Apparatus to demonstrate Joule's law, 19th-20th century.
© Science Museum / Science & Society Picture Library
James Prescott Joule (1818-1889) is responsible for the experimental establishment of the mechanical theory of heat. Joule's law states that the heat (Q) produced when an electric current (i) flows through a resistance (R) for a time (t) is given by Q = R x t x i squared. An electric current flowing through a high-resistance wire heats the wire which warms the water. The temperature rise of the water over a certain time, compared to the electric current pased through the wire, shows the relationship between the two.