In a sensing unit according to the present invention, a spring portion having a support portion and a movable portion is conductive. The derivatized phenol is a di-tert-butyl phenol, preferably a 2,6 tert-butyl phenol substituted at least at the 4-position with an aliphatic, aromatic or aliphatic-aromatic moiety, of C2 or greater. When the transmitter is operated under a first mode, both the first mixer and the second mixer receive oscillation signals. The second connector has second terminals configured to make contact with the first terminals, and a second housing which has second fitting guide portions formed at both ends in the longitudinal direction of the second housing and configured to engage with the first fitting guide portions.