A fool proof and pocket size electronic thermometer is described that is particularly useful for obtaining oral or rectal body temperatures. The thermometer includes a casing on which is mounted a scale for visually indicating temperature readings. A simplified plug-in connector is used to connect a miniature electrical cord to the casing. Such cord terminates in a disposable semiconductor junction through which a constant current is made to flow so that the voltage developed across it is dependent upon its temperature in a straight line function. The casing encloses small operating batteries and low power consumption circuitry for providing the constant current flow through the semiconductor junction and translating voltage changes developed across it into temperature readings.Circuitry is provided for disabling the measuring circuit and driving the indicator off scale to indicate battery voltage below a predetermined level.