Making bulbs brighter and dimmer
If you think about it, a light bulb is a strange object. Just look at one. It is a hollow ball of glass with a curly wire in the middle of it. We take light bulbs for granted but how did they come to be such odd looking devices?

At the beginning of the nineteenth century, not long after the battery was invented, a scientist called Humphrey Davy found a way of using electricity to make a very bright light. His invention was called an arc lamp. In this lamp, the current of electricity flowed between two carbon rods. Davy's first lamp also produced a great deal of heat and smoke. However there was a problem with an arc light: the carbon rods burnt down as they produced light and had to be adjusted to keep the same distance so the spark could leap between them.

The search was on for a better way to make an electric light and in 1848 an English scientist called Joseph Swann began experimenting with strips of carbonised or slightly burnt paper. When electricity was passed through the paper in air the paper burnt away completely very quickly. To make the paper last longer Swann put it in a glass tube from which air had been removed. Unfortunately the battery he used was not very strong and the paper only produced a dull red glow when electricity was passed through it. If he had used a stronger current, the paper would have glowed much more brightly and Swann could have seen how useful the device could be. Swann also had problems with keeping air out of the tube. Despite his limited success, Swann continued to work on his ideas over the years and in time better ways were invented of keeping air out of the tubes. This helped Swann to make a better bulb.

By this time another scientist, called Thomas Edison, who lived in the United States, was at work on trying to make a useful light bulb too. At this stage, neither scientist knew of the work of the other. Edison thought that the metal called platinum would help make a bright light in an airless tube but after a year he looked for other materials. Eventually he decided on the same material that Swann had also started to use - a thread of scorched cotton. Amazingly both scientists introduced their lamps to the world at the same time - in 1879. In 1880 Edison - celebrated his success by lighting the main street of a town called Menlow Park in the United States. Thousands of people turned out to see the lamps, including newspaper reporters from all around the world. By 1881, Swann's light bulbs were used in England to light the House of Commons in Parliament and in 1882, Swann's light bulbs were also used to light the British Museum.

The bulbs invented by Swann and Edison did not provide many hours of light before the filaments burnt out. In the early twentieth century a scientist called Irving Langmuir showed that tungsten filament was more useful than the one made of cotton. He also showed that the metal filament would last longer if it was not put in an airless tube but one filled with a gas which would not make the metal change. At first, the gas called nitrogen was used. Today a gas called argon is used in many light bulbs. In 1931 the filament of tungsten wire was made into coils like the ones we see today. By coiling the wire, a greater length can be used to make light and this makes the bulb shine even brighter.

When you use bulbs in a circuit, the filaments give you an indication of the current flowing through them. A dimly glowing filament indicates a small flow of electricity while a brightly glowing filament indicates a large flow of electricity. If you have two bulbs in a circuit you can make them glow more brightly by adding another battery. If you want them to glow more dimly you could take away a battery or add another bulb in line with the other two.

Next time you look at a bulb, think of the scientists who worked on making airless tubes and batteries and struggled to find a filament that would keep alight for a thousand hours like the light bulbs in our homes today.

Why had scientists made airless glass tubes in the first place?
An airless space is known as a vacuum. If you look back through the centuries you will find that scientists seemed to work on different things at different times. For example, in the seventeenth century some scientists were interested in studying air. A question which arose in this study was, "What would happen if you could pump air out of something?" A scientist invented a pump and found he could remove air from containers and create a vacuum. In the next century, when scientists were investigating electricity, they wanted to know if electricity could flow through a vacuum since earlier scientists had shown they could be made. Swann and Edison simply used the vacuum tubes to try out their ideas about electricity flowing in a filament in a vacuum.

What did people use to light their homes before electric lights?
They used candles, oil lamps or gas lights. In fact when electric lights were introduced into hotels a notice had to be put by the light switch to tell people not to try and light the electric light with a match! This was because people were so used to using matches to light the candles or lamps in their homes.

Did arc lamps ever have a use?
Yes, they did. They were used in lighthouses in the nineteenth century. Some were used to provide powerful spot lights in the twentieth century.

Why does adding batteries make the bulbs brighter?
A battery gives a push to the electric current. This push is measured in volts and is called the voltage. The batteries you use in circuit work are usually cylindrical and have a voltage of 1. 5 volts (V). If you put two batteries together, they make a voltage of 1.5V + 1.5V = 3V. If you add another 1.5V battery to the two the voltage rises to 4.5V and the bulbs shine even more brightly.

Does it matter which way round you arrange the batteries?
Yes, it does. The batteries must be arranged in a line with their positive terminals all facing the same direction.

What would happen if you arranged two batteries so that their positive terminals faced each other?
The voltage or push of one battery would work against the voltage or push of the other. If both batteries were evenly matched no current would flow.

Can you just keep adding batteries to make the bulbs brighter and brighter?
Each bulb is designed to work best at a particular voltage. For example a 3V bulb would work with a 1.5V battery but it would not shine at its brightest. If another 1.5V battery were added to the circuit the bulb would shine brightly without being in danger of burning out. If another 1.5V battery is added to the circuit the voltage would now be 4.5V. This would make the bulb shine even more brightly but it would soon burn out.

How does a dimmer switch work in a home?
The electricity in it is generated in a power station not by batteries. So a dimmer switch does not control the number of batteries in a circuit. There is a long coil of wire in a dimmer switch. As you turn the switch you either increase the amount of wire the electricity has to flow through or you decrease the amount of wire. When electricity flows through a wire it has to do some work and loses some power. If you turn the switch so the electricity has to flow through a large amount of wire the current has to do a great deal of work and not much power is left for the lamp. This means that the lamp glows dimly. If you turn the dimmer switch so the electricity has to flow through only a small amount of wire, there is more power to use on the light and it shines brightly.