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.
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.
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.
