Joule Magnetostriction

Take the Strain – Joule Magnetostriction

Transformer Hum

Whether you realise it or not you have probably already encountered magnetostriction at some point in your life.

The characteristic ‘transformer hum‘ is due to magnetostriction: the alternating electric current induces a changing magnetic field which causes the iron core of the transformer to change dimensions slightly with each cyle of AC. Hence the hum.

Generous Joule

James Prescott JouleThe discovery of the magnetostriction effect is now attributed to James Prescott Joule who published his findings in 1847. Rather generously, Joule actually attributes the initial observation of magnetostriction to one Mr. F. D. Arstall, a machinist in Joule’s town.

Joule’s paper comes from the before the time of graphs and diagrams. This means two things: firstly that the language used seems to have a beautiful flow and turn of phrase that you just don’t see these days; secondly it means that you have to read the text rather closely to work out what is going on.

Ingeneous Experiment

The core of Joule’s investigations into magnetostriction was measuring the strain induced in an iron bar forming the core of a solenoid. To me the most ingeneous of these is one that I cannot imagine being in any lab today.

To investigate the change in volume of the iron core, Joule has a 40 inch long glass cyclinder wrapped in copper wire. An iron bar 36 inches long is placed in this cyclinder before it is filled up with water. Joule was then able to measure any change in volume of the iron as the solenoid was turned on and off.

joule magnetostriction

My interpretation of some of Joule’s apparatus used to investigate volume change during magnetostriction.

To think that such a large and ungainly piece of equipment could contribute to physics that would go on to help develop the micro- and nano-electronics industry is remarkable.

Magnetostriction Today

We now think of magnetostriction in terms of the magnetic field affecting the behaviour of the electron’s spin, which affects the electron’s orbit, which in turn affects the crystal lattice. It is the sum of these changes to the crystal lattice on an atomic scale that add up the the macroscopic changes which cause transformer hum. This has also been put to use in sonar sensors [pdf] and magnetostrictive speakers.

In spintronics magnetostriction can be a nuisance (often minimised by using permalloy). Lately, however there has also been work to utilise magnetostriction in so called ‘straintronic‘ devices, which offer a route to low power computing and signal processing.


Image: Steve Snodgrass; Some rights reserved

Joule J.P. (1847). XVII. On the effects of magnetism upon the dimensions of iron and steel bars , Philosophical Magazine Series 3, 30 (199) 76-87. DOI:

Roy K. & Jayasimha Atulasimha (2011). Hybrid spintronics and straintronics: A magnetic technology for ultra low energy computing and signal processing, Applied Physics Letters, 99 (6) 063108. DOI: