These, like ordinary chemical thermometers, are made from special tubing with a capillary bore. The bulb is blown by a mechanical blower. The arrangement for preventing the mercury running back into the bulb is very simple. A very small bulb is blown so that the capillary tune becomes somewhat widened a little above the bulb. While the tube is still hot it is nipped or pressed so that the enlargement becomes much flattened; the flattening of this bulb breaks the thread of the mercury, so that on cooling the mercury in the tube above the constriction remains, while that below runs back into the bulb. On heating, the mercury easily rises through the constriction.