We often imagine that the creation of Nickel/Iron alloys of “Invar", "Elinvar" and the likes to be a unique magical, mystical metallurgy created by "mad scientists" for the Watch business. But in fact a HUUUGE larger demand for such a "near zero" coefficient of expansion (with temperature) metal was far more important in the world of electronics, not the least of which was for wire leads fed through Glass radio tubes. In truth, the “invention” of invar came soon after the first “antimagnetic” wires and some materials for Glass thermometers resulting in “Dumet” seals and then advanced quickly to Kovar which is big brother to Invar, etc.

Beginning in the 1960’s I used “Kovar” Nickel Iron extensively in my designs for their superior mechanical, electrical and highly permissive magnetic properties.

Read on . . . (Thanks to Wickipedia)
Dumet wire seal
In 1911 the Dumet-wire seal was invented which is still the common practice to seal copper leads through soda-lime or lead glass. If copper is properly oxidised before it is wetted by molten glass a vacuum tight seal of good mechanical strength can be obtained. After copper is oxidized, it is often dipped in a borax solution, as borating the copper helps prevents over-oxidation when reintroduced to a flame. Simple copper wire is not usable because its coefficient of thermal expansion is much higher than that of the glass. Thus, on cooling a strong tensile force acts on the glass-to-metal interface and it breaks. Glass and glass-to-metal interfaces are especially sensitive to tensile stress. Dumet-wire is a copper clad wire (about 25% of the weight of the wire is copper) with a core of nickel-iron alloy 42, an alloy with a composition of about 42% nickel.[7] The core has a low coefficient of thermal expansion, allowing for a wire with a coefficient of radial thermal expansion which is slightly lower than the linear coefficient of thermal expansion of the glass, so that the glass-to-metal interface is under a low compression stress. It is not possible to adjust the axial thermal expansion of the wire as well. Because of the much higher mechanical strength of the nickel-iron core compared to the copper, the axial thermal expansion of the Dumet-wire is about the same as of the core. Thus, a shear stress builds up which is limited to a safe value by the low tensile strength of the copper. This is also the reason why Dumet is only useful for wire diameters lower than about 0.5 mm. In a typical Dumet seal through the base of a vacuum tube a short piece of Dumet-wire is butt welded to a nickel wire at one end and a copper wire at the other end. When the base is pressed of lead glass the Dumet-wire and a short part of the nickel and the copper wire are enclosed in the glass. Then the nickel wire and the glass around the Dumet-wire are heated by a gas flame and the glass seals to the Dumet-wire. The nickel and copper do not seal vacuum tight to the glass but are mechanically supported. The butt welding also avoids problems with gas-leakages at the interface between the core wire and the copper.

"Kovar" (Iron-nickel-cobalt)
Kovar, has low thermal expansion similar to glass and is frequently used for glass-metal seals. It can bond to glass via the intermediate oxide layer of nickel(II) oxide and cobalt(II) oxide; the proportion of iron oxide is low due to its reduction with cobalt.

Fascinating Dave! I've read where Hamilton,not being satisfied with the elinvar that was being produced in the common world, started making their own. Which , in turn, led to Hamilton manufacturing scores of things for the U.S. government. In fact, by the end of WWII, watch manufacturing was taking a back seat to the other things they were making there. Regards, Brad