[Illustration: 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? Ans. Its action depends upon the heating of a conductor by the current flowing through it, causing it to expand and move an index needle or pointer, the movements of which, by calibration, are made to correspond to the pressure differences producing the actuating currents. Ques. What are the characteristics of hot wire instruments? Ans. Voltmeters of this type are not affected by magnetic fields, and as their self-induction is small, they can be used on either direct or alternating currents; but they possess certain serious defects: they consume more current than the other types; cannot be constructed for small readings; are liable to burn out on accidental overloads; and are somewhat vague in the readings near the zero point and are sometimes inaccurate in the upper part of the scale. Ques. Describe the construction and operation of the Whitney hot wire instruments.