This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Before entering upon a discussion of the merits and demerits of the utilization of electricity as a means of producing heat, either for cooking or warming, it would be well to consider the causes which have led to its adoption. Let us then, for a moment, consider the means usually employed for cooking in the kitchen of an ordinary home, i.e. the use of coal in an open rang The efficiency of such a method in the case (say) of roasting before the fire, is usually considered to be as low as from 10 to 15 per cent of the heat produced, and when we consider that the remaining percentage is either actually lost by going up the chimney, or is spent in making it generally uncomfortable for the cook we feel that we have hardly got value for our fuel. Some slight improvement has been obtained by enclosing coal-fires in stoves. But even they, although not allowing the heat of the room to become excessive, still permit an enormous waste in the shape of hot air going up the chimney.
Another method of cooking very much in vogue consists in the use of gas-stoves. Careful experiments, however, have shown that, although obtaining a much higher efficiency than coal, with the additional advantage of no surrounding heat, about four-fifths of the total heat generated is, in the case of roasting, uselessly dispersed. In addition to this, food cooked over gas frequently has a disagreeable flavour, while the stove, even if it has ventilators attached, almost invariably gives off a quantity of noxious gases, - sometimes that known as "acetylene", - which are the direct cause of the headache, which usually on after entering a room in which a gas-stove has been burning.
A good cooking-apparatus should, in addition to being of moderate cost while actually in use, possess the following advantages: - 1. No smell: 2 NO external or radiant heat; 3. No generation of noxious vapours; 4. The quality of being able to be turned off or quenched, either partially or wholly, as cooking proceeds or ends. These advantages, especially the last, may appear at first sight to savour somewhat of Arcadia, but, as I hope presently to point out, not only IN they to be obtained, bat are now actually within reach of the ordinary householder, by means of electricity.
A very popular impression seems to be that electricity, in its commercial form, can give no best So far, however, is this from being the case, that it is at present utilised to melt and weld metals which cannot be affected by other methods. This idea has no doubt arisen from the fact that buildings lighted by electricity are quite cool, when compared with those lit by other illuminants; the reason of this, however, is that electricity employed to give light yields such extraordinary brilliancy when compared with the actual size of the illu-minating space, that such space, though in itself extremely hot, conveys hardly any additional beat to the surrounding atmosphere.
The theory upon which electrical heating is based is very simple. The reader is doubtless aware that all generators of electricity require an external circuit or path. In this path the work to be done invariably lies, whether it be in the shape of lighting, power-transmission, or any other means of absorbing electrical energy. The easier this path becomes, the greater is the rush of current down it; hence, as some metals are known to conduct the current with much greater than others, we Bay that they offer less resistance. To understand this aright, let us compare the flow of electricity along a wire with the How of water in a pipe. We know that, with a certain head and pressure of r. a large pipe will allow an easy flow, while a small one will necessitate the forcing of the water through it with considerable difficulty. Electricity in a wire acts in a somewhat similar way; when the current is large and the conductor small, the latter becomes hot, owing to the resistance offered. Different metals, however, offer different degrees of resistance; thus, if we substitute an iron for a copper wire, the iron, although perhaps of exactly the same diameter as the copper, will get hot with the same current that the copper was able to conduct without heating. This fact - that resistance to a current produces heat - is the basis of cooking and wanning by electricity.
The next point to be considered is, how this heat can be produced without allowing any to escape. We have already seen that the wires, by whose agency heat is to be produced, must be of high resistance; but few metals capable of offering this high resistance can withstand any great degree of heat, especially when exposed to the atmosphere. The commoner metals, otherwise most suit-able, generally oxidize when subjected to an unusual degree of heat. The only method of preventing this oxidizing is so to enclose the resistance-wires that the atmosphere cannot get at them; if such enclosure is perfect, the commoner metals may be used with impunity.
Resistance-wires used for heating-purposes are generally wound in spiral coils, as a great length of metal, and so of heating-surface, can thus be packed in small compass. Strange though it may seem, more difficulty has been experienced in obtaining an air-tight jacket for the coils than in perfecting the electrical part of the apparatus. The great diffculty was to find a composition which, while admitting no air, was capable of expanding, when heated, exactly in an equal ratio with the wires it enclosed. It will Be seen that, given some such suitable compound, heat generated in the enclosed coils could, through contact with the compound, be much more readily conveyed to any pan or dish placed on it than if this were merely held over hot wires. And not only that, but such unprotected wires would be a constant source of danger from the possibility of a short circuit being formed. By a "short circuit" is meant a more direct path, and may be illustrated by a pan placed on two resistance-springs, through the total length of which the current would, in the ordinary course of things, be made to flow; the pan, being also of metal, would offer a much shorter circuit for the current than the many convolutions of the springs, and would, by thus allowing the electricity to bridge across the springs, or that part of them short circuited, to become dead, and so lose the heat they possessed. The composition, therefore, must not only possess certain ratios of expansion, and be impervious to the passage of air, but it must also l»e made of insulating material; that is, of a materia] of the very worst conducting capacity, so that a short circuit cannot possibly occur through contact with the compound itself.