S.O.S

Open Circuits

Open Circuits

In an open circuit the flow of electricity is controlled by a r switch. A switch is the simplest way of controlling the flow of current. When the switch is open, or off, it creates a gap in the circuit. When the switch is closed, or on, the electricity can flow. Telegraphy is the transmission of electric signals. When it was invented in 1838, it allowed people to communicate directly with each other over long distances. In the same year Samuel Morse introduced his Morse Code - a dot and dash code of short and long electrical signals. These were passed along the wire and decoded at the other end. In 1910, telegraphy was used for the first time to capture a notorious murderer, called Dr Crippen. The international Morse Code distress signal has always been S.O.S. - three short, three long, three short flashes. Soon ships in distress will transmit a unique identification number via satellite instead.

S.O.S

1. You will need two sheets of polystyrene, plasticine, two light bulbs and holders, four 1.5v batteries, 6 short lengths of insulated wire, 4 drawing pins and 2 paper clips.

2. Place a bulb in each bulbholder and stand on the polystyrene boards. Connect the two bulbhoders and attach wires to the other side of each.

3. Use the drawing pins and paperclips to make a switch for each board. Connect the loose wires to the drawing pins, connecting a paperclip to one of them.

why it works

The paper clips act as switches. Both must be in contact with the drawing pins to allow electrons to flow round the whole circuit, lighting up the bulbs. The sender must raise and lower one paper clip to turn the bulb on and off- the receiver must keep the other down to complete the circuit. A mechanical switch in the home is slow to work and produces a spark. It joins and separates electrical contacts in the circuit. The spark or arcing produced creates high temperatures. A relay is an electrically controlled switch; it can be operated by various means, but the most common is an electromagnet called a solenoid (see page 28). The solenoid uses an electromagnet to move a metal rod through a short distance. This opens or closes the relay circuit.
4. Rest two batteries end to end on each board and connect them into the circuit using plasticine. Check the bulbs work by holding down both clips.

5.The bulbs are connected in series. When one bulb lights, both light. A gap in the circuit and the bulbs will not glow.

Bright ideas

• BS.O.S. is 3 short, 3 long, 3 short flashes - now try sending a whole message. Find a copy of the Morse Code. Can you 'translate' a reply? Can you build a two-way circuit that will work from another room? If you use bulbs, the wiring must be long enough to link the two - what happens to the light from the bulb if you use longer wire? How can you tell when each word and sentence ends? Try working out a special code, then you can send secret messages.
• Make a burglar alarm system with a pressure switch made out of folded aluminium foil. Hide the switch underneath a rug and connect it into an electrical circuit with a buzzer or bulb. Which kind of alarm is most effective - a bulb or buzzer?
• How many switches are there in your home? Where are they? What does each switch operate? Carry out a survey.