Popular Science Monthly/Volume 38/March 1891/Non-Conductors of Heat
|NON-CONDUCTORS OF HEAT.|
IT is a matter of common observation that a hot body continually gives off its heat to things around it, until at length the giver and the receivers all come to a common temperature. This gradual equalization may be brought about in three different ways: In the first place, heat is thrown off in every possible direction from every point of a heated body by what we call radiation. Secondly, when air, water, or any other fluid is in contact with a hot surface that is not directly over it, the touching particles become warm and light, and move away to give place to others. This carrying away heat by the successive particles of a fluid is called convection. In the third place, when a solid substance is placed against anything of a higher temperature, its nearest parts are warmed and give up a portion of the heat received to those parts lying next to them; and these, again, share their gain with those next in order; and so on, till finally the outer surface heats whatever is in contact with it. Such conveyance of heat from particle to particle, without sensible motion, is termed conduction.
Strictly speaking, according to modern theory, radiant heat is a peculiar kind of undulation communicated to a supposed exceedingly subtile, all-pervading ether; and conduction is an oscillation of the molecules of the conductor itself. But, though we no longer consider heat to be a substance, it is convenient to use the old terms figuratively in describing the phenomena, just as we still say the sun rises and sets, though it is the earth that moves.
When we sit near an open fire, we are warmed by radiation through the intervening air, while the air itself is heated by contact with the fire and passes up the chimney. So radiation and convection, or radiation and conduction, may go on at the same time, and when cooling takes place it is not always easy to tell how much of the effect is due to each of the causes respectively. Hence, substances that are put around hot bodies to retard the change of temperature are often called indiscriminately non-conductors, though in fact they may act partly by preventing convection or by intercepting radiation. Practically, indeed, it is of little consequence to decide exactly how the loss of heat is prevented, but, in the full study of retentive coverings, we must not altogether lose sight of the distinction between mere conduction and general transmission.
It is a matter of much interest as well as of economical importance to find out what substances are most suitable to keep hot bodies warm and cold bodies cool; and several methods have been devised for making either absolute or comparative trials. After due consideration of the plans used by different experimenters, the writer has adopted, for the many determinations which he has had occasion to make, an apparatus which may be arranged in three different ways: First, a short, cylindrical metallic vessel, with the flat ends vertical, is kept at a constant high temperature by a continual current of steam or hot water passing in at the bottom and out at the top. The non-conductor, of a regular thickness, say one inch, is applied to one of the flat faces of the heater. The other surface of the covering is in contact with a thin brass box, or calorimeter, filled with a known quantity of water to receive the transmitted heat. The number of degrees which the water is raised in an hour gives a definite measure of the amount of heat that the covering allows to pass through.
Secondly, in trying liquids or air for their conducting power it is desirable to get rid of convection by heating from above, so that the hottest part of the fluid shall be and remain at top. Therefore the heater is suspended with its used face downward and exactly horizontal. The calorimeter, with its face also horizontal, is placed at any chosen distance "below the heater, and is furnished with a curb of well-varnished pasteboard extending up a little higher than the face of the heater. This curb is of a somewhat larger diameter than the hot box, so that there is a free space all around, and very little heat can be conducted by it.
Thirdly, in making practical tests of coverings for steam-pipes the non-conductor is put entirely around the pipe and the calorimeter is made in two parts with concave sides to fit the covering. Of course, in all cases the whole apparatus is surrounded by cotton-wool or woolen blankets to prevent the disturbing influence of the surrounding air.
With the first arrangement, if the space between the calorimeter and the heater is filled with air only, which is confined by a curb of paper, but is free to circulate within the inclosure, the heat passes over rapidly, especially when the heater is at a very high temperature, while in the second apparatus the transmission is slow. In the former case, convection has full scope; in the latter, the air is stagnant and the heat passes downward by conduction and radiation. Therefore, still air has very little transmitting power, and confined air which is free to move around within the inclosure conveys heat readily.
