a quick registering thermometer utilizing a sensor that has electrical resistance varying with temperature located at the tip of a probe , the sensor having relatively small thermal mass and short thermal time constant . within seconds of contact with the patient , the circuitry communicating with the sensor determines the rate of change in resistance of the sensor and , in turn , the temperature that the sensor would reach if allowed the length of time it would need to reach equilibrium with the contact surface of the patient . based on the equilibrium temperature determined for the surface of the patient , the circuitry calculates and registers the core body temperature . included in the circuitry is an error adjustment mechanism for changing the calculated temperature as a function of the number of displays of the thermometer in a given period of time .