Ans. As shown in fig. 638, a wire AX, of non-oxidizable metal, of high resistance and low temperature coefficient, passes over a pulley B mounted on the shaft C. The ends of the wire are attached to the plate E at its ends F and G, the wire being insulated from the plate at G. A spring H holds the wire in tension and takes up the slack due to the expansion caused by the heating of the wire when a current passes through it. The current flows only in the portion of the wire marked A, between the plate E and the pulley B up to the point K where the connection is shown. When a current flows through the wire A, the spring takes up the slack, pulls A around B, and causes B to rotate upon its shaft C. It is clear, that a pointer attached to C, would indicate on a scale the movement of B and C, but as this movement is very slight, a magnifying device will be required. This device consists of a forked rod L, rigidly attached to the shaft C, and carrying at its lower end a silk fibre fastened to the fork and passing around a pulley M, to which a pointer N is attached. For direct current measurements only an electromagnetic system is used. [Illustration: Fig. 638.--Diagram showing principle and construction of the Whitney hot wire instruments. The action of instruments of this type depends on the heating of a wire by the passage of a current causing the wire to lengthen. This elongation is magnified by suitable mechanism and transmitted to the pointer of the instrument.] Ques. What is the principle of electrostatic instruments? Ans. The action of these instruments depends upon the fact that two conductors attract one another when any difference of electric pressure exists between them. If one be delicately suspended so as to be free to move, it will approach the other. [Illustration: Fig. 639.--Kelvin electrostatic voltmeter; a form of instrument designed for measuring high pressures up to 200,000 volts. The instrument, as illustrated, consists of fixed and movable vanes with terminals connecting with each. These vanes which act as condensers take charges proportional to the potential difference between them, resulting in a certain attraction which tends to rotate the movable disc against the restraining force of gravity. In the figure _aa_ and _b_ are two fixed vanes and _c_ a movable vane, carrying a pointer and having a proper weight at its lower end.] Ques. Describe the Kelvin electrostatic voltmeter. Ans. A simple form consists, as shown in fig. 639, of a metal case containing a pair of highly insulated plates, between which a delicately mounted paddle shaped needle is free to move. When the needle is connected to one side of a circuit and the stationary plates to the other side, the needle is attracted and moves between them as indicated by the pointer. Adjusting screws at the lower end of the needle allow it to be balanced so that its center of gravity is somewhat below the center of suspension. Gravity then is the restraining force.