Wednesday, June 18, 2008

An Arc Lamp


Scientists Just Beginning to Study Its Heat and Light Characteristics.

Long as the arc lamp has been in use the nature of its phenomena is still far from being known or understood. In a communication recently made by M. Violle to the French Academie des Sciences interesting evidence was adduced to show that the temperature of the arc increased with the current. Photographs of the crater of the carbon, on which the little oval of white light appears to stand, showed that its intrinsic brilliancy was the same with 1,000 or 1,200 amperes as with 10 amperes.

Examining the spectra of the arc and of the positive carbon, M. Violle found a large number of the bands of the spectrum of the arc stood out brilliantly against the continuous spectrum of the crater. They were, however, unsteady and varied in brightness, being brighter the greater the current. It is doubtful whether the brilliancy of the bright bands forming the spectrum of a gas light are related to its temperature in the same way as are the corresponding portions of the continuous spectrum of a solid body. The doubt is increased when the gas is illuminated under the action of electricity, which seems capable of converting itself into light without heat.

On the other hand, if the arc behaves like a conductor carrying a current, it must be the seat of an evolution of heat proportional to the energy consumed, so that its temperature should increase with the current. In any case the cause which limits the temperature of the crater does not apply to the arc. M. Violle tried to determine the temperature of the arc by introducing into it a thin rod of carbon. A carbon rod introduced into the arc produced between two poles of the same metal burns away differently with different metals, slowly with copper, quickly with zinc, showing, however, a much higher temperature than that of the volatilization point of zinc.

M. Violle concludes that the temperature of the arc is, generally speaking, higher than that of the positive carbon, and that it increases with the energy consumed. — Philadelphia Press.

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