Circuit diagrams
Could you make a circuit if you were given some batteries, wires, bulbs and a switch? The chances are that you could soon arrange the batteries end to end with their positive terminals pointing the same way and you could also connect the wires and bulbs and switch together into a loop. When you turn on the switch, you would probably think that there is a good chance that the bulbs will shine brightly. As you make circuits today have you ever wondered what batteries looked like in the past or how electric currents were discovered in the first place?

If you could travel back in time about one hundred and sixty years you would find that the battery you had to use was much larger than the small cylinders you use today. It was a jar filled with acid in which there were two metal plates. One plate was made of copper and the other one was made of zinc. You made a circuit by connecting the end of one wire to the top of the copper plate and the end of another wire to the top of the zinc plate just as you connect the ends of wires to the terminals of batteries today.

If you could travel back in time about two hundred years you would be working with the first battery to be invented. This looked rather like a pile of coins ready to be knocked over. In fact it was made of discs of copper and of zinc and of paper soaked in saltwater. The discs were arranged in an orderly fashion. At the bottom of the column was a copper disc. Above this disc was a disc of soaked paper. On top of the soaked paper was a zinc disc then above the zinc disc was another copper disc, paper disc, zinc disc and so on until the top of the pile was reached. When a wire was connected to the bottom disc and a wire was connected to the top disc and their free ends brought close together, a spark of electricity leapt between them.

You may wonder why anyone should want to make a battery like this or to make a battery at all. The reason the battery was made was to test an idea. Some years before the battery was built, a chance observation in a laboratory led to a series of important experiments. The laboratory belonged to the Italian scientist Luigi Galvani. One day, one of his students was dissecting a frog when another student set off an electrical machine and made a spark. The frog's leg kicked even though the frog was dead. When Galvani investigated what had happened he found that the student dissecting the frog had been touching the frog's leg with his knife. Galvani also discovered that the spark had not touched the student, the knife or the frog. He reasoned that somehow the frog's leg could detect the electricity and he carried out a number of experiments to test his ideas.

One of his ideas was to see if the frog's legs could detect the lightning in a thunderstorm. His experiment to test this idea was to set up some frog's legs on brass hooks and suspend them on an iron fence. When a thunderstorm occurred and lightning flashed, the frog's legs kicked but Galvani also discovered something else. When the sky was clear the frog's legs sometimes kicked. One day Galvani had set up some frog's legs and none kicked. After a while he pressed a brass hook close to the iron to make sure the two metals were in contact and a frog's leg jumped. Galvani then found that whenever he pressed the two metals together the frog's leg jumped. He found there was no need for a spark or lightning.

Galvani thought that the electricity came from the frog's leg. He reasoned that animals produced electricity but other scientists were not so sure. One such scientist, who also lived in Italy, was called Allesandro Volta. He agreed that two metals were needed to generate electricity but not the leg of a frog. Instead of a frog's leg he used paper soaked in salt water. He arranged discs of paper, zinc and copper into a pile and found it could produce a current of electricity in a wire. He showed that electricity could not only be produced by frog's legs but by metals and chemicals. His battery, called the voltaic pile, led to the development of the battery you use in your circuits today.

Don't animals make electricity?
Yes, they do. Some animals, such as the electric eel and the electric ray, make large amounts of electricity, which they use for defence or stunning food. All animals, including ourselves, generate electrical currents in our bodies. They flow through our brain, spinal cord and nerves.

Why do animals have electric currents?
The currents carry messages round the body, which tell the muscles what to do. For example, the heart muscle gets an electrical current less than once a second to tell it to keep the heart pumping.

How could the frog's leg kick when the spark did not touch it?
If you rub a piece of plastic you can generate an electrical charge on it. The power of this charge allows you to pick up tiny pieces of paper with the plastic. If you hold the plastic away from someone's hair, the hair stands up and the strands move apart. The power of the electrical charge passes through the air and affects the hair. In a similar way Galvani reasoned that the frog's leg detected the power of electricity in the spark.

Was Galvani's idea proved wrong?
Yes, it was. This is not unusual in science and all scientists must be prepared to be proved wrong. The important thing is to have an idea and test it. If Galvani had not had the idea and tested it, Volta would have had nothing to disagree with and the battery would not have been invented.

Why are symbols used for all the parts of a circuit?
Has there been a time when everyone in the class drew the same object? If there has, you probably found that each person drew the object in their own way. Now imagine them all drawing a circuit you have made. You would get many different pictures and some of them would not be accurate. The purpose of using symbols is to make sure that everyone draws a circuit in the same way.

Do people in other countries have different symbols?
No. The symbols are international. This means they are used all over the world. If two electricians met who could not understand a word of each other's language they would both be able to understand a circuit diagram that was given to them.

Can I add things to a circuit diagram such as lines to show a bulb is lit?
No. You must only use the symbols. If you were to add lines to show that a bulb is lit, a buzzer is sounding or a motor is working, it would only cause confusion to other people who had to look at your diagram. Keep to the symbols and draw them neatly.