[Bored chemist]

What would stop you using a switchable magnetic clamp like this one
http://www.axminster.co.uk/product-Axminster-Magnetic-Welding-Clamp-with-Release-Switch-715039.htm
(bigger than that, obviously) and having a little electromaget flick the switch?

[lyner]

I imagine that it works with a permanent magnet and an iron 'yoke'. The 'switch' will be a lever which pulls the permanent magnet away from the yoke and putting a 'magnetic short circuit' into the reluctance path. It could work on a bigger scale but might require a lot of force to do this mechanical action with a magnet which can lift a car.