movement. As the carbons burn away the arc lengthens, the current weakens, the electro-magnets lose their grip, and the carbons move together until the increasing current puts on the brake again. Some of these lamps are 'semi-automatic' only, that is to say, the arc has to be struck by hand, while others perform this operation automatically as well, usually by an additional magnet which draws back the carbons by the correct amount after the arc is struck.
My frank advice to intending lanternists is to leave these lamps alone. Some of them are satisfactory up to a point, but they are all apt to be 'jumpy,' and on the whole the hand-fed type is in my opinion to be preferred.
Arc Lamps on Alternating Currents.—The alternating current is not so good as the continuous for lantern work with arc lamps: the light per ampère is not so great, the light has an irritating habit of travelling round the carbons and there is always a slight 'hum.'
The sum total of these drawbacks is nothing very serious, provided that proper arrangements are adopted, and I have frequently manipulated arc lamps on alternating circuits with such good results that professional lecturers have at first refused to believe that the circuit really was alternating.
As it is frequently stated that to obtain a steady light with an alternating current is impossible, I can understand their surprise, and I can also understand the statement in question, as the problem is usually tackled on entirely wrong lines.
It is almost always stated that arc lamps for alternating currents should be arranged with the carbons vertical, and many makers actually so construct their lamps as to allow of this.
To obtain a steady light under these conditions is impossible and I pity anyone who attempts it; but the statement that this is the best method of working has been repeated so often that it seems to have been taken for granted.
The best arrangement (in my hands at any rate) is to
slant the carbons as for the continuous current, and also to have the upper carbon cored and the lower one solid, but to use a rather larger lower carbon than would be correct if the main were continuous.
Also the upper carbon should not be quite so far back as with D.C.; to have the front edges of the two carbons practically in line is about correct, but the exact position should be carefully adjusted to obtain the steadiest light, and it will be found that a slight alteration makes a considerable difference.