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Home arrow Archives   The American Surveyor     

Invar—The Breakthrough for a Low Expansion Alloy Print E-mail
Written by Jerry Penry, PS   
Friday, 16 November 2012

A 2.495Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

The discovery of the low expansion alloy, Invar, established a new standard in the way precise surveying measurements were made, both in reliability and accuracy. It became the first successful attempt to produce a metal alloy exhibiting a nearly zero coefficient of thermal expansion. In 1889, James Riley of Glasgow, Scotland, brought before the Iron and Steel Institute his investigations into the making of an alloy through a series of tests which combined up to 49 percent nickel with iron.

Seven years later, in 1896, Charles Edouard Guillaume, a Swiss-born metallurgist and employee with the International Bureau of Weights and Measures near Paris, began looking specifically for an alloy to be used for surveyors' wires that would not noticeably change when exposed to temperature variations. While experimenting with nickel contents between 30 and 60 percent, Guillaume discovered the coefficient of expansion at room temperature was lowest when mixing a nickel content of 36 percent with 64 percent iron. Since his new alloy exhibited the least amount of thermal expansion, and because Guillaume considered it invariable, it quickly became known as "Invar". In 1920, Guillaume was awarded the Nobel Prize in Physics for his discovery of Invar which by then was common in many precision instruments such as clocks, seismic gauges, and surveyors' steel tapes. Guillaume is the only scientist to be honored with the Nobel Prize for a metallurgical achievement.

Swiss watchmakers began using the new alloy for clock pendulums and in the balances in pocket timepieces at the beginning of the twentieth century. In 1901, a Swiss watchmaker, Achille Hirsch, founded the Invar Watch Company. For many years preceding the discovery of Invar, watchmakers struggled with even the best compensated watches and chronometers because of the non-linear variation of the elasticity of the steel in the hair spring. Invar was the answer to this problem. Watches produced by the Invar Watch Company began appearing with the word "Invar" printed on the dial and stamped on the metal inside the case, designating a sign of quality. The Hamilton Watch Company used the name Elinvar which was stamped on the inside of their watches which utilized the Invar alloy.

The United States Coast & Geodetic Survey (C&GS) used steel tapes as an additional method to measure precise base lines in triangulation networks as early as 1891. The variance of the temperature was a problem when using steel tapes since expansion and contraction continually had to be factored in during the measurements. It had always been a goal of C&GS to develop an instrument that would not be affected by temperature change. The Ice Bar, invented in 1891, is arguably the most precise instrument ever invented which remained a constant length, but it could only measure a distance of 5 meters at one time which was time consuming. (See The American Surveyor, April 2007).

Geodetic agencies in other countries began hearing the favorable applications in which Invar could be used for precise measuring. Serbia obtained a set of Invar wires and measured a base line with the Jăderin apparatus in 1904. Sweden tested a length of Invar wire without making any corrections for temperature on one of their previously measured base lines and determined a difference of only 19mm in 10 kilometers. The countries of Argentina, Australia, Germany, France, Japan, Mexico, Romania, Russia, and Switzerland also purchased lengths of Invar wire for their own geodetic surveying. It soon became apparent that the United States would lose its international edge in precise measuring if it didn't at least begin its own experiments with the new alloy. On April 1, 1904, a French company trademarked the word Invar. Successors to that company have kept the trademark going, so it is active today with the fifth renewal in 2007.

Invar became the answer C&GS had been seeking for measuring base lines, but the alloy tapes needed to be fully tested. In December of 1905, C&GS purchased six Invar tapes from J. H. Agar Baugh in London, England, for the purpose of subjecting them to the rigors of field work while comparing them to existing steel tapes under the same conditions. Unlike the alloy wire used by other countries, the United States obtained lengths of flat Invar ribbon similar to the steel tapes they had already been using in the field. The new Invar tapes were numbered 437 through 442. The Invar tapes were the color of nickel and they did not oxidize like steel tapes. The downside was that they were soft and easily bent which required larger aluminum reels, 16 inches in diameter, to prevent permanent bending of the Invar tapes when not in use.

