Blog: The Discovery of Invar

First published: 1 June 2025, last updated 26 April 2026.

In the late nineteenth century, nickel steel was a hot metallurgical topic. It made tough armour for battleships, but also contained a mystery uncovered by Dr Charles Guillaume. The balance of atomic and magnetic forces within the alloys gave rise to strange properties, such as an absence of thermal expansion in one alloy that was named Invar. The investigation of these strange properties opened a new field of material science, made Guillaume famous and won him the Nobel prize for physics in 1920.

I make additions and corrections to this web site frequently but, because they are buried somewhere on one of the pages, the changes are not very noticeable. I decided to create this blog to highlight new material.

Note that these articles also get updated, especially soon after they are posted when additional information may be added. Check the “last updated” date to see when the article was last updated.

The article in this blog is from a series explaining how temperature compensation is achieved in modern watches without bimetallic compensation balances. This is achieved by using an unusual property of nickel steel alloys to make balance springs that, unlike steel springs, get stiffer as their temperature increases.

The breakthrough occurred in 1897, with the invention by Paul Perret and Dr Guillaume of the first nickel steel compensating balance springs. The development of nickel steel balance springs began with these first Paul Perret balance springs, leading to Elinvar, and ultimately to Nivarox in the 1930s.

Six articles in this series are currently planned;

The highlighted links will jump straight to the ones that have been published.

The articles are from the page about Temperature Compensation by Nickel Steels.

As always, if you have any comments or questions, please don't hesitate to get in touch via my Contact Me page.

The Discovery of Invar

Armour plates tested at Annapolis, MD, in 1890
Image from Scientific American, 1898. Click image to enlarge

Nickel is a metallic element with the symbol Ni and atomic number 28. Slightly yellow in colour, resembling very pale brass, it is corrosion resistant due to the formation of a thin, protective nickel oxide layer on its surface.

Nickel alloys well with steel. In ancient times, nickel steel was obtained from meteorites and used to make items such as the dagger found in Tutankhamun’s tomb. However, because nickel is difficult to extract from ore, its applications remained limited until the development of efficient extraction methods in the 19th century.

As technological advancements made it possible to extract and refine nickel, the use of nickel in steel alloying gained prominence. Pioneering research by Michael Faraday in the 1820s demonstrated that even small additions of nickel improved steel’s toughness. By the 1880s, nickel steel was replacing traditional steel as a material for battleship armour.

Tests conducted by the US Navy at Annapolis in 1890 demonstrated the superiority of nickel steel over both traditional steel and compound armour plates. The trial compared three plates: a steel plate and a nickel steel plate, both manufactured by Schneider & Co., Le Creusot, and a compound plate made by Cammell & Co., Sheffield. The plates were approximately 10½ inches thick.

Four shots were fired into the corners of each plate from 28 feet using a 6-inch breech-loading rifled naval gun. The projectiles were Holtzer 6-inch armour-piercing shells weighing 100 lbs, propelled by 44½ lbs of brown prismatic powder manufactured by DuPont, with a striking velocity of 2,075 feet per second. A fifth shot was then fired into the centre of each plate by an 8-inch breech-loading rifled naval gun using Firth armour-piercing shells weighing 210 lbs, with an 85 lb powder charge and a striking velocity of 1,850 feet per second. It must have been great fun!

The figure from the Scientific American Coast Defense Supplement of 1898 shows the final result. The compound plate was perforated by all projectiles, and its steel face was destroyed. Both Schneider plates kept out all projectiles. The steel plate showed slightly greater resistance to penetration, but it cracked severely under the 8-inch shell, whereas the nickel steel plate remained uncracked. This demonstrated the superior toughness of nickel steel, leading the Board to conclude that the nickel steel plate was the best.

Another French company, the Société de Commentry, Fourchambault et Decazeville, also played an important role in the development of nickel steel. Formed in 1853 through the merger of several companies involved in steel production, it included the steelworks at Imphy, located in the Nièvre department of the Bourgogne-Franche-Comté region of central-eastern France. The Imphy works, with origins dating back to the 17th century, emerged in the late 19th century as a centre of innovation in metallurgy, particularly in nickel steel alloys.

