Phase-change (dot matrix) thermometers
A non-electrical alternative is to use samples of inert chemicals which melt at progressively higher temperatures from 35.5 °C to 40.5 °C in steps of 0.1 °C.
They are mounted as small dots in a matrix on a thin plastic spatula with a protective transparent cover. This is placed under the patient’s tongue. After a short time the spatula is removed and it can be seen which dots have melted and which have not: the temperature is taken as the melting temperature of the last dot to melt.
These are cheap disposable devices and avoid the need for sterilising for re-use.
Other devices operating on similar principles are available for applying to patients’ foreheads, but they are less reliable because the forehead is more remote from the core body temperature, and hence its temperature is more variable.
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