Electric sources of light

Electric light, which has so thoroughly changed our life in the hours of darkness, is, in the main, produced in two ways, either by the filament lamp or the electric discharge lamp. The filament lamp depends on heating a filament of metal or carbon to incandescent heat inside a glass bulb which has been evacuated or into which an inert gas has been introduced to prevent burning out of the filament.

The electric discharge lamp began as an arc between carbon electrodes in air, the carbon being more or less rapidly consumed. It is now more commonly an arc in an atmosphere of mercury vapour, sodium vapour or neon gas inside a glass tube or bulb. The glass may be coated with fluorescent material to change the colour of the light.

The first practical electric lamp was the carbon arc. This was demonstrated by Humphry Devy in London in 1810, but there were no machines to provide the electric current required. In spite of this, the arc lamp, fed by batter­ies of wet cells (accumulator battery with moist elements), was used for theatrical effects at the Paris Opera in 1846.

The Russian inventor P. N. Yablochkov also worked at the problem of carbon arc lamps, and it is largely to him that we owe the application of the lamp for practical purposes.

Arc lamps were used in ever increasing numbers for street lighting and other applications up to the end of the First World War. Yet it was a very inefficient source of light con­suming much current and requiring constant attention. The arc lamp was obviously of no use for the home.

The aim of many early experimenters was the production of light sources small enough to be used in the home. Many experiments were made with carbon filament lamps from 1840 onwards. No filament lamp could last long enough to be economical in use, until the discovery of the technique of exhausting the lamp bulb to a fairly high degree of vacuum by means of a special air-pump.

The practical carbon filament lamps produced with that instrument proved successful. Still they were only about one-six as efficient as arc lamps.

Carbon was not a suitable material for filaments, and at the end of the century research was directed towards making filaments of metal which would be both fine enough and strong enough. Tantalum was used with some success but it was tungsten that gave what is still the best of filaments. Efficiency was improved by coiling the filament and putting it in an atmosphere of inert gas (argon).

Automatic machines mount the filament assembly, exhaust the air, introduce the argon gas and seal the bulbs.

Research followed into the possibility of changing the quality of the light by means of fluorescent materials. In 1939 the first practical high efficiency fluorescent lamp was introduced. It was the result of work that had been carried on over many years and in various parts of the world on the materials that can be excited by an electric discharge.

The researches continued and lamps have been progres­sively improved ever since.