A calibration system adapted to calibrate a resistance of an electrical device having a lead wire comprises a resistance detector adapted to detect the resistance of the electrical device, a first container containing an etching solution adapted to etch the lead wire, and a heater configured to heat the electrical device. If a first resistance of the electrical device detected by the resistance detector at a first temperature is within a first predetermined range, the electrical device is heated with the heater to a second temperature higher than the first temperature. A second resistance of the electrical device is detected by the resistance detector at the second temperature. If the second resistance is beyond a second predetermined range, the lead wire is etched by the etching solution to adjust the resistance of the electrical device until the second resistance at the second temperature is within the second predetermined range.

A thermally protected metal oxide varistor includes a body, a first electrode, a thermal fuse, and a glue. The body is made up of a crystalline microstructure including zinc oxide mixed with one or more other metal oxides. The first electrode is located on one side of the body and is connected to a first lead wire. The thermal fuse is connected between the first electrode and the first lead wire. The glue is to be deposited over the thermal fuse as well as over a top portion of the first lead wire.

A resistor material including a plurality of crystalline phases having a positive temperature coefficient of resistance, and an amorphous phase having a negative temperature coefficient of resistance and having a resistivity higher than the crystalline phase, in a mixed state, is provided. Moreover, a resistor element having a resistor film configured by the resistor material described above, and a method of manufacturing a resistor element by forming a film of an amorphous material having a negative temperature coefficient of resistance and subjecting this film to an annealing treatment to obtain the resistor element described above, are provided.