Ans. It consists of a small piece of soft iron or _vane_ mounted on a shaft that is pivoted a little off the center of a coil as shown in fig. 634. The principle upon which the instrument works is that a piece of soft iron placed in a magnetic field and free to move will move into such position as to conduct the maximum number of lines of force. The current to be measured is passed around the coil producing a magnetic field through the center of the coil. The magnetic field inside the coil is strongest near the inner edge, hence, the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible. A pointer, attached to the vane shaft moves over a graduated dial. [Illustration: Fig. 634.--Magnetic vane instrument. A soft iron vane, eccentrically pivoted within a coil carrying the current to be measured, is attracted toward the position where it will conduct the greatest number of magnetic lines of force against the restraining force of a spring or equivalent.] Ques. Describe an inclined coil instrument. Ans. As shown in fig. 635, a coil carrying the current, is mounted at an angle to a shaft to which is attached a pointer. A bundle of iron strips is mounted on the shaft. A spring restrains the shaft and holds the pointer at the zero position when no current is flowing. When a current is passed through the coil, the iron tends to take up a position with its longest sides parallel to the lines of force, which results in the shaft being rotated and the pointer moved on the dial, the amount of movement depending upon the strength of the current in the coil. The coils for large sizes are generally wound with a few turns of flat insulated copper ribbon. The instruments are adapted to either direct or alternating currents but are recommended for alternating currents. [Illustration: Fig. 635.--Thompson inclined coil ammeter. It is constructed on the magnetic vane principle in which an iron vane is attracted by the magnetic field due to the coil, so as to turn itself parallel with the axis of the coil, the latter being inclined with respect to the axis of the vane. The voltmeter of this type has a similarly placed stationary coil, but in place of the iron vane, is provided with a moving coil in series with the other coil. The restraining force in each case being that due to springs. Figs. 636 and 637 show the actual construction of inclined coil instruments.]