The Davy Lamp (or Davey Lamp) - a method of preventing a flame from igniting surrounding gas. Invented by Sir Humphrey Davy in 1815.

The need for a safety lamp

The invention of the steam engine in the 1790's for lifting and pumping had allowed mines to be dug much deeper than previously. Unfortunately, the problem of ventilation hadn't been solved, and the new deeper mines were much more at risk of gas buildup. Gases such as methane ('firedamp' or 'minedamp' are the miner's names for gases which build up in mines) are released when mining takes place, and in an enclosed, subterrenean space, build up rapidly to dangerous levels. Miners needed a method of lighting that didn't carry the risk of igniting flammable gases. Rotten fish (believe it or not, rotten fish do glow in the dark1) were used as a light source if the danger of explosion was thought to be great, but they only glowed dimly and also smelt appalling. A spate of colliery explosions in the north of England in the 1800's raised attention for the need of a safe light source.

Invention of the 'Davy Lamp'

There is some dispute over who invented the first miner's safety lamp, Dr. William R. Clanny, Sir Humphrey Davy, and George Stephenson were all independently working on the safety lamp at the same time, and all had some knowledge of each others work.

Dr. Clanny came up with the principle of isolating the flame in 1813, his first lamp designs involved enclosing the flame within a water reservoir, and using a pair of bellows to supply the flame with air. This clumsy design never saw use in a mine, but proved the principle of isolating the flame.

At the same time, Sir Humphrey Davy was conducting his own experiments on flame isolation, and discovered that if two containers were filled with explosive gas, they might be connected together by a narrow tube, and the gas in one of the chambers could be ignited without the flame reaching the adjoining chamber.  This meant that a flame in a lamp, fed mine air through small holes, would not ignite the surrounding air of the mine.

Further experiments showed that a fine metal gauze of no more than 24 holes per inch acted in the same way. The flame is unable to pass through the gauze, because given a large enough surface area, the maximum temperature of the gauze screen is below the ignition temperature of the flammable gas. This effect can be observed by holding a piece of gauze over the flame from a gas jet, the gauze will dissipate the heat fast enought that the the flame will only burn above the gauze.

Davy's safety lamp was born, consisting simply of a naptha-fuelled lamp encased in a cylinder of gauze. This prototype safety lamp was first trialled at Hebburn Colliery on January 9, 1816. This principle of enclosing the flame with gauze was used in almost every flame safety lamp since.

When the Davy lamp was introduced into general use, miners were given strict instructions never to open the gauze while underground, and to be very wary of damaging the gauze. Any larger holes in the gauze would render the lamp useless. A further revision of the Davy lamp was to add a hood over the gauze and to enclose the flame itself behind glass. These revisions prevented damage to the gauze and allowed the lamp to emit more light. Safety lamps of this type continued to be used right up till the introduction of electric lighting in the mines.

Gas Measurement

Another effect of being able to regulate a flame underground was that the lamp could be used for the measurement of the gas concentration in the air. The flame would change colour - it would grow larger and develop a blue tinge if there was firedamp in the mine. The flame would also go out if there wasn't enough oxygen in the air (called chokedamp). This superseded the use of caged animals for gas detection, and safety lamps continued to be used for this right up until the development of modern gas detection technology.

This writeup is part of wertperch's nodeshell challenge!
Note 1. Rotten sea-fish glow in the dark because of the presence of luminous bacteria. The light emitted by such decomposing sea creatures is greenish white in color and is believed to be caused by a chemical process within the bodies of the microscopic bacteria. This process involves the oxidation of a substance, luciferin in the presence of a stimulating agent, luciferase. UK Institute of Biology Editorial, August '01: