A clinical temperature standard

A temperature standard in the body temperature (clinical) region would be helpful in calibrating fever thermometers, now largely electronic. Temperature standards (fixed points) commonly use the freezing point of a pure material, often a metallic element. A check of the periodic table reveals only one suitable candidate, rubidium, which melts at 38.89° C(102.00°F). Rubidium, however, is an awkward material to handle. A hydrocarbon plastic (polystyrene, polyethylene, polypropylene) would make a suitable container for the same reason. The device should require only a small quantity of rubidium. Detecting the melting/freezing transition should not be difficult, as rubidium has two helpful properties, either one of which may be used for the purpose. On melting its volume expands 3.7%, and its electrical resistivity rises 58%, from 14.21 to 22.52 μohm-cm. A unity ratio ac bridge might be used, one arm being a solenoid surrounding the rubidium container, and the other a solenoid surrounding a metal of similar dimensions, with resistivity intermediate between that of liquid and solid rubidium, e.g., tin-lead alloys. A three-coil arrangement of the type used in linear displacement transducers might also be used with the two metals