The working principe of the bimetallic thermometer is simple: a bimetallic strip is constructed by bonding together the two thin strips of different metals is in it's core. These two metals in the bimetallic thermometers are joined together at one end with the help of the welding. 

The bonding is kept in such a way that there is no relative motion between the two metals. The physical dimension of the metals varies with the variation in temperature in the bimetallic thermometers

Since the bimetallic strip of the thermometer is constructed with different metals thereby, the length of metals changes at different rates. 

When the temperature measured by the bimetallic thermometer increases, the strip bends towards the metal which has a low-temperature coefficient.

And when the temperature decreases, the strip bends towards the metal which has a high-temperature coefficient.

The linear strip shows the small deflection. If the length of strip increases, the size of thermometer also increases. For keeping the size of the thermometer in the manageable limit, the helix or spiral strip is used for