Incandescent Light Bulb

An incandescent lamp makes light by passing current through a thin filament until it glows white-hot, inside a glass bulb emptied of air so that the filament does not burn. Joseph Swan and Thomas Edison made it practical in 1879, and it lit homes and streets for more than a century.

What it is

The incandescent lamp is a glass bulb with a fine thread inside, the filament, which glows when an electric current passes through it. Before it, homes were lit by flames: candles, oil lamps and gas jets, which smoked and often started fires. Electric arc lamps existed, but they were far too bright for a room.

The lamp on the plate is the kind Edison’s factory made in the early 1880s. Its descendants were the usual household lamp for most of the twentieth century, until compact fluorescent and then LED lamps took their place.

How it works

Any material heated enough gives off light. Electrical resistance turns the energy of the current into heat, and a thin filament passes from dull red through orange to white heat. The hotter it runs, the whiter and more efficient the light, and the faster the filament evaporates.

In air, a white-hot carbon or metal thread burns up in moments, so the bulb is emptied of air, or later filled with a gas that does not react. The pump is attached through a narrow glass tube at the top, which is melted shut afterwards; that is the small exhaust tip on old lamps.

In a Sprengel pump each falling drop of mercury traps a little air and carries it down the tube.

The wires that carry the current to the filament must pass through the glass. Glass and metal expand by different amounts when heated, and a mismatched seal cracks as the lamp warms. Platinum expands at almost the same rate as the glass used in early bulbs, so those lamps had short platinum wires through a glass stem, joined to cheaper copper inside the base.

Edison designed the lamp together with the supply system. A filament of high electrical resistance draws little current at around 100 volts, so lamps can be wired in parallel, each with its own switch, on copper mains of reasonable size.

Only about 5 to 10 percent of the electricity in an incandescent lamp comes out as visible light; the rest leaves as heat.

How to build it

A lasting lamp needs a glassblower, a vacuum pump that reaches a very low pressure, and a filament sized for its supply voltage. Only the first step below is a safe home demonstration; the others outline how the first lamps were made in a workshop.

See the principle. Clamp a short piece of thin pencil lead between two metal clips on a ceramic tile and cover it with an upturned glass jar. Feed it from a bench power supply set to about 12 volts with its current limit turned down, then raise the limit slowly. The lead glows orange, then burns through as it meets the oxygen under the jar, which shows why the bulb must be emptied of air.

Carbonise the filament. Workshops packed loops of bamboo or cotton thread in powdered charcoal in a lidded, vented crucible and heated it red-hot in a furnace for hours. Starved of air, the fibre turns to carbon instead of ash; the escaping gases are flammable and contain carbon monoxide.

Seal the leads. Two short platinum wires, or another metal matched to the glass, were fused into a glass stem, the filament was clamped to their tips with carbon paste, and the stem was fused into the neck of a blown bulb with a thin exhaust tube at the top. A low-voltage filament must be shorter and thicker than a 100-volt one.

Pump and flash. With the bulb on the pump, a gentle current through the filament drove gas out of the carbon and the glass. When the pressure stopped falling, the tube was melted shut with a flame.

Fit the base. A brass screw base was cemented to the neck with plaster, one wire soldered to the shell and the other to the centre contact.

Safety. Mains voltage can kill: run experiments only from a current-limited bench supply under 24 volts, never from a wall socket, and never short a car or lithium battery across a filament. Filaments, glass and clips get hot enough to burn skin and set paper alight, so work on a tile, away from anything flammable. An evacuated bulb can implode and throw glass; wear goggles and work behind a clear shield. Mercury vapour is toxic: do not build or use a mercury pump at home. Never heat a sealed container: carbonising gives off flammable gas and carbon monoxide and belongs in a vented furnace outdoors, not in a home.

History and evolution

Carbonised bamboo fibres lasted far longer than the paper and thread used in the first lamps.

Humphry Davy made a platinum strip glow with current from a large battery in 1802, and in 1840 Warren de la Rue placed a platinum coil inside a vacuum tube. Platinum was too costly; carbon was cheap but burned in the air that the pumps of the day left behind.

In England, Joseph Swan worked on carbonised paper filaments from around 1850. An 1860 trial in a poorly evacuated bulb failed. From 1875 he used the new Sprengel mercury pump, and between December 1878 and February 1879 he showed working carbon lamps in Newcastle upon Tyne. In December 1880 he fitted lamps at Cragside, the home of the industrialist William Armstrong.

Around 1910 drawn tungsten wire replaced carbon and gave a whiter, more efficient light.

At Menlo Park in New Jersey, Thomas Edison’s laboratory chose a high-resistance carbon filament. On 22 October 1879 a lamp with a carbonised cotton thread burned for 13.5 hours; the team then raised the current, and about an hour later the bulb overheated and cracked. On the evening of 31 December 1879 Edison lit his laboratory and the street outside for visitors, using horseshoe filaments of carbonised cardboard. In 1880 he settled on bamboo, with reported lives of around 1,200 hours, and in 1882 the Pearl Street Station began supplying lamps in lower Manhattan. After a patent dispute in Britain, Swan’s and Edison’s companies merged in 1883 as “Ediswan”.

Around 1910 William Coolidge at General Electric found how to draw brittle tungsten into fine wire. At white heat tungsten evaporates much more slowly than carbon, so lamps could run hotter. Irving Langmuir then filled bulbs with an inert gas, nitrogen at first, which slows evaporation further, and coiled the filament. From 2009 the European Union, followed by many other countries, began phasing out ordinary incandescent lamps; compact fluorescent lamps and later LEDs replaced them.