# Joule effect

Joule effect and Joule's law are any of several different physical effects discovered or characterized by English physicist James Prescott Joule. These physical effects are not the same, but all are frequently or occasionally referred to in literature as the "Joule effect" or "Joule law" These physical effects include:

## Joule's first law

Main article: Joule's first law

Between the years 1840 and 1843 Joule made a careful study of the heat produced by an electric current. From this study, he developed Joule's laws of heating, the first of which is commonly referred to as the Joule effect. Joule's first law expresses the relationship between heat generated in a conductor and current flow, resistance, and time.[1]

## Magnetostriction

The magnetostriction effect describes a property of ferromagnetic materials which causes them to change their shape when subjected to a magnetic field. Joule first reported observing change in the length of ferromagnetic rods in 1842.[2]

## Joule expansion

In 1845, Joule studied the free expansion of a gas into a larger volume. This became known as Joule expansion.[3] The cooling of a gas by allowing it to expand freely is occasionally referred to as the Joule effect.[4]

## Gough–Joule effect

If an elastic band is first stretched and then subjected to heating, it will shrink rather than expand. This effect was first observed by John Gough in 1802, and was investigated further by Joule in the 1850s, when it then became known as the Gough–Joule effect.[5][6]
Examples in Literature:

• Popular Science magazine, January 1972: "A stretched piece of rubber contracts when heated. In doing so, it exerts a measurable increase in its pull. This surprising property of rubber was first observed by James Prescott Joule about a hundred years ago and is known as the Joule effect."[7]
• Rubber as an Engineering Material (book), by Khairi Nagdi: "The Joule effect is a phenomenon of practical importance that must be considered by machine designers. The simplest way of demonstrating this effect is to suspend a weight on a rubber band sufficient to elongate it at least 50%. When the stretched rubber band is warmed up by an infrared lamp, it does not elongate because of thermal expansion, as may be expected, but it retracts and lifts the weight."[8]