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the end of a reel. Time both positive and negative carbons the first time you use them.
New carbons may contain a certain amount of moisture, which is the cause of the arc "spluttering". The moisture quickly boils and blows off minute particles of carbon, which, impinging on the mirror or condenser, produce a form of damage known as pitting. To prevent pitting, a special shield is first brought into position in front of the mirror when the arc is struck and it is kept there until the spluttering stops. At the end of each reel the mirror should be brushed with a special brush of the type which cannot harm the surface.
But to prevent any such damage as pitting, new carbons should be made perfectly dry before they are used, so purchase them in bulk and store them in the dry est place you can find. The next set of carbons to be used can be placed on top of the lamphouse for a few minutes, so that they thoroughly dry out.
Complete instructions on the use of arc projectors are issued with each machine purchased, but for the reader who requires further information on the subject of arcs, we recommend "The Complete Projectionist" by R. H. Cricks, and Richardson's "Blue-book of Projection". These books are of course written principally for 35-mm. users, but there is little difference between 35 or 16-mm. arcs — it is merely a question of size.
Coloured Films and Slides
The chief thing to remember when using coloured films is that they usually require a little more light, and that this must be considered before starting a show. More light usually means more voltage to the lamp, but as very few projectors are fitted with a variable lamp control, it is usual to control the voltage to the lamp by means of the transformer tappings. The necessity for a transformer having an adjustable tapping device which may be used during projection at once becomes obvious, for without such a device, the operator may have to stop the show in order to adjust the transformer. But this should not be necessary when using a 750 or 1,000-watt lamp, unless, of course, the length of throw is excessive, or the print unusually dark.
The "average programme" now often includes at least one coloured film among a number of monochrome ones, and for such a programme the fixed-type transformer should be ad
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