Yet it is a not uncommon belief that, as air is a poor conductor of heat, a mere inclosed air-space around a hot or a very cold body suffices to prevent change of temperature. It is said by some that an ice-pitcher or a refrigerator needs only a double wall and no filling between. And we occasionally meet with loose statements like the following: "Confined air has long been regarded by scientific and practical men as one of the best non-conductors of heat." But it should be remembered that imprisonment is not always close confinement. The air must be fettered so that it can not stir.
Now, if we fill the space in either the first or the second apparatus with cotton or fine wool, we shall find the transmission even less than with still air. And yet the fibrous matter may actually occupy only a hundredth part of the space which it apparently fills, and the fibers can touch the heated surface and each other only in a few points. Therefore the specific conducting power of wool or cotton can have very little to do with their capability of keeping back heat. We know not precisely what the conducting power of the solid matter of cotton may be, for we can not compress the fibers far enough to destroy their elasticity and expel all the included air. But the woods are very similar in substance, and some of them are two thirds as dense as fully compacted cotton would be. One of the dry, hard woods, heavy enough to sink in water, was found to have about four times the transmitting power of loose cotton. Were the transmission due to the substance of the fibers themselves, it would be increased by packing more in the same space. But, in fact, it is found that it is somewhat diminished by moderate crowding.
It would appear, then, that the efficiency of light non-conductors must be owing mostly to the imprisoned air which really occupies all but a small fraction of the space; and the stiller the air is held, the better is the effect.
Of course, the amount of friction which fibers can oppose to the motion of the entrapped fluid depends on their minute structure and arrangement. Thus in cotton they are long, fiat, twisted, irregular in breadth, and variously bent. And as to fineness, it was found by counting and weighing some Sea Island cotton fibers averaging about an inch and a half in length, that it would take seventeen thousand to weigh a grain. Wool is scaly and very crinkly. Down is made up of flat threads with innumerable short, loose branches. The heads of the common cat-tail (Typlia latifolia), which make a good non-conductor, consist of brown seeds, each having a stalk with very spreading branches. The seeds, with their appendages, are so very fine that eight hundred of them weigh only one grain. They may well float, as each one, for its weight, presents a very extensive surface to the air; and, for the same reason, in mass they serve to keep the air stagnant.
Ground cork and some other barks, and the sawdust of the soft woods, as well as the charcoal made of these substances, are very good retainers of heat. Lampblack also works well. When the thing to be kept hot is at a very high temperature, some light, incombustible powders are very suitable. Among the best of these are fossil meal and the calcined magnesia and magnesium carbonate of the druggists. Fossil meal consists of the silicious skeletons of microscopic vegetables, called diatoms, exceedingly various in shape and size, the very largest of them hardly reaching the length of the hundredth of an inch. It is found abundantly in some peat meadows and in the bottoms of ponds. Both fossil meal and magnesium carbonate have been largely used in covering steam-pipes.
Obviously, when the same light substance is tried in both the first and second apparatus above mentioned, and the results differ, it must be owing to the inability of the substance to hold the included air still in the first arrangement. So powdered plumbago, or black lead, which is very slippery, shows nearly twice as much transmissive power in one case as in the other. Loosened asbestus fiber also lets through about twice as much heat in the vertical arrangement as in the horizontal. Yet this fiber may be split up exceedingly fine; but the great difference in its behavior as compared with cotton or wool must be owing much less to its own greater specific conducting power than to the smoothness and inelasticity of its fibers. It has too slight a hold on the included air. The more finely shredded it is the better it works; but our experiments have proved that it is not to be recommended as a non-conductor. And yet asbestus is often spoken of as though its excellence in this respect were unquestionable; but, because this wonderful mineral is very useful in many ways by reason of its incombustibility, it does not follow that it has any magic virtue in its other relations to heat. Asbestus paper intercepts heat somewhat better than the loose fiber; but a great many layers must be put together, and then the virtue is by no means commensurate with the cost. It is sometimes recommended as a suitable article to put between floors to prevent the spreading of a possible fire; but those who propose it for this use seem to overlook the fact that the efficiency of non-conductors is nearly proportional to their thickness, and, though an inch might be of some service, one fiftieth of an inch can do very little good.
