[Illustration: Fig. 620. Moving iron type instrument. The essential parts are: N, soft iron needle; C, coil; M, permanent magnet; P, pointer; S, scale. Current passing through the coil acts on the needle, causing it to turn against the restraining force due to the influence of the permanent magnet.] [Illustration: Fig. 621.--Moving coil type instrument. The essential parts are: A, spiral spring; C, coil; K, soft iron core; M, permanent magnet; P, pointer; S, scale. Current passing through the coil causes the moving system to turn against the restraining force due to the influence of the permanent magnet.] Ques. Describe a moving coil instrument. Ans. This type of instrument is shown in fig. 621. It consists of a moving coil C, to which is attached the pointer, and which is pivoted between the poles of a permanent magnet M. The coil moves between these poles and a fixed soft iron core K, and is held in the normal position by two spiral springs A, above and below the core. The springs also serve to make electrical connection with the coil C. When a current passes through the coil, magnetic lines are set up in it which are at an angle to those passing from one pole of the permanent magnet to the other. The lines of force, which formerly passed from one pole of the magnet to the other by straight lines or by short curved ones, are "stretched" on account of the field produced by the current in the coil, and, in trying to shorten themselves, tend to twist the coil through an angle. This tendency to move is resisted by the two spiral springs, hence the coil moves until equilibrium is established between the two opposing forces. The amount of deflection of the pointer depends, either upon the current strength, or the voltage according to the winding of the coil. [Illustration: Fig. 622.--Keystone voltmeter; view showing the moving element being withdrawn by loosening one screw. These instruments are constructed on the d'Arsonval system, the moving element being shown in detail in figs. 623 and 624. The entire system is mounted upon a solid metal base plate. The permanent field magnet is made of a single piece of magnet steel, and the pole pieces are of soft steel, permanently secured to the magnet in order that the distribution of the magnetic flux will not be changed by removal and replacement of the pole piece. Accordingly the moving mechanism is mounted separately from the field, so that it can be readily lifted from the field without removing the pole pieces. The function of the core is to secure a uniform field. The moving coil is wound upon a form of aluminum, which serves the purpose of damping by the generation of eddy currents. The winding of the coil is of fine copper wire, to which current is conveyed by means of the controlling springs and which, in the case of a voltmeter, is connected in series with a resistance, and in the case of an ammeter, across the terminals of a shunt.]