D.C. Measuring Instruments

This is the instrument most generally used in circuit work. It consists of a rectangular coil of a number of turns of fine copper wire which is pivoted on jewelled bearings between the poles of a permanent horseshoe magnet (Fig. 25). The electromagnetic field produced around the coil when a current is passed through it is increased by the use of a soft-iron cylindrical core. The coil attempts to turn so that its magnetic axis will line up with the field of the horseshoe magnet; a spiral spring opposes this movement, and the coil will set in some intermediate position. A pointer is attached to the coil and moves over a scale marked in divisions.