What is a notable characteristic of the watch balance spring developed by Charlie Edouard Guillaume?

The notable characteristic of the watch balance spring developed by Charlie Edouard Guillaume is that it was made of Invar. Invar is a nickel-iron alloy that was discovered to have a very low coefficient of thermal expansion, which means it does not change size significantly with temperature fluctuations. This property is crucial for maintaining accurate timekeeping in watches, as temperature variations can cause traditional materials to expand or contract, leading to timing inaccuracies.

By using Invar, Guillaume significantly improved the stability and precision of watch movements, making it a revolutionary development in horology. This advancement allowed watches to provide more reliable performance across varying environmental conditions, ultimately enhancing the overall accuracy of mechanical timepieces.