At the time of the US Navy test in 1890, Schneider & Co. of Le Creusot was France’s principal producer of armour-grade nickel steel. In the years that followed, the Imphy steelworks, which was part of the Schneider group, became increasingly important in the production of specialised nickel steels. By the late 1890s it was supplying high-quality nickel steel to the French Navy and exporting to other countries, including the United States and Japan.

Dr C. E. Guillaume

Charles Édouard Guillaume was born on 15 February 1861 in Fleurier, Switzerland, in the heartland of the Swiss watchmaking industry. His grandfather, Charles Frédéric Alexandre Guillaume, had left Switzerland to establish a watch business in London. His three sons joined the business but one, Édouard, returned to Fleurier and had a son, Charles Édouard. He, Charles Édouard, studied physics at the Polytechnikum in Zurich (the precursor of the Eidgenossische Technischc Hochschule). At the age of 22, Charles Édouard joined the Bureau International des Poids et Mesures, at Sèvres, a suburb of Paris.

The Bureau international des Poids et Mesures (BIPM), or International Bureau of Weights and Measures, was established by the Metre Convention in 1875 and began operations around 1881. Its principal task was to standardise international measurements and to distribution to signatory countries of the Metric convention standards of length and weight. From the very outset, it had a problem, the cost of the platinum-iridium alloy used for length standards, chosen because it has low thermal expansion, was prohibitively high.

Guillaume was given the task of finding a cheaper alloy with low thermal expansion that would be suitable for length standards. There were no instruments that could measure dimensions of materials at different temperatures to determine their coefficients of thermal expansion with sufficient accuracy, so one of his first tasks was to design an accurate dilatometer.

Guillaume presented the results of his studies in 1892. He had examined pure nickel, a nickel-iron alloy and three types of bronze. The nickel-iron alloy he eliminated because it rusted quickly in the presence of water. The pure nickel seemed the most promising, but lengths of four metres were required and no manufacturer could produce bars of pure nickel longer than about two metres.

Nickel Steels

In 1895, the bureau received a request to calibrate a metre length standard belonging to the Technical Branch of the French Artillery. Extensive research in the 1880s to find improved alloys for armour had resulted in a steel alloy with 22% nickel and 3% chromium, from which this standard had been made.

During the calibration, the standard did not behave well. Guillaume measured its coefficient of thermal expansion and found that it was about 18 parts per million (ppm), that is 18 × 10⁻⁶, per degree Celsius. This is greater than bronze and much higher than expected from the rule of mixtures. An alloy of 75% steel and 22% nickel with a small amount of chromium would be expected to have a thermal expansion about three-quarters of the way between steel at 11 to 13 × 10⁻⁶ per degree Celsius and nickel at 12.5 × 10⁻⁶ per °C. It was determined that the material was not suitable for a length standard, and no further work was done.

In 1896, a bar of an iron alloy with 30% nickel was supplied to the Bureau by the Société de Commentry-Fourchambault for the possible construction of precision weights. Much to his surprise, Guillaume found that its thermal expansion coefficient was only about one-third of that of the earlier alloy, lower than platinum and much lower than expected.

Guillaume realised that the unexpected rates of thermal expansion of the two alloys showed a previously unknown phenomenon was taking place at an atomic lever that required significant further study, a reaction that would ultimately make his name world famous and lead to a Nobel Prize. When the thought initially occurred to him, he told his wife that he would spend the next ten years studying the effect. In fact, it was to occupy him for the next thirty years.

Guillaume found that both nickel steels had been made by the Commentry-Fourchambault steelworks at Imphy. He contacted the Company and spoke to its Managing Director, Henri Fayol, who was not only willing but eager to help, saying “What do you need to continue? I'm with you all the way.” This was the start of a long collaboration, during which more than six hundred alloys were been provided free of charge to the International Bureau.

Nickel steels linear alpha and non-linear beta thermal expansion coefficients:
Click image to enlarge

Guillaume initially asked for seventeen different alloys of iron and nickel covering the range from pure iron to 44% nickel. He measured the expansion coefficients of these, which gave the results plotted on the graph here. The horizontal x axis shows increasing nickel content from zero at the left to 100% at the right. The verical y axis shows the expansion coefficient in parts per million per °C.

