From 1840, Joule began to study the thermal effect of current, and wrote papers such as "On the Heat Produced by Voltaic Electricity" and "The Heat Released by Metal Conductors and Batteries in the Process of Electrolysis", pointing out that the heat generated by a conductor in a certain period of time is directly proportional to the square of current and the product of conductor resistance. Shortly thereafter, 1842, the famous Russian physicist Lengci independently discovered the same law, so it was called Joule-Lenz Law. This discovery laid a foundation for revealing the equivalence of electric energy, chemical energy and thermal energy, and opened the door to the law of conservation of energy. Joule also noticed the quantitative relationship between various natural "forces" that produce heat. He did many experiments. For example, he put a coil with an iron core in a closed water container and connected it with a sensitive galvanometer. The coil can rotate between the magnetic fields of the strong electromagnet. The electromagnet is powered by a battery. In the experiment, the electromagnet alternately turns on and off the current 15 minutes, and the coil speed reaches 600 times per minute. In this way, we can compare friction heat generation with current heat generation, and Joule proves that heat is proportional to the quadratic power of current. He also tested three methods, such as shaking hands and falling heavy objects, and finally came to the conclusion that "the heat of 1 pound of water rising1f is equal to and may be converted into the weight of 65438 lifting 838 pounds." Summarizing these results, he wrote a paper on the thermal effect of magnetoelectricity and the mechanical value of heat, which was read out at the meeting of the Mathematics and Physics Group of the British Science Association on August 2 1. He emphasized that the energy in nature is equivalent and will not be destroyed. Where mechanical energy or electromagnetic energy is consumed, considerable heat can always be obtained in some places. This is an excellent proof and support for the power of heat. Therefore, it caused a sensation and fierce controversy.

In order to further convince those scientists affected by the heat theory, he said, "I intend to repeat these experiments with more effective and accurate equipment." Later, he changed the measurement method, for example, comparing the work required to compress a certain amount of air with the heat generated by compression to determine the mechanical equivalent of heat; The mechanical equivalent of heat is determined by the heat released by water through the movement of fine tubes; The most famous propeller experiment is still considered to be the most accurate. The falling weight drives the blades in the calorimeter to rotate, and the heat generated by the friction between the blades and water can be accurately measured by the temperature rise of water. He also replaced water with other liquids (such as whale oil and mercury). The mechanical equivalent of heat obtained by different methods and materials is 423.9 kg/m/kcal or close to 423.85 kg/m/kcal.

During the period of 1840 ~ 1879, Joule spent nearly 40 years studying and measuring the mechanical equivalent of heat. He did more than 400 experiments in different ways and came to the conclusion that the mechanical equivalent of heat is a universal constant, which has nothing to do with the way of doing work. His own 1878 test results are the same as 1849. Later, I learned that this value is 427 kg weight m per kilocalorie. Thus, Joule deserves to be a real experimental master. His experimental constants provide unquestionable evidence for the law of conservation and transformation of energy.

1847, when 29-year-old Joule reported his achievements again at the meeting of the British Science Association held in Oxford, Lord Kelvin, who wanted to refute after listening, was completely convinced by Joule. Later, they cooperated very well. * * made a porous plug experiment (1852) and found that the gas temperature dropped after the porous plug expanded, which is the Joule-Thomson effect. These experimental results of Joule are summarized in his important book on the mechanical equivalent of heat published in 1850. His experiment was repeated by many people from different angles and different methods, and the conclusion was the same. Joule was elected as a member of the Royal Society in 1850. Since then, he has continued to improve his experiment. Engels listed "the proof of energy conversion caused by the mechanical equivalence of heat (Meyer, Joule and Keldin)" as the first of the three major discoveries of natural science in the second half of the 9th century.