The working
of a hot wire type measuring instrument is based on the heating effect of the
current. These type of instruments are not much in use nowadays because if
their instability in working. Nevertheless, they paved the way for further
advancements of their own type.
CONSTRUCTION –
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| HOT WIRE TYPE INSTRUMENT |
The adjacent figure shows the constructional features of a hot wire type instrument. It
consists of a platinum-iridium wire AB stretched between a fixed end B and
tension adjusting screw at A. The platinum-iridium wire used here can withstand
oxidation at high temperatures. Now, a phosphor bronze wire CD is attached or
hooked up to the centre of AB. This wire CD is further attached to a silk fiber
which after passing round the pulley is attached to a spring S.
HOW DOES IT WORK?
When the
current is passed through the fine wire AB, it gets heated up and expands. The
sag of the wire is magnified and the expansion is taken up by the spring. This
causes the pulley to rotate and the pointer to deflect, indicating the value of
the current.
The
expansion is proportional to the heating effect of the current and hence to the
square of the rms value of the current.
DEFLECTION –
The
deflection of the pointer is proportional to the expansion of AB which in turn
is proportional to I2.
Hence, $deflection\propto
{{I}^{2}}$
As, spring
control is used, ${{T}_{c}}\propto \theta $
\[\therefore
\theta \propto {{I}^{2}}\]
These
instruments read the rms value of current and their readings are independent of
its form and frequency.
DAMPING –
Damping is
provided by eddy currents produced in the aluminium disc. This thin disc is
attached to the pulley such that its edge moves between the poles of the
permanent magnet M.
ADVANTAGES –
- Suited for both AC and DC work.
- Readings are independent of waveform and frequency.
- Unaffected by stray fields.
DISADVANTAGES –
- Sluggish response due to temperature dependent working.
- High power consumption.
- Mechanical shocks.
- They are fragile.
- Inability to withstand overloads.
