Magnetic petrol gauge, 1927.
© Science Museum / Science & Society Picture Library
A gauge for indicating the level of fuel in a motor car petrol tank, made by Simms Motor Units (1920) Ltd. This type of instrument was invented by T Martin in about 1912. The dial contains a pivoted magnetic needle which acts as a pointer. As the level of petrol in the tank falls, and the float with falls it, the motion of the float arm rotates a magnet by means of gears at the back of the dial, and this in turn moves the magnetised pointer.