Apparatus for showing the mechanical equivalent of heat, 1884.
© Science Museum / Science & Society Picture Library
This device, invented by the physicist Charles Vernon Boys (1855-1944), gives an accurate figure for the mechanical equivalent of heat. The amount of heat energy needed to raise the temperature of 1kg (2.2lb) of water by one degree is one kilocalorie, or about 4000 joules. This is the same amount of mechanical energy needed to lift a four tonne weight by 10cm.