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Contact in the classroom

Science education suppliers offer a wealth of switches for classroom use, some more useful than others. I've a primary science catalogue in front of me, presenting 15 or more different switches on a single page. Some are simple onoff switches. Some look identical, but can be push-to-make (like computer keys) or push-to-break (like the fridge light switch). Some even toggle - the first push switches them on and the second switches them off.

Then there are the fancy ones. A "double pole" switch can be used to reverse the flow of current, and so change the direction of an electric motor. A micro-switch is just that - a tiny switch that costs pennies to buy but pounds for a washing-machine engineer to replace. Many of these have three terminals. One is "in" and the other two are "out"; using one or other terminal means that the same switch can be used to either make or break the circuit.

Reed switches are special switches that respond to a magnet. Putting one of these in a circuit and bringing a magnet close to them will switch the current on or off. They are useful in models - a magnet on a wheel will work the switch every time the vehicle passes a reed switch. Reed switches are contained in a tough glass tube and need handling with care.

Burglar alarm pads - operated by the weight of the boot of a careless burglar - are featured in the curriculum for science and design and technology. "Tilt" and "tremble" switches that respond to movement are used to detonate bombs. You can make your own - but be careful you're not arrested for running a terrorist training school.

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