Hot - wire Instruments
Hot - wire Instruments
are based on heating effect of current. It consists of platinum iridium
wire which is stretched between a fixed end and tension adjusting screw. When
current is passed through the stretched wire, it expands and a sag is
produced and slack in phosphor bronze wire which is attached on the middle of
the stretched wire. This slack is taken up by the silk fiber and is
passing around the pulley. The pulley is attached to the spring. As
the silk thread is pulled by spring, the pulley moves by deflecting the pointer.
The deflection of the pointer is proportional to the points of the stretched
wire which is itself proportional to I2. Hence deflection is α I2.
If spring control is used then Tc α Θ and Θ α I2
These instruments work with rms value
of current their readings are independent from its form and frequency.
Damping
A light aluminum disc is attached to
the pulley and its edges moves between the poles of a permanent magnet
and eddy current is produced in it and provides damping. These
instruments are designed as an ammeter but it can also be used be used as
voltmeters by connecting high value resistance in series on its disc.
Advantages
These instruments can be used on direct
current.
Their readings are independent.
Frequency
is uniform
These are unaffected by stray losses.
Disadvantages
These instruments take time to heat up the coil
and then deflects the pointer.
These instruments consume high power
as compared to moving coil instruments.
The zero position needs adjustment.
These instruments are breakable.
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