Traffic Control for Electrons

In our last Fact Sheet No 9 we talked about conductors. One important part we need in almost all applications is the SWITCH.

In its simplest form (or put it differently: "in PRINCIPLE") any two conductors that can be pushed together and pulled apart again are a switch. Whenever those two conductors touch, they allow current to flow by closing the electric circuit.

In practice, switches have hundreds of different shapes, sizes, - and uses. The simple ones you use often and they are easy to understand how they function. We all operate light switches and door bell buttons, switch on and off our television sets and computers.

It gets more interesting once we look into the more complex switches. They have been designed for special functions and here we look at some of them.

An interesting switch you may have seen in houses where there is a temperature control for a heater or an air conditioner is the 'mercury switch'. Mercury is a metal that is liquid at normal temperatures. It is inside an evacuated glass tube with two conductors - electrodes - built in on one side. As the tube turns sideways and the mercury moves around the two electrodes, the switch is closed and current flow is assured. 'Bi-metal- strips' consist of two layers of different metals that are chosen for different expansion rates. When the temperature changes they bend and tilt the mercury tube to either open or close the switch.

A mercury switch. As it tilts to the left, the mercury moves to the left and the contacts are open.

--- Just a reminder here: Should such a glass tube ever break, mercury is a poison and should be treated with great care!---

Another interesting variety is the 'reed switch'. Two magnetically sensitive reeds, again inside a glass tube, will open or close. In connection with a small bar magnet they are often used in doors and windows to switch on the alarm in case a burglar should decide to come for an unexpected and most unwelcome visit. As soon as the magnet moves away from the reed switch, say, when a door is opened, the switch either closes or opens (according to the circuit in the system) and puts the siren and flashing lights into action.

ARRANGEMENT OF A REED SWITCH

By the way: pumping the air out of such devices prevents oxidisation as sparks develop during the operation of the switch. Such oxidisation can contaminate the metals and might destroy the switch in time.

These are symbols as used in circuit diagrams

Then there are the relays. They are operated by electro-magnetism and often switched from a remote location. We'll talk about those later in the series.

The smallest switches we know of are of almost atomic size. They are in the memories of our computers. Millions and thousands of millions of them are snuggled together in a small space. Also, there are diode and transistor switches and many, many more variations on the topic 'switches'.

One last word as to the most important factors in designing a switch:

• 1) How much current it has to be able to conduct without getting hot. Heavier currents need larger conductors and surface areas.
• 2) Its voltage rating. A switch for a 9V battery can be small, whereas a switch for mains power has to be much better insulated and the conducting parts have to be at a larger distance from each other.
• 3) How are they required to operate? Turning a knob, pushing in and out, or using a lever of some description.

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This concludes our first ten Electronics Fact Sheets for Juniors. At the time of writing, the total number of prepared topics is in the order of 100. This number may well get bigger in time. So may the pages on this site - in time.

Also remember, I am always interested in your ideas, questions and feedback. You are most welcome to write to me: Peter, at my email address.

For those of you who are really curious, here is a jump to a very controversial subject in Fact Sheet 31.

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Copyright © 1997-2010 Peter Schmedding, Child Development Projects, Canberra, Australia.