Specifications

104
rates welded together. The metal strip expands when heated and contacts when cooled. One
metal in the bimetal strip has a high rate of expansion when heated and the other has a low rate.
When a bimetal strip is heated both the metals in the strip expand but the one at the bottom as
shown in Fig.9.7, with a high rate of expansion, expands faster and forces the upper half to curl
up or bend away from the contact point. The strip curls or bends enough to break the contact,
i.e. opening the circuit. As the strip cools, it straightens and restores contact with the stationary
point. The bending of the bimetal strip on heating is towards the side that has smaller expansion
rate. By adjusting the size of the gap between the strip and the contact, you control the
temperature. A simplified sketch of Adjustable Thermostat is shown in Fig.9.8
Working: Automatic Irons are fitted with a thermostatic switch to regulate the heat to a specific
predetermined value. The thermostatic switch disconnects the supply when the predetermined
value is reached and reconnects the supply when iron cools down. A turning knob with a dial just
below the handle marked as a rayon, cotton, silk, wool, etc., can be operated to select the
preset temperature depending upon the particular fabric to be ironed. The kind of cloth and the
required heat are given in table I. A lamp fitted in the handle goes off when the desired temperature
is attained.
Fig.9.6 - General layout of Bimetal Thermostat
Fig.9.8 - Simplified sketch of Adjustable Thermostat
Fig.9.7 - A fixed bimetallic strip bends when it
is heated
Nylon 70
o
C to 90
o
C
Rayon 100
o
C to 120
o
C
Silk 130
o
C to 150
o
C
Woolen 160
o
C to 180
o
C
Cotton 200
o
C to 220
o
C
Lynun 230
o
C to 260
o
C
Cloth Heat required in degree Centigrade