Variable electrical resistance device

A variable electrical resistance device for monitoring displacement of a mechanical component, said device comprising:

• a housing;
• an electrically resistive track supported on or in said housing;
• an electrically conductive wiper moveably supported in said housing and adapted to traverse said resistive track, said wiper being adapted to be mechanically coupled to said external component to follow displacement thereof;
• first and second terminals electrically connected to ends of said track;
• a third terminal electrically connected to said wiper;
• said device being characterised in that means are provided to effect relative displacement of said wiper and said track away from and towards one another whereby contact between said wiper and said track may be broken, or the contact pressure therebetween reduced, during periods when said monitoring by said device is not required;
• said electrically resistive track being secured to a support, said support being moveable to effect displacement of said track away from and towards said wiper;
• said support for said track being mounted on one or more springs, or being spring biased, in such a way that the support is normally held with the track in contact with the wiper and is caused to be retracted and held against the action of the one or more springs or the spring bias, to break contact, or reduce the pressure of contact, between the wiper and the track;
• and a solenoid arrangement is used to retract and hold the support against the action of the one or more springs, or the spring bias, to break or reduce the pressure of the contact between the wiper and the track.