England · Late 18th century; practical forms widespread by c. 1780–1800
Bimetallic Compensation Balance
Partially verifiedTemperature compensation was essential to achieving chronometer-grade accuracy before the development of temperature-stable alloys such as Invar and Elinvar in the twentieth century.
A balance rim made of fused steel and brass (or similar dissimilar metals) curves inward or outward as temperature changes, altering the effective radius of the balance to counteract the tendency of a plain steel balance spring to run slow when warm.
Several makers contributed refinements; Earnshaw and Arnold are both credited with practical compensation-balance forms adopted for marine chronometers, and the type was progressively refined through the nineteenth century (including cut bimetallic balances to reduce middle-temperature error).
Background
Temperature sensitivity in early timepieces was a major source of inaccuracy, as the weakening of steel balance springs at higher temperatures caused watches to lose time. John Harrison initiated temperature compensation for balance wheels in 1753 by implementing a bimetallic compensation curb on the spring in his marine chronometers. A more practical solution was later devised around 1765 by Pierre Le Roy and further refined by Thomas Earnshaw and John Arnold.
The design functions by allowing the balance wheel diameter to shrink in response to heat, which reduces the rotational inertia to offset the weakening of the balance spring. The rim consists of a fused layer of brass on the outside and steel on the inside, causing the arms to bend inward as the brass expands more than the steel.
Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.
How it works
- The balance rim is cut open at two points to allow the bimetallic arms to move.
How to verify this yourself
Consult the National Maritime Museum's chronometer catalogue and The National Archives patent index for named claims; contested-priority details should be checked against Betts's scholarly work rather than watch-trade folklore.
Manufacturers known to have used it
- Marine chronometer makers generally, 19th century
- high-grade pocket watch manufacturers
Photographs
Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.
Sources & references
- Balance wheel, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
- Betts, J., Arnold & Earnshaw: The Practicable Marine Chronometer (National Maritime Museum)
- Gould, R. T., The Marine Chronometer: Its History and Development
Where sources disagree, the disagreement is stated rather than resolved silently.