When alternating current is available such a lamp will work well with a transformer to step down the voltage to the required degree, and the arrangement is simple, cheap, and efficient, and produces a light at least equal to that from
acetylene. In comparatively small halls, where the current is alternating, this is undoubtedly the best method of working, as it is simpler than the arc and amply brilliant enough for all practical purposes.
With the continuous current the problem is not so simple, as transformation of voltage is not an easy matter, and a glow-lamp on; say, a 200-volt circuit involves a long and fragile filament, which it is difficult to arrange in a small space.
Many years ago the Ediswan Company produced a 'Focus' lamp for the purpose, with the filament arranged in the form of a square grid, and this lamp gave a light of about 100 candles, and was fairly successful for a small room. More recently the Osram Company introduced a similar lamp with a metal filament arranged somewhat in the form of a cone, and this lamp also sufficed for a small class-room. It was, I believe, made in Germany and was practically unobtainable during the war. I understand the Osram Company are at present arranging to manufacture it in this country, but up to the time of writing it has not made its appearance.
None of these lamps worked direct on a public lighting circuit can be regarded as really satisfactory, as it has been found impossible so far to get a concentrated light; the 100-volt lamps have, of course, been superior to those made for 200 or 250, but they are all for lantern purposes far behind low voltage lamps, which are really good when a suitable current can be obtained.
The Pointolite Lamp.—This lamp produced by the Ediswan Company is in reality a miniature arc with tungsten electrodes in a highly exhausted vacuum bulb. To attempt a technical description would be beyond the scope of this book; it will suffice to say that the action depends upon the same principle as the various wireless vacuum valves or the Coolidge X-ray tube.
This lamp requires a peculiar starting device which is supplied with it, and gives a good, intense, and concentrated
light, not equal to the ordinary arc or to limelight, but comparing well with any other form of illuminant. It can only be used with the continuous current.
The Nernst Lamp.—This lamp at the present moment is practically non-existent in this country, having been made exclusively in Germany. Also as recent improvements in metal filament lamps have rendered it almost obsolete for ordinary lighting purposes, it is, I think, very doubtful whether it is still manufactured even in that country, and hence I do not propose to waste space in an extensive description.
It will suffice to say that the lamp consists of one or more straight rods or filaments of a refractory material, which are semi-conducting to the electric current when hot, but non-conducting when cold. To commence with the filament must be heated, and in the lamps as supplied for lantern work this is usually done by means of a spirit lamp, which can be removed immediately the current begins to pass, as the filament is thereafter maintained at a white heat automatically.