The upper graph in the figure labelled α (alpha) shows the linear coefficients of thermal expansion. The expansion coefficient of steel at normal temperatures is marked on the left y axis as point A. The expansion coefficient of nickel is marked on the right y axis as point B. If the rule of mixtures was followed, the expansion coefficient of an alloy at would be expected to lie between these points, e.g. the expansion coefficient of an alloy of 50% nickel and 50% steel would be expected to be exactly at the mid-point between points A and B. The dotted line joining A and B shows this rule.

However, instead of following the rule of mixtures, Guillaume found that the expansion coefficients of the alloys takes a “wild path”. From the left hand axis, as the nickel content increases from zero to about 28%, the expansion coefficient increases until it reaches the line drawn from C to B near 24%. Point C on the left y axis is the expansion coefficient of iron at high temperatures. The curve then falls sharply until at about 36% nickel it reaches a low point of about 0.9 ppm per degree. This was a quite astonishing result, a metal alloy with such a low coefficient of thermal expansion was previously unknown and unexpected.

Shortly after the discovery was announced, in the Swiss Journal of horology Professor Marc Thury pointed out that the alloy with very low expansion would be useful for pendulum rods, requiring only minimal compensation for the effect of changes in temperature. He proposed the name ‘Invar’ for this alloy, derived from invariable dimensions. The name immediately caught on and made Guillaume famous outside of the small world of professional metrology.

Guillaume continued to study the nickel steel alloys. He discovered that an alloy containing 44 to 48% nickel has the same thermal expansion as glass. Wires made from this alloy could be substituted for the expensive platinum that was previously used to form the lead wires for incandescent lamps, hence the name “platinite” was given to this alloy.

The careful and rigorous studies of the properties of nickel steel alloys and investigations into the causes of their unusual properties led to Guillaume being awarded the Nobel Prize for Physics in 1920.

Non-linear Effects

Thermal expansion doesn't follow a direct, linear, relationship to temperature changes. As atoms are given more energy and heat up, they vibrate more, which causes the bonds between them to lengthen. This is what causes thermal expansion. Longer bonds cause weaker interactions between atoms, and the rate of bond weakening isn’t linearly proportional to temperature; it increases at higher temperatures. This non-linear behaviour means that the rate of expansion of most metals accelerates as the temperature rises.

Using his very accurate dilatometer, Guillaume was able to measure the departure from linearity of the thermal expansion of the nickel steel alloys, which is shown by the curve in the lower graph of the figure labelled β (beta). An interesting feature of this curve is that it crosses the zero axis at exactly the same point at which the curve in the upper graph is at a minimum, the Invar point. The zero line has been highlighted in red to show this clearly.

To the right of the Invar point on the lower graph, between the nickel contents of 36% and about 50%, the curve is below the zero line. This is a range of nickel steel alloys that have negative non-linear expansion, meaning that their rate of thermal expansion decreases with increasing temperature.

This is an extremely rare property; no other metal alloys with such a strong negative non-linear effect are known.

What is Invar

Invar is the name of a nickel steel alloy containing 36% nickel and having the unusual property of extremely low thermal expansion. The name Invar was suggested by Professor Marc Thury because of the alloy's almost invariable dimensions.

Other nickel steel alloys have more normal rates of thermal expansion and should not be referred to as Invar.

Invar in Watches

Invar has never been used in watches, despite what is sometimes claimed. The name Invar was never registered as a trademark by Guillaume or Thury and was therefore open to anyone to copy. This led to the name Invar being used by some companies that were not connected with Guillaume for purely marketing purposes. Nickel steel components in watches were sometimes called Invar even though they were a different alloy. One of these was the term “Invar balance” used for balances invented by Guillaume that used a different nickel steel alloy called Anibal. Guillaume remarked that “Invar balance” was “a decidedly erroneous name, Invar not entering into the composition of the balance.”

Invar balance: “a decidedly erroneous name, Invar not entering into the composition of the balance.”

If you have any comments or questions, please don't hesitate to get in touch via my Contact Me page.

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