Incandescent Electric Lamps.—Incandescent electric lamps of the ordinary metal or carbon filament type are also frequently used in small class-rooms, and should be mentioned here, as they provide approximately the same illumination as a gas mantle, or in some cases rather better. It will, however, be more convenient to deal with the question of electric lighting as a whole in the chapter devoted to it.
It will suffice here to say that lamps are made for the purpose with a special filament arranged to provide a concentrated light, the ordinary type being almost useless in this respect, and that small battery lamps, worked by a suitable accumulator, can also be used, but except under very special circumstances are hardly worth the trouble of keeping the batteries charged.
CHAPTER IV
ACETYLENE
There is no doubt that at present acetylene holds second place to electric light in popularity for optical lantern work. The light is good; not, it is true, so good as limelight or the electric arc, but still sufficient for a picture up to 12 feet in diameter at a working distance from the screen of not more than 30 feet, and this suffices for the large majority of halls.
It has great advantages over limelight in convenience and cheapness, although on both these points it must yield place to the electric arc, always providing that current is available, and therefore it is chiefly used in country districts and in gas-lit halls in large towns.
Acetylene gas is formed, as is well known, by the action of water upon carbide of calcium, and the generators constructed for lantern work are essentially the same in construction as for other purposes.