James Prescott Joule
Discoverer of Mechanical Equivalent of Heat
Joule was born on December 24, 1818, at Salford, near Manchester, England. His father was a wealthy brewer who hired private tutors for James and his brother. By about age 18, he was attempting to build an electrical perpetual motion machine, a device that could generate its own power to keep running. He soon decided that the task was impossible, although some people are trying even today. He studied at the University of Manchester in 1835 where he worked with English chemist JOHN DALTON.
Soon he turned his attention to studying the energy lost to heat in the resistance of wires carrying the electricity. By 1840 he had demonstrated that the power lost in a circuit is proportional to the resistance in the circuit times the current squared. This basic relationship is called Joule’s Law.
His greatest discovery was derived from ongoing experiments on the nature of heat and how it can be produced by moving or falling objects. To aid in this investigation, Joule produced an experimental device featuring an insulated container with paddle wheels inside, connected by pulleys to large falling weights. As the weights descended, the paddles spun, churning up the liquid in the container and raising its temperature. Joule found that the actual amount of heat produced did not depend on what fluid he used, but only on how much weight was falling and how far it fell. In his best trials, he determined that 4.15 units of mechanical energy (the sum of the energy of movement and energy stored in parts of a system) were equivalent to one calorie of heat: Joule’s measurement was very close to our current calculation of 4.184. Thus he established the mechanical equivalent of heat and published his findings in 1843. The unit of energy has been named the joule, in his honor, and is abbreviated as J.
In Joule’s time, energy was called vis viva, or `”living force.” In a public lecture in 1847 Joule explained how this living force could be convened to heat, or the other way around. He stated that nothing is lost in such conversions: a given amount of heat will be converted into the same amount of “living force.” He was the first to dearly understand and explain what we now call the Law of Conservation of Energy, which is the first law of thermodynamics.
His last major scientific contribution came in 1852, when he and physicist William Thomson (later known as LORD KELVIN) worked together on the thermal properties of gases. They discovered that if a gas is allowed to expand into an empty chamber, the temperature of the gas decreases. The principle is now known as the Joule Thomson effect.
Joule lived as a wealthy amateur scientist until 1875, when he lost most of his fortune. His health also began to fail and his scientific work slowed to a stop. He lived until 1889, dying in Sale, England.
James Joule’s Legacy
Joule’s exploration of the connection between heat and other forms of energy laid a crucial foundation for future theoretical work. Joule’s ideas also had extensive practical applications, including innovations in refrigeration and electrical systems.
Two of Joule’s contemporaries, Hermann von Helmholtz and RUDOLF CLAUSIUS, expanded on his work by establishing the mathematical principles that underpinned Joule’s experiments. Their formulas are still in use by contemporary scientists and engineers.
The practical applications of Joule’s work are significant. Refrigerators, freezers, air conditioners, and dehumidifiers all make use of the Joule Thomson effect, which states that a gas cools as it expands. The Joule Thomson effect is also at work in electric heat pumps, which are used to collect diffuse, low temperature heat from outdoor air or from underground and concentrate it to warm homes.
Joule’s Law, relating the heat loss in a circuit to resistance and current, has an important application today in distributing electricity. Calculations of how much electricity is lost because of resistance as it is carried across wires helps electric utility companies supply power at the right voltage to homes. The efficiency of our electrical systems is a consequence of Joule’s Law and a tribute to his work.
James Prescott Joule – 1818-1889