INCANDESCENT MANTLES
By W.A. Atkinson
Chatterbox Annual (1908)
When the electric light was discovered, many people thought that gas would soon be superseded as a means of lighting houses and streets. But at the present time gas is used nearly, if not quite, as much as ever, and, by means of a new invention, its light, which used to be yellow and dim by comparison with the electric light, is now as white and bright as the electric light itself. This invention is known as the incandescent mantle.
We are apt to think that new inventions are the result of bright ideas that flash through the brain in a moment, without any apparent cause. But if we only knew the circumstances in which any particular discovery was made, we should usually find that the invention was really only a little step forward from what was already well known, and that very often it was made by a man who was well acquainted with all that had been already done, and was simply seeking to make just one little improvement more. This was true, at any rate, of the discovery and invention of the incandescent mantle.
The light of the incandescent electric lamp is produced, as you will remember, by a charred thread, which the electric current causes to glow. You will remember too, that this thread is enclosed in a small glass globe, from which the air has been withdrawn. If the thread were heated by the current without being enclosed in this way, it would simply burn up in a moment, just as a lighted cotton thread would.
All substances do not, however, burn up so quickly as this, and it occured to an inventor, called Auer von Welsbach, that he might, perhaps, be able to discover some material which could be heated by an electric current without burning away quickly, even though it were exposed to air. He was shrewd enough to see that if such a discovery could be made, the tedious and expensive work of manufacturing air-tight globes and pumping the air out of them would be dispensed with. He made diligent and careful investigations, and success rewarded his efforts. From certain rare minerals, found in Norway and America, he obtained the substances which he required. They gave out a bright, incandescent light when they were made hot, and they burned or wasted away very slowly. The substance now most commonly used has been named thoria.
Since these substances became incandescent as soon as they were made sufficiently hot, it was soon seen that it did not really matter in what way they were heated. It was not necessary to use electricity at all. The heat of a gas-flame would do quite as well if it could only be utilised in a suitable way. And, as nearly every house already had all the pipes, brackets, burners, and other fixtures for gas-lighting, it was worth some effort on the part of inventors to find out a means by which thoria and the other new incandescent substances could be fitted for use with an ordinary gas-light, because many people would then be induced to adopt this new improvement, which could be effected at so little trouble and cost.
The first incandescent gas-light which I remember to have seen was produced by a small comb of white, earthy material, which was suspended like a little bridge over the gas-flame. The teeth of the comb hung downwards, and the flame of the gas burning between them made them white-hot, and they gave out a more brilliant light than the gas itself.
The incandescent matle which is now most in use is somewhat bell-shaped, and it is hung round the flame of the gas, being supported by a slender rod which stands up in the midst of the flame. If we examine the mantle as it comes from the maker, we find that it looks as if it were made of starched or glazed muslin. If we put it in its place on the gas-burner, and bring a lighted match to it, we find that it burns with a rather alarming flame, which soon dies out. The mantle is not entirely consumed. There remains a bell of white ash, so frail and brittle that the lightest touch will break it. We must even put a globe round it to protect it from the draughts of air, lest it should be broken. Yet this frail bell of ash, when it is made incandescent by the lighted gas, glows with a brilliant light, which may be six times as bright as that of the gas alone, and it resists the heat so well that it will not wear out for many weeks or even months.
A few words about the way in which these mantles are made will enable you to understand their peculiarities much better. They are woven or knitted of cotton-thread or some kind of slender grass fibre. A single thread hangs down from the top of the knitting-machine, and is whirled round and round a ring of needles or hooks which loop the thread and knit it into a long sleeve. This is cut into lengths, each of which is dipped into a solution of the thoria and other incandescent earths, and afterwards dried. It is then exposed to a flame, which burns away the cotton-thread, but leaves all the earthy materials with which it was saturated, and these form, as it were, a skeleton of the knitted mantle, which is carefully and skillfully worked to the required shape. When this is accomplished, the frail skeleton is dipped in collodion, which, when it dries, makes the mantle stiff and firm enough to bear gentle handling. It is then packed in a box ready for sale. It is the collodion which flames up, when we first apply a light to the mantle, and when this is consumed, the frail skeleton of earthy ash, the true working mantle, is left.
The light of the incandescent mantle, as I have said, far out-shines the ordinary gas-light, and the latter could, indeed, be dispensed with, were it not needed to heat the mantle. The gas-light, in fact, ceases to be of use as a source of light, and becomes merely a little furnace, as it were, in which the mantle is made hot.