A maximum reading device for measuring temperature requires some means for storing or retaining the maximum value until it can be read. In producing clinical thermometers, it is current practice to modify the thermometer tube to inhibit return flow of the liquid until desired. This has taken the form of a constriction in the bore of the thermometer tube.
Forming of the constriction has not lent itself to a machine operation. Rather, it has required a high level of manual skill, and frequently a considerable amount of reworking to obtain a successful product.