Excerpt from Recent Developments in Drawing by the Aid of Projection Apparatus, Used on the House Lighting System
In the first place one must possess an arc lamp in which the carbons are held at right angles to each other; and, in passing, it is gratifying to remember that the now so widely used right angled arc lamp for projection, was devised in 1894 by Mr. Albert T. Thompson of Boston. Without a lamp for holding the carbons in this position so that the one giving the light can remain constantly in the optic axis, the small drawing outfits would be practically impossible.
The carbons must be small, from the small current used, viz 3 to 6 amperes; and they should be soft cored. A small arc lamp is a convenience, but any arc lamp designed for the carbons at right angles can be used by employing bushings or adapters to hold the small carbons.
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