In this topic, you study the construction and working of hot wire instruments. And also the advantages, disadvantages and applications of the instrument.
The principle of working of these instruments is “whenever the Current is passed through the metallic wire, it increases in its length (principle of expansion Of metals) due to heating effect”.
The diagram, Fig. 23.10 shows the simple construction of a hot wire instrument. A wire of platinum-irridium of about 16 cm in length is used. The wires is connected at both A and B contacts one contact is fixed and other is adjustable with a screw. A fine phospher bronze wire is attached with the platinum irridium wire at a point C. The other end of the wire is fixed with the point D to the base of the instrument. It should be clearly understood that the temperature coefficient of expansion of the frame on which the hot wire is mounted and the hot wire, be made of an exact match to avoid the error in exhibiting the leading and uneven expansion between wire and its fixture. A fine silk thread is attached at a point, this thread is passed over a pulley and through a spring. A permanent magnet and alutninium disc is also provided for eddy current damping.
Fig. 23.10. Hot wire instrument.
Whenever the current is passed through the platinum irridium wire it expands in length. The increase in length is proportional to the square of current (H a 12). The sag of the hot wire will result the slack in the phospher bronze wire. The silk thread which is provided with a spring, will be pulled by the spring and the pulley through which it passes, will move. Now due to this tension the pointer will move over the calibrated scale. As the aluminium disc moves in between the permanent magnet, the eddy currents are induced in the aluminium disc and the eddy current damping is obtained. Generally gravity control is used in this instrument. Following are the advantages and disadvantages of the instrument.
Advantages
(i) The expansion is proportional to the square of current, so the instrument can be used on A.C. and D.C. both.
(ii) There is no induction effect so it can be effectively used for higher frequencies.
Disadvantages
(i) The readings are not obtained quickly.
(ii) On higher current the wire may burn out.
(iii) Zero position is to be adjusted frequently.
(iv) Power consumption is relatively high.
(v) The scale is not uniform.
Uses. It is used as an ammeter, voltmeter. These are appreciably used for higher frequencies.