Hot
wire instruments work on the principle that the length of
a wire is increased by the heat produced in it when current flows through it.
Hot wire Instrument construction and working
The main parts of the instrument are as shown.
It consists of a platinum-iridium wire streched
between a fixed end B and the tension adjusting screw at A. Platinum-iridium wire
is used because of its ability to withstand high temperatures without getting
deteriorated by oxidation.
When current passes through AB, it expands due to the
heating effect of the current (the wire molecules vibrate more due to heat
energy and distance between them increases). A sag is produced in AB which
produces a sag in wire CD. The wire CD is attached to AB at the centre. This sag in CD is taken up by the silk fibre
which after passing around the pulley is attached to a spring S.
As the silk
thread is pulled by the S, the pulley moves, thereby deflecting the pointer.
The deflection produced is directly proportional to
the wire extention which is directly proportional to the heat produced in the
wire.
The heat produced in the wire is given by , H= Irms2 Rt
So, deflecting torque Td is proportional to
the square of rms value of the current flowing through the wire i.e
Td ∝ Irms2
Now if the instrument uses spring control, then controlling torque is, Tc =KÏ´
When pointer is in equilibrium, Td
= Tc
Ï´ ∝ Irms2
So, the hot
wire instruments have a non-uniform scale.
Hot wire instruments read rms value of the current and the reading is independent
of the waveform and frequency.
Damping
system
A thin light aluminium disc is attached to the pulley
such that it rotates with the pointer, between the poles of a permanent magnet M. Thus, eddy currents
are produced which tend to oppose the the motion of the disc, providing
necessary damping.
These instruments are used as ammeters and the
underlying principle for hot wire ammeters is same. Hot wire ammeter measures rms value of currents .
