Arc lamp

A arc lamp is a system getting of the Lumière using electricity in the form of a electric arc.

The electric arc

Since the discovery of the electric arc by the Chemist English Sir Davy Humphry, in 1809, research was numerous. It had obtained a 8 cm length arc, after having brought in contact two coal rods connected to the two poles of a battery of elements Volta; between the two rods a flame occurred which curved in the shape of arc of circle under the effect of the current of ascending hot air, this is why it gave to this flame the name of electric arc, name which was preserved since.

Provision of the electrodes

For an arc with D.C. current, positive coal is always placed at the upper part and grows hollow at her end in form of Cratère which is carried to Incandescence, it is him which produces the greatest part (85%) of the light emitted by the arc. Negative coal placed below form a blunted point which recovers nodosities; it is carried to the red and produces approximately 10% of the emitted light. The arc him even, i.e. the vapors located in the space ranging between the two electrodes provides approximately 5% of the emitted light. The arc must be laid out so that the crater is turned towards surface to light and negative coal must be thinnest possible in order to form a minimum screen for the light produced by the crater. The arc remains very unstable with the variations of tension or current. To reduce this disadvantage it is enough to place in series an ohmic resistance.

Lighting of the electric arc

To light an arc, it is necessary to approach coals, to bring them in contact and to draw aside them then. This operation is carried out automatically.

Technical evolution

Consumption of the electrodes

With the free air

As coals wear, a device brings them closer to maintain their spacing constant. Two coals wear by combustion, but as positive coal is hotter, it wears more quickly. In a 10 mm length arc under 10 amps and, positive coal wears of 3 cm per hour. On average the duration of a pair of charcoal, for an arc with free air, is 7 a.m. The too fast wear of coals remains the main issue, it involves an expensive labor and the arcs with free air completely disappeared.

In vase semi-close

While making burn the arc in an almost closed vase, the allowed quantity of oxygen is just sufficient to ensure combustion. Coals burn much less quickly (up to 150 hours), even if the luminous output is less in comparison with the arcs with free air. The dynamo of Gram was used to supply the arc lamps with the regulator of Serrin, therefore in D.C. current. The consumption of the coal electrodes is too much fast so that one can consider an industrial use the more so as their cost is far from being negligible. One is thus far from a true innovation.

The candle of Jablochkoff

At this point in time a certain Paul Nicolaïewich Jablochkoff with the simple idea, to place the electrodes, either in glance but side-by-side, vertically. The two electrodes are isolated by a coating from clay and the electric arc takes place at the top of the two ends. A small coal band ensures starting. In order to obtain an equal wear of coals, Jablochkoff already thinks of using the alternative course. It was one of the first to arouse the interest of the industrial use of the alternative course. In 1876, one year before the bulb with incandescence of Edison, of the arc lamps of Jablochkoff are brought into service for the lighting of the streets of the department stores in Paris and London. Jablochkoff still improves its arc lamps in order to be able to replace easily and quickly worn coals.

On some imperfections of the candle of Jablochkoff - Universal Newspaper of Electricity, June 15th, 1880.

electric lighting by the Jablochkoff system - Universal Newspaper of Electricity, August 10th, 1881.

Current

The arc lamps were replaced by the gas-discharge lamps bases or high pressure thanks to the evolutions of the Technologie S.

The arc lamp in the maritime headlights

The arc lamps are hardly any more used today. A particular application of the very powerful arc lamps is the lighting of the Phare S. the headlight of Ushant for example had been installed, on a purely conclusive basis, the Palate of the Lumière during the exposure “Arts and Techniques”, in Paris in 1937. By bad weather, four arcs of 500 amp S provide a white, visible light beyond the horizon. A system Optique produces glares some 270 ms all the 10 S. The turning unit weighs 36 ton S; it is placed in a lantern of 5,50 m of Diamètre to 12 m of Haut.

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