Arc, Voltaic.
The voltaic arc is the arc between two carbon electrodes slightly
separated, which is produced by a current of sufficient strength and
involving sufficient potential difference. The pencils of carbon are
made terminals in a circuit. They are first placed in contact and after
the current is established they are separated a little. The current now
seems to jump across the interval in what sometimes appears an arch of
light. At the same time the carbon ends become incandescent. As regards
the distance of separation with a strong current and high electro-motive
force, the arc may be several inches long.
The voltaic arc is the source of the most intense heat and brightest
light producible by man. The light is due principally to the
incandescence of the ends of the carbon pencils. These are differently
affected. The positive carbon wears away and becomes roughly cupped or
hollowed; the negative also wears away, but in some cases seems to have
additions made to it by carbon from the positive pole. All this is best
seen when the rods are slender compared to the length of the arc.
It is undoubtedly the transferred carbon dust which has much to do with
its formation. The conductivity of the intervening air is due partly,
perhaps, to this, but undoubtedly in great measure to the intense
heating to which it is subject. But the coefficient of resistance of the
intervening air is so much higher than that of any other part of the
circuit that an intense localization of resistance occurs with
corresponding localization of heating effect. This is the cause of the
intense light. Thus if the carbons are but 1/32 of an inch apart as in a
commercial lamp the resistance may be 1.5 ohms. The poor thermal
conductivity of the carbon favors the concentration of heat also. The
apparent resistance is too great to be accounted for by the ohmic
resistance of the interposed air. A kind of thermoelectric effect is
produced. The positive carbon has a temperature of about 4,000° C.
(7,232° F.), the negative from 3,000° C. (5,432° F.) to 3,500° C.
(6,322° F.). This difference of temperature produces a
counter-electro-motive force which acts to virtually increase the
resistance of the arc. The carbon ends of an arc can be projected with
the lantern. Globules are seen upon them due to melted silica from the
arc of the carbon.
Fig. 19. EXPERIMENTAL APPARATUS FOR PRODUCING THE VOLTAIC ARC.