A thermometer is constituted by a body member and a tip member secured to the body member. A thermal sensor is mounted on the inside of the tip member, adapted for sensing a temperature and producing a temperature signal. A set of leas wires is coupled to the thermal sensor for transmission of the temperature signal. A display unit is disposed on the body member and a processor is disposed to electrically connect the set of lead wires to receive the temperature signal and drive the display unit for display of a corresponding temperature reading. A rear cover is secured to the body member. A mechanical selector is disposed between the processor and the rear cover and the type of temperature scale displayed in the display unit is selected by pushing the mechanical selector from the rear cover.