[Illustration: Fig. 515.--Bunnell tangent galvanometer. This instrument is mounted on a circular hard rubber base, 7-3/8 inches diameter, provided with levelling screws and anchoring points. The galvanometer consists of a magnetized needle 7/8 inch in length, suspended at the center of a rubber ring six inches in diameter, containing the coils. There are five coils of 0, 1, 10, 50 and 150 ohms resistance. The first is a stout copper band of inappreciable resistance; the others are of different sized copper wires, carefully insulated. Five terminals are provided, marked, respectively, 0, 1, 10, 50 and 150. The ends of the coils are so arranged that the plug inserted at the terminal marked 50 puts in circuit all the coils; marked at the terminal 50--all except the 150 ohm coil; and so on, till at the zero terminal only the copper band is in circuit. Fixed to the needle, which is balanced on jewel and point, is an aluminum pointer at right angles, extending across a five inch dial immediately beneath. One side of the dial is divided into degrees; on the other side, the graduations correspond to the tangent of the angles of deflection.] Ques. How may correct readings be obtained? Ans. The instrument may be calibrated, that is, it may be ascertained by special measurements, or by comparison with a standard instrument, the amounts of deflection corresponding to particular current strengths. Thus, if it be once known that a deflection of 32° on a particular galvanometer is produced by a current of 1/100 of an ampere, then a current of that strength will _always_ produce on that instrument the same deflection, unless from any accident the torsion force or the intensity of the magnetic field be altered. [Illustration: Fig. 516.--Tangent galvanometer. It consists of a short magnetic needle suspended at the center of a coil of large diameter and small cross section. In practice, the diameter of the coil is about 17 times the length of the needle. If the instrument be so placed that, when there is no current in the coil, the suspended magnet lies in the plane of the coil, that is, if the plane of the coil be set in the magnetic meridian, then _the current passing through the coil is proportional to the tangent of the angle by which the magnet is deflected from the plane of the coil_, or zero position--hence the name: "tangent galvanometer."] The Tangent Galvanometer.--It is not possible to construct a galvanometer in which the _angle_ (as measured in degrees of arc) through which the needle is deflected is proportional throughout its whole range to the strength of the current. But it is possible to construct a very simple galvanometer in which the _tangent of the angle of deflection_ shall be accurately proportional to the strength of the current. [Illustration: Fig. 517.--Horizontal section through middle of tangent galvanometer, showing magnetic whirls around the coil and corresponding deflection of needle.]