An electrical clinical thermometer system including a probe for insertion into a body cavity. A temperature sensor is carried by the probe whose electrical resistance changes with temperature. This probe is connected into a wheatstone bridge circuit which is restored to a balanced condition by an adjustable resistor which is mechanically connected to a readout device that indicates the temperature at the sensor when a meter in the bridge circuit is at its reference setting. The system is insensitive to voltage changes such as from a battery whose voltage declines. The readout device holds its reading when the probe is removed from the cavity, and may be either mechanical or digital. A sensor for this system comprises an elongated metal body with a discontinuity which is bridged by a plurality of fuse-like resistances of different values which can selectively be destroyed so as finely to adjust the resistance of the sensor. Calibration means for the adjustable resistor enable the use of inexpensive, relatively low-tolerance components to secure a very accurate measurement.