Fig. 514.--Central Scientific Co. tangent galvanometer. A 9 inch brass ring is mounted on a mahogany base which rotates on a tripod provided with levelling screws. The needle has an aluminum pointer and jewelled bearing. The winding consists of 300 turns of magnet wire so connected to the plugs in front that 20, 40, 80, or 160 turns or any combination of these numbers may be used. For heavy currents a band of copper is used by connecting to the extra pair of binding posts in the rear of the instrument.
Ques. Why does the instrument not give accurate readings for large deflections?
Ans. The needles are not so advantageously acted upon by the current, since the poles are no longer within the coils, but protrude at the side. Moreover, the needles being oblique to the force acting on them, part only of the force is turning them against the directive force of the fibre; the other part is uselessly pulling or pushing them along their length.
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.