The field test came in 1906 under the direction of Owen B. French when C&GS decided to re-measure six existing baselines: Point Isabel Base, Texas; Willamette Base, Oregon; Tacoma Base, Washington; Brown Valley Base, South Dakota; Stephen Base, Minnesota; and Royalton Base, Minnesota. Three existing 50-meter steel tapes 403, 405, and 406, and three of the new 50-meter Invar tapes, 438, 439, and 440 were used for the experiment. A fourth tape of each type was utilized as a check while the others were held in reserve in the event of damage. To calibrate the tapes, a comparator line was established in the tunnel joining two buildings at the Bureau of Standards. The line was repeatedly measured with the Ice Bar to ensure the probable error of the derived 50-meter length did not exceed +/- 0.03 mm. The steel and Invar tapes were then used to measure the comparator line at least four times each. All of the tapes were also standardized at the Bureau of Standards before beginning the field work to test strength and elasticity. During part of the standardization process, Invar tape 438 was subjected to one hundred reelings and unreelings, on several occasions, without showing any change in length. Another Invar tape, 437, was kept outdoors when not in use and subjected to a temperature range of 14° to 86° F. It also did not show any change in length.

At the conclusion of the field tests, it was found the Invar tapes could be handled and manipulated in identically the same manner as the steel tapes except for placing them on larger reels. The Invar tapes, when properly manufactured, were found to vary less than one part in 500,000 after six months of use in the field. Measurements could be made with the Invar tapes during the heat of the day and repeated standardization was not necessary. This was a savings of time in both the field and laboratory. Measuring with an Invar tape was therefore deemed superior to using a standard steel tape.

During the 1915 field season, C&GS began using a meter-long Invar bar to determine if any changes were occurring with their leveling rods. This bar provided calibration information to determine how the leveling rods were affected by changes in temperature. With the obtained information, C&GS made the decision to use strips of graduated Invar on their geodetic level rods beginning with the 1916 field season. A manufacturer in the United States, Midvale Steel and Ordnance Company of Philadelphia, began producing Invar. Previously, Invar was only produced in Europe. The equivalent American alloy was named "Gamma steel" so as to not infringe upon the trademarked Invar manufacturing name. C&GS ordered several alloy strips which were graduated in metric and inset into the face of nine wooden rods and anchored to the hardened steel feet. The alloy strips were 26mm wide, 0.9mm thick, and 122 inches long and fastened to the rod with six pairs of thin brass plates. The entire leveling rod, named the Fischer Rod in honor of the chief of the instrument division, Ernst G. Fischer, weighed 14 lbs. An additional 14 rods were brought into the agency for use during the 1920-1922 seasons.

The United States Geological Survey also began using Invar tapes to precisely measure their base lines. While the exact year that Invar tapes came into use for that agency is not clear, the Manual of Topographic Instructions published in 1913 by USGS included a 300-foot Invar tape among the standard items for a surveying crew.

By the 1930's, alloy tapes were purchased by various government agencies, and private and municipal engineering companies in lengths of 100 feet, 300 feet, 50 meters and 100 meters. To meet the needs of surveyors for a lower priced alloy tape, at least two manufacturers began producing tapes similar to Invar that were of a slightly different composition and coefficient of expansion. The Keuffel & Esser Company began using the name "Lo-Var" on May 24, 1934, and trademarked the name the following year. The name "Minvar" was introduced by the Lufkin Rule Company in the mid 1960's, but was never trademarked. These names were acronyms of the words Low Variation and Minimum Variation. The precision tapes manufactured by these companies were individually numbered and had to be calibrated by the National Bureau of Standards. The name "Invar" has become synonymous with the alloy despite the various other names.