Fibrous matters and powders in the loose state are somewhat troublesome to confine in the form of coverings, and hence they are sometimes consolidated into sheets or blocks which can be handled without breaking and applied easily. Hair-felt, which is made in thick sheets from the hair which tanners scrape from hides, is cheap and is very serviceable when the heat is not scorching. Paper pulp has been formed into very thick, hollow, half cylinders to put around steam-pipes. Carbonate of magnesium and fossil meal cohere when moistened and slightly compressed, and they may be made into slabs with the addition of a very small percentage of hair or asbestus to give toughness. Such a paste may be plastered directly on steam pipes or boilers and allowed to dry, the fiber serving to prevent cracking; but the greater compactness of light materials so consolidated renders them less effective, especially when a heavy cementing substance is added, like clay or plaster of Paris.
Of non-conducting substances that are already in the solid form, the light woods are often used advantageously. It should be noticed that most of them conduct heat much better along the grain than across it. Thus a cross-section of Liriodendron, or yellow poplar, was found to transmit heat nearly twice as fast as a board of the same thickness sawed lengthwise. Cork is expensive and hard to get in large pieces; but it is far preferable to wood, as it is lighter and more elastic and does not absorb water. Very porous and light bricks confine heat much better than those that are hard burned, but they must be kept dry.
The presence of moisture in a non-conductor greatly impairs its usefulness, as every one knows who has attempted to hold a hot body with a damp cloth. Count Rumford, who long ago did much valuable work in the experimental study of heat, concluded that fluids have no conducting power at all, but transmit heat solely by convection; and, accordingly, water is still sometimes spoken of as an exceedingly poor conductor. But later investigators have disproved the correctness of that idea. Our own trials show that, when convection is obviated, water transmits in a given time six times as much heat as hair-felt of the same thickness, and nearly eight times as much as still air. Others have found that bisulphide of carbon and ether transmit heat even better than water; but most liquid substances are slower conductors. Thus it takes more than twice as long for a given amount of heat to pass through cotton-seed oil or lard oil as through water.
As to the gases, some physicists seem to have proved that heat passes through air more readily than through a vacuum, while hydrogen has six times as much transmissive power, and carbonic acid half as much as air; but none of them used apparatus that would give absolutely certain results.
To show more clearly the retentive power of various substances, we subjoin the following table, in which the first column of figures shows the net percentage of solid matter in a given space. The second column of figures gives the number of English units of heat transmitted in one hour through one square foot of the covering one inch thick, the average difference of temperature between the heater and the water in the calorimeter being 100° Fahr. By the English unit of heat is meant as much heat as will raise the temperature of one pound of water 1° Fahr. Of course, the smaller the number in the last column the better is the substance for keeping a body warm or cold.
In some of the experiments the source of heat was steam at 310° Fahr. In the others a stream of water at about 176° Fahr. was kept running through the heater.
It is plain that in choosing non-conductors for practical service we should take into account something more than their heat-retaining power. They should be of materials that are abundant and cheap; clean and inodorous; light and easy of application; not liable to become compacted by jarring, or to change by long keeping; not attractive to insects or mice; not likely to scorch, char, or ignite at the long-continued highest temperature to which they may be exposed; not liable to spontaneous combustion when partly soaked with oil; not prone to attract moisture from the air; and not capable of exerting any chemical action on surfaces with which they are placed in contact. There is no one thing which combines all the desirable good qualities, but there is a considerable range of substances which fulfill most of the
requirements. For steam-pipes there have been many more or less suitable coverings in the market. But one should receive with much allowance the representations of dealers, who sometimes continue to advertise what has been proved to be of inferior value. Not uncommonly they are anxious to sell that on which they can make the most profit rather than that which is most efficient.