Switches


A switch turns things on and off.
Every switch you use has the same basic parts. There are two pieces of metal called contacts and a device that can make them touch or move apart. In a light switch in a wall this device is called a rocker and the switch is called a rocker switch.
When you press on the rocker to light a lamp the lamp lights up. This change happens because when you pressed the rocker down it pushed the two metal contacts together and electricity could flow into the lighting circuit in the room.
To turn off the lamp you move the rocker switch again. This time the light goes out. This change happens because when you pressed the rocker up it allowed the two springy metal contacts to move apart and the flow of electricity stopped.
A switch can stop the flow of electricity because the air in the gap between the contacts is an insulator.

When can electricity cross an air gap such as that in a switch?
Air is a very good insulator and so electricity does not pass through it easily. However, electricity can flow when the voltage across the gap is very high or when the gap is very small.


This should never happen in any switch you are using, although, as the contacts close or open, you might here a slight snapping sound because a small spark will jump across the contacts.

What kinds of switches are found in the home?
The rocker switch is the main kind of switch. It is mounted in the wall and used for switching lights on and off. Many wall sockets have rocker switches too.
A door bell is a push switch and a radio may have a rotary switch.

Why do some switches have cords?
Switches with cords may be found in a bedroom and a bathroom. The switch in the bedroom is in the ceiling over the bed and the cord just makes it easy for a person to switch the lights on or off without getting out of bed.
The cord switch in the bathroom has a much more serious purpose. It prevents a person with wet hands from touching the switch. If they did there is a danger that water would get inside the switch. Water is a conductor of electricity. As a result, a person touching an ordinary switch with wet hands could get an electric shock.

Why does the switch on a pelican crossing not work straight away?
A pelican crossing is a crossing for people across a road. It has traffic lights that are operated by people waiting to cross.
The traffic light switch is connected to a timing device. When you press the switch you set off the timer and eventually it switches on circuits which change the lights. If the switch worked straight away it would give drivers very little time to stop. By having a timer drivers can see people waiting at the crossing and prepare to stop if the lights begin to change.

Can switches be used to send messages?
Yes, and they have been for over a hundred years.
In the 1830's the British inventors Sir William Fothergill Cooke and Sir Charles Wheatstone invented the first telegraph system, which used pointers that moved when the circuit was made and broken. A few years later, American inventor Samuel Morse invented a way of sending messages through a circuit which stamped, or embossed, code on a strip of paper. Later, devices were developed to turn the signals into sound clicks, which could be received faster. As a result, people became used to listening to Morse Code with its now familiar dots and dashes.


Morse worked out a code of long and short clicks called the Morse Code. In 1844 a circuit 64 kilometres long was set up between Washington and Baltimore in the USA and Morse sent coded messages along it. In 1850 a circuit was set up between Great Britain and Europe through a cable that was laid on the floor of the English Channel so that messages in Morse Code could be sent between people in different countries. In 1858 a circuit 3680 kilometres long was set up between Great Britain and the USA by laying a cable across the floor of the Atlantic Ocean. This allowed people in different continents to send messages in Morse Code to each other.