A 1962 Keuffel & Esser catalog stated their Lo-Var tapes had a coefficient of thermal expansion approximately 0.00000022 per degree of Fahrenheit which was 1/30 that of carbon steel. Lengths available at that time were 50, 100, and 150 feet, and 30 and 50 meters. Prices for these alloy tapes in 1962 ranged from $86.00 to $170.00. Today, that price would exceed $1,300.

Today, few if any surveyors still use alloy tapes due to the advance in EDM and GNSS technology. The leveling staff rods and bipods, however, are still in use for first order leveling. One manufacturer states the most severe irregular graduation errors on their precision Invar leveling staffs are less than +/- 0.005 mm. This is achieved by directly engraving the graduation into the lacquering with an interferometrically controlled laser beam. The cost for a 3-meter Invar staff can be as much as $3800 today. Lengths of Invar rod have also been used as supports for GPS base antennas.

Invar is also known as FeNi36, Invar 36, NILO 36, Pernifer 36, and Invar Steel. Most Invar produced today continues to have a chemical composition of 36 percent nickel and slightly less than 63 percent iron. The remaining elements are typically small traces of aluminum, carbon, chromium, cobalt, magnesium, phosphorus, silicon, sulfur, and titanium added for machinability and because a pure mixture is difficult to obtain.

The word "Nilvar" was trademarked by the Driver-Harris Company in 1932. "Kovar" was trademarked by Westinghouse Electric & Manufacturing Company in 1936. Today, the Kovar name is held by CRS Holdings, Inc., and contains 29 percent nickel, 17 percent cobalt, and the balance iron. Super Invar, more difficult to obtain, contains 32 percent nickel and 5 percent cobalt, with the balance being iron. Most manufacturers of the nickel/iron alloy sell it by the pound. One source recently contacted had a scrap piece of 1-inch diameter Invar rod 18 inches in length available for $150.

Today, uses for Invar continue to advance even outside of the surveying industry. Invar is used in microscopes, telescopes, thermostats, computers, wave guide tubes, orbiting satellites, lasers, gyroscopes, and many other high-tech applications.

Jerry Penry is a licensed land surveyor in Nebraska with nearly thirty years of experience. He is also a public speaker and a published author.

Sidebar:
Invar Watches

From various sources on the Internet:

"Invar, if memory serves, is a metal alloy with an unusually low coefficient of thermal expansion (that is, it doesn't grow or shrink much when it gets hot or cold). Its use in key components of a watchworks means the watch will not change speed significantly with changes in temperature. Apparently it can also make the watch less susceptible to interference from strong magnetic fields."

"Invar is a wonderfully odd alloy. If I remember right, one of its first commercial uses was in surveyor's chains."

"I used to work for a company which made a high-tech soldering iron which used an alloy similar to Invar to control when the tip was heated by current and when it wasn't. By adjusting the alloys used, they were able to control the temperature at which the crystal structure, and thus the magnetic properties, changed."

"The Invar watch company was founded in 1901 by Achille Hirsch in Le Chaux de Fonds, but they didnt' register the name until 1925. Your watch has medallions from the 1906 Worlds Fair, so it would have been made after that. The timing of the company's founding is interesting, because Invar (the material) was only invented in 1896. Mr. Hirsch was a rather prolific watch distributor around the turn of the century, who registered literally hunderds of trademarks, so to some degree this watch represents an attempt to profit from the new material. Around that time, there was a fairly large industry in what's known as "Swiss Fakes"...low-grade swiss watches that were meant to look like better quality watches. One of the ways these "Swiss Fakes" kept costs down was to not use the split bi-metallic balance wheels that better quality watches used to help offset temperature differences. The new Invar material made the split bi-metallic balance obsolete, but looked similar to the cheap balances used by the swiss fakes. So branding the watch "Invar" meant that the consumer didn't have to worry about that."

"As far as the watch itself goes, it's a good (but not great) example of a swiss bridge design, with somewhat fanciful bridge modifications. The crescent shaped regulator is quite nice, if rather impractical. Well worth getting serviced and passing down through the generations! Protect that case; that style is rare, and good quality examples have a hard time surviving the years."

A 2.495Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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