. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. hanging ones, and have no ad-vantages except, perhaps, in the factthat the lamp can be printing the photographic negative on to themetal plate the same lamps may be used, but it is THE ELECTRIC LIGHT INSTALLATION. 133 generally more convenient to keep the printing depart-ment separate, and to use a single lamp of higherpower. Not less than thirty amperesis advisable, and forty or forty-fiveamperes is better if quick work isrequired. Although it is quite theusual practice to use on
. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. hanging ones, and have no ad-vantages except, perhaps, in the factthat the lamp can be printing the photographic negative on to themetal plate the same lamps may be used, but it is THE ELECTRIC LIGHT INSTALLATION. 133 generally more convenient to keep the printing depart-ment separate, and to use a single lamp of higherpower. Not less than thirty amperesis advisable, and forty or forty-fiveamperes is better if quick work isrequired. Although it is quite theusual practice to use one lamp onlyfor printing, it means an absolutewaste of current on a 100 voltcircuit, because just as much energyis wasted in the resistance as isused in the lamp. It will cost nomore to run two lamps in series,and put them close together if onlyone frame is being exposed. Thedouble carbon lamp previouslyreferred to makes an excellent print-ing lamp, and entails no waste ofcurrent. The enclosed lampsare also much used, and it isbelieved that quicker exposuresare obtained by their means,the sensitive. owing to being strongly film affected by the violetrays which these lampsthrow out. Fie-66- A word must be said about the resistance coils,which are in all cases necessary for cutting down thevoltage of the supply. These should always be ordered 134 THE HALF-TONE PROCESS. with the lamps, as the vendor has no interest in supply-ing anything but that which is suitable for the purpose,and they will usually be sent out adjusted to suit thelamps. Although these resistances are made with anadjustment which permits more or less current beingpassed, it is not wise to tamper with them, as they maybe very easily short circuited and burnt out. Anyadjustment of the resistance should be done with anammeter in circuit. The resistance coils generallyused for arc lamp work consist of a German silverwire wound round a porcelain cylinder. Such coils canbe used either for continuous or for alternating
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