Guillaume, Charles Edouard, gee YOHM, sharl ay DWAR (1861-1938), a Swiss physicist, specialized in improving the accuracy of measurement. Guillaume was awarded the Nobel Prize for Physics in 1920. Guillaume measured the amount by which different alloys (combinations of metals) expand with increases in temperature. He discovered an alloy of iron, nickel and carbon, called Invar, which hardly changes in size as temperature changes. Invar and other similar alloys were used in precision instruments, such as the pendulums or springs of accurate clocks. Ordinary clocks ran fast or slow according to the temperature, but clocks using invar remained very accurate. Guillaume also did important work to establish the offical volume of the liter. It was especially for his work on nickel steel that he was awarded the Nobel Prize.