown in the illustration. In this instrument a reading is made by the use of a ray of light reflected upon a screen from a mirror attached to the needle so that even the small est motion is shown. The Thomson galva nometer consists of a pair of astatic needles attached by shellac or other adhesive material to a mirror made of very thin microscope glass. This is super seded by a single fibre of raw silk in the cen tre of a coil containing many thousand turns of fine wire. The whole is suitably protected from currents of air by a glass case, and the base is mounted upon leveling screws, so that the hanging needle may be adjusted to swing freely in the centre of the coil. The needle is caused to point lo zero of the scale by a pow erful magnet outside of the case which is ad justable as to direction by a tangent screw, and may be removed to any distance to weaken its effect upon the needle or increase its sensitive sists essentially of a thick strip or wire of copper bent into the form of a circle, from one to two feet in diameter, with a small magnetic needle with pointers of thin glass fibres moving on a graduated circle, at its centre, supplied with a mirror. When the needle is small compared with
the ring, it may be assumed that the needle, in whatever direction it lies, holds the same relative position to the disturbing power of the ring. This being the case, it is easy to •prove that the strengths of currents circulating in the rings are proportional to the tangents of the angles of deviation of the needles. Thus, if the deflec tion caused by one voltaic cell was 45°, and of another 60°, the relative strengths of the currents sent by each would be as the tangent of 45° to the tangent of 60°, viz., as 1 to 1.73. The needle can never be deflected 90°, for as the tangent of 90° is infinitely large, the strength of the de viating current must be infinitely great, a strength manifestly unattainable. The tangent galvanometer can consequently be used to measure very strong currents.
A common er detector galvanometer is an in strument used in ordinary shop work, and for outdoor testing, where a portable instrument is required, and the other forms are too delicate. It contains a large magnetic needle or compass swinging upon a pivot. A small cavity formed in an agate let into the centre of the needle is usually employed to prevent friction in swinging upon the pivot. The coils of wire are placed un derneath the dial bearing the graduations over which the needle swings, and the whole is in closed in a round brass box, with a glass cover over the needle. For convenience, a circuit-clos ing key for admitting current to the coil is often built into the case and permanently connected with the coils. Such a galvanometer is often used in connection with a set of resistances for making measurements of resistance by the Wheatstone bridge (q.v.), and in that case the apparatus is known as a combhiation or portable testing set.