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Joule had the idea that he could use the electric motor to convert the family brewery from steam power to electric power. As a course of his experiments he decided that it would be too expensive to do this, as the work done by the motor did not come out very highly when the cost of the zinc used in the batteries that powered it was taken into consideration. It was still cheaper to produce the steam power by burning coal. James Joule was very painstaking and paid great attention to detail. He spent a great deal of his life in determining the mechanical equivalent of heat, inspired from his belief that heat was derived from work. William Thompson (later to be Lord Kelvin) worked with Joule between 1852 and 1862 on experiments which led to the discovery known as the Joule-Kelvin cooling effect.

Joule began a series of experiments involving mechanical work, electricity and heat. He sent a paper entitled “On the Production of Heat by Voltaic Electricity.”

In this paper, he showed that the amount of heat produced per second in a wire carrying an electric current equals the current (I) squared multiplied by the resistance (R) of the wire. The heat produced is the electric power lost (P). (That is, P=I²R.) This relationship is known as Joule’s Law. Joule's Law, it states that heat is produced in an electrical conductor. In the experiments behind this law, he had simply placed coils of different kinds of metal in jars of water and measured the change in temperature. Joule published the results of further experiments on the production of heat by friction in the Royal Society’s Philosophical Transactions in 1850 and gave the most accurate determination of the constant yet.

Joule-Thomson effect, whereby an expanding gas is cooled as work is done to separate the molecules. Joule was patient and ingenious in his experiments. These attributes greatly assisted him in avoiding errors and in obtaining results far more accurate than those of previous experimenters.