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.
In the earlier experiments with heating-resistances, cement of different kinds was largely used, but owing to its breakable and porous nature, air very soon got to the wires, oxidation set in, and the apparatus became useless. The material in most frequent use now is a kind of enamel, which while possessing the necessary qualities mentioned above, is also a good conductor of heat, and is capable of adhering firmly not only to the wires imbedded in it. but also to the hot-plate to which the warmth is to be conducted. It will at once be seen that heat obtained by conduction is, for cooking, far superior to that due to radiation only.
The wires can be built in any shape to suit the requirements of the article to be heated, thus preventing the great loss of energy usually expended in uselessly heating the surrounding atmosphere; and not only this, for in the case of light goods, such as kettles, flat-irons, etc, the heating-springs are actually built into the article itself, thus not only doing away with the atmospheric medium, but giving only one intervening plate to be heated, instead of at least the two used in the case last mentioned.
On coining into a kitchen where an electric oven is in use (see Fig. 555), we are at once struck with the great difference between its surroundings and those pertaining to the old-fashioned range. Among the things which have disappeared are the smell of smoke, the risk of falling soot, and the