Construction of Hot-Wire Instruments

Construction of Hot-Wire Instruments Points : Construction of Hot-Wire Instruments The constructional details of hot wire, double sag type instrument. The hot wire “W” of 0.1mm diameter is usually made of platinum iridium so that it may withstand high temperatures without deterioration on account of oxidation. This hot wire “W” is stretched between a fixed point “B” and a certain adjustment point “A”. Another wire “WI” of phosphor bronze is attached to wire “W” at point “C” and other end of it is fixed at point “D”. A fine silk thread “G” is passes round a pulley “E” and the other end of it is fixed to spring “S”. A light pointer “P” and thin aluminium disc “L” are carried by the spindle upon which the pulley “E” is also mounted. When the current to be measured is passed through the wire “W”, it expands due to heating effect of the current and slack is produced in it. The slack in “W” wire causes a slack in “W”, which is taken up by spring “S” through silk thread “G” and the moving system is rotated due to motion of pulley “E”. Thus the pointer “P” moves over a calibration scale. The, hot-wire instruments are the oldest form of the thermal instruments, which were used for the measurement of high frequency current. Eddy current damping is provided in the instrument by means of an aluminium disc “U” rotating between the poles of a permanent magnet.