A thermometer includes a body member and a tip member with a thermal contact surface secured to the body member. A thermal sensor mounted on the inside of the tip member is adapted for sensing a thermal contact surface and producing a temperature signal. A set of lead wires is coupled to the thermal sensor for transmission of the temperature signal. A display module is disposed in the body member. An electric generator with a start voltage and an operation voltage is disposed in the body member. A capacitor is electrically connected to the electric generator for storing the electric energy generated by thereof and supplying the electric energy to the processor. A voltage switch with a threshold voltage is electrically connected between the capacitor and the processor, in which the threshold voltage is lower than the operation voltage.