Why does an Electric Bulb Give off Light?

A light bulb gives off light because an electric current is passed through its filament, a thread of tungsten metal thinner than a human hair, which then becomes white hot.

Sir Joseph Swan (1828-1914) in England and Thomas Alva Edison (1847-1931) in the United States constructed the first incandescent electric lamps in 1879. They succeeded in preventing the rapid burning up of the filament by oxidation, but their problem lay in the choice of a suitable material for the filament.

Edison sat in his laboratory watching a filament of charred cotton thread glow in a glass bulb, which had been exhausted of air, for 40 hours. But the thread was too fragile to sustain the heat provided by the electric current. A material was needed that would stand great heat, for the hotter the filament, the brighter is the light given by the lamp.

But even with the present day use of tungsten, the problem still remains in that the more a filament is heated, the sooner it will burn away. To solve this problem, gas discharge lamps were invented.

These consist of glass tubes filled with sodium or mercury vapor, or neon gas. At each end of the tubes are electrodes, or contacts. When an electric current is applied to one of the contacts it passes through the gas to the other contact, causing the gas to glow and give out light.

When does an Electric Short Happen?

An electric short or short circuit can occur when two wires which are supposed to be separated come into contact, perhaps through loose connections in a socket or worn out insulation. A screwdriver inserted into a socket can also cause a short circuit and severe injury to the person holding it.

Ordinarily the circuit followed by an electric current forces it to overcome certain resistances by generating energy which which heats an electric toaster or lights an electric lamp. But when the two sides of an electric circuit are accidentally connected by a path which cuts out this resistance, a short circuit occurs, because electric current always flows through the path of least resistance.

When this happens the current flow becomes so high that it melts the piece of wire or fuse inserted in the circuit as a safety measure and so cuts off the voltage. Damage to an appliance is thereby avoided, for if the excess current continued to flow, it would cause the appliance to become very hot.