This paper was prepared to assert that coil springs could be useful as displacement sensors as well as mechanical
parts. We have investigated the displacement dependence on the inductance of a coil spring and resonance
frequency of a simple LC circuit in which a coil spring took a part as an inductor. The changing ratio of inductance
to displacement was 6.6 %/mm. Inductance and/or inductance change of a coil spring was explained with
FEM simulation. The changing ratio of the output voltage, which was obtained with the LC circuit, to displacement
was estimated to be 6.23 %/mm.