Differential thermometers refer to thermometers capable of measuring or indicating the (generally small) difference between two temperatures. Often called thermoscopes, they were commonly used to study thermal radiation. They consist of a capillary U-tube closed at both ends by two glass bulbs. In the tube is inserted a drop of liquid serving as an indicator. When one of the two bulbs (which is often blackened) is exposed to a source of thermal radiation, the air in the bulb expands, pushing the drop along the capillary tube toward the second bulb.
Ref: 86525/2006-09-15

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Thermometer (Messmittel zur Anzeige von Temperaturen. Da ...)
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