engineering

Permalloy

Permalloy is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Permalloy rather than just read about it. In short: Permalloy () is a nickel–iron magnetic alloy, with about 80% nickel and 20% iron content. Invented in 1914 by physicist Gustav Elmen at Bell Telephone Laboratories, it is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields.

Key takeaways

Permalloy belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Permalloy to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Permalloy from memory before moving on to harder problems.

Reference excerpt

Permalloy () is a nickel–iron magnetic alloy, with about 80% nickel and 20% iron content. Invented in 1914 by physicist Gustav Elmen at Bell Telephone Laboratories, it is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields. Commercial permalloy alloys typically have relative permeability of around 100,000, compared to several thousand for ordinary steel. In addition to high permeability, its other magnetic properties are low coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. The low magnetostriction is critical for industrial applications, allowing it to be used in thin films where variable stresses would otherwise cause a ruinously large variation in magnetic properties. Permalloy's electrical resistivity can vary as much as 5% depending on the strength and the direction of an applied magnetic field. Permalloys typically have the face-centered cubic crystal structure with a lattice constant of approximately 0.355 nm in the vicinity of a nickel concentration of 80%. A disadvantage of permalloy is that it is not very ductile or workable, so applications requiring elaborate shapes, such as magnetic shields, are made of other high permeability alloys such as mu metal. Permalloy is used in transformer laminations and magnetic recording heads.

Development

Permalloy was initially developed in the early 20th century for inductive compensation of telegraph cables. When the first transatlantic submarine telegraph cables were laid in the 1860s, it was found that the long conductors caused distortion which reduced the maximum signalling speed to only 10–12 words per minute. The right conditions for transmitting signals through cables without distortion were first worked out mathematically in 1885 by Oliver Heaviside. It was proposed by Carl Emil Krarup in 1902 in Denmark that the cable could be compensated by wrapping it with iron wire, increasing the inductance and making it a loaded line to reduce distortion. However, iron did not have high enough permeability to compensate a transatlantic-length cable. After a prolonged search, permalloy was discovered in 1914 by Gustav Elmen of Bell Laboratories, who found it had higher permeability than silicon steel. Later, in 1923, he found its permeability could be greatly enhanced by heat treatment. A wrapping of permalloy tape could reportedly increase the signalling speed of a telegraph cable fourfold. This method of cable compensation declined in the 1930s, but by World War II many other uses for Permalloy were found in the electronics industry.

Other compositions Other compositions of permalloy are available, designated by a numerical prefix denoting the weight percentage of nickel in the alloy, for example "45 permalloy" means an alloy containing 45% nickel, and 55% iron by weight. "Molybdenum permalloy" is an alloy of 81% nickel, 17% iron and 2% molybdenum. The latter was invented at Bell Labs in 1940. At the time, when used in long distance copper telegraph lines, it allowed a tenfold increase in maximum line working speed. Supermalloy, at 75% Ni, 20% Fe, and 5% Mo, is also well known for its high performance as a "soft" magnetic material, characterized by high permeability and low coercivity.

Applications Due to its high magnetic permeability and low coercivity, Permalloy is often used in applications that require efficient magnetic field generation and sensing. It exhibits low energy loss, which is beneficial for improving the performance of magnetic sensors, transformers, and inductors. Permalloy is also used in the production of magnetic shielding materials, which help protect electronic equipment from external magnetic interference.

See also Loading coil Mu-metal Sendust Supermalloy (a material with even higher magnetic permeability)

Notes

References Richard M. Bozorth, Ferromagnetism, Wiley-IEEE Press (1993 reissue), ISBN 0-7803-1032-2. P. Ciureanu and S. Middelhoek, eds., Thin Film Resistive Sensors, Institute of Physics Publishing (1992), ISBN 0-7503-0173-2.

Illustrations

Worked examples

Example 1 — a first encounter with Permalloy

Start with the simplest possible case. Write down what Permalloy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Permalloy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Permalloy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Permalloy

In research: Permalloy appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Permalloy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Permalloy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ferromagnetic materials, Magnetic alloys, Nickel alloys, so understanding it makes those chapters shorter.
In everyday life: Look for Permalloy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Permalloy in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Permalloy means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Permalloy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Permalloy in simple terms?

Why does Permalloy matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Permalloy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Permalloy.