A liquid-in-glass thermometer read by an interferometer

In mercury thermometers, the meniscus moves erratically, limiting resolution to about 1 mK. We describe a miniature liquid-in-glass thermometer in which the working fluid is 10 μL of water, topped by a thin layer of oil. By measuring the level of the oil meniscus with an interferometer, the thermometer can detect temperature changes of less than 2 μK at room temperature. We believe the smooth motion of the meniscus is due to a wetting film formed above it. We intend to modify the device for use as a microcalorimeter.