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.
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.
Figs. 636 and 637.--Thompson inclined coil portable indicating instruments. [Fig. 636], ammeter type P interior; [fig. 637], wattmeter, type P, interior. These instruments, though primarily designed for use on alternating current circuits, may also be used on direct current circuits, by making reversed readings and taking the mean as the true indication. The voltmeters and wattmeters are constructed on the dynamometer principle and the ammeters, on the magnetic vane principle. The voltmeters and wattmeters are provided with a contact key which may be locked in position, enabling the instruments to be left constantly in circuit. The movements of the pointer are damped by means of an air vane; there is also a friction damping device operated by a small button to check excessive oscillations of the pointer. The inclined coil instruments are so designed that the torque is sufficiently high to insure the pointer assuming a definite position with each change in current value.
Ques. What is the principle of the hot wire instrument?