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All 16-mm. arcs, with only rare exceptions, are known as horizontal arcs (Fig. 29), which merely describes the fact that both carbons are positioned tip-to-tip and in line with each other.
POSITIVE
CAHBON
CONCENTRATED SPOT OF LIGHT FORMED OVER. GATE APERJURi.
I
CHATEfc
Fig. 29
HORIZONTAL MIRROR ARC The crater, formed in the tip of the positive, faces the mirror.
One carbon is known as the positive and the other as the negative, and the positive may be recognised by its slightly greater diameter and by the crater which forms in the tip when it is burning. Note that the crater of the positive always faces the mirror, or if there is no mirror, the condenser lens. But condensers are seldom fitted in horizontal arcs, which explains why they may also be known as mirror arcs.
Positive and negative carbons may be slightly different in their material composition and must not be interchanged in the arc-lamp. And they are still called negatives and positives even though they may be used on A.C. mains.
The composition of A.C. carbons may also be different from carbons used on D.C. Those supplied with the projector will be of a certain length, diameter and composition, and they must only be used on a current supply recommended in the makers' instructions. Serious loss of light may occur if the arc is overrun or under-run, so the projectionist must always keep an eye on the ammeter governing the arc carbon-circuits.
As the carbons burn, they are slowly moved inwards, usually by an automatic "feed", so that the gap between them remains the whole time equidistant. The life of a carbon rod may be from 30 to 50 minutes, or perhaps about three hours, the longer carbons, of course, being used only in those projectors which can accommodate them.
Do not be caught out by letting a carbon burn away before
ill