. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. most cases a coat of whitewash serves verywell. Bright reflecting sur-faces do not illuminate thecopy evenly. In regard to the choiceof lamps, some care mustbe exercised to get a suit-able type. If left to theelectrical contractor he mayput in a form which maybe very well suited for streetor general lighting, but notfitted to the needs of theprocess worker. The lampitself may not have beenmade to carry so heavy acurrent as 25 amperes,street lamps generally run-ning about 10 to 15 amperes,and
. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. most cases a coat of whitewash serves verywell. Bright reflecting sur-faces do not illuminate thecopy evenly. In regard to the choiceof lamps, some care mustbe exercised to get a suit-able type. If left to theelectrical contractor he mayput in a form which maybe very well suited for streetor general lighting, but notfitted to the needs of theprocess worker. The lampitself may not have beenmade to carry so heavy acurrent as 25 amperes,street lamps generally run-ning about 10 to 15 amperes,and although the lamp maybe adapted by changing the coils and carbon holders,it is never so satisfactory as a lamp designed for thecurrent it is intended to carry. Again, the street lampsare not expected to be so sensitive in feeding the car-bons together as they are consumed away, and in somecases this operation is performed in jerks, or, in popularparlance, by fits and starts. Another disadvantageis that these lamps are usually big and clumsy with re-spect to the case at the top of the lamp, so that. 128 THE HALF-TONE PROCESS. where there is little head room the lamps cannot besuspended high enough. It may also be difficult toremove the case in the event of anything going wrong. One most important point to bear in mind is thatwhatever type of lamp is selected it must be one inwhich both the carbons move together—what is calleda focussing lamp. In some lamps the bottom carbonis fixed and only the top one moves. In such a caseit would obviously be useless to have a parabolicreflector, as the arc could never be kept central in it,and even without a reflector the lamps would haveto be raised and lowered as the carbons were burntaway. The form of lamp to be recommended, therefore,is known to electricians as a differential self-focussinglamp. The word differential implies that there willbe a pair of coils in the lamp which will balance againsteach other, any sudden change in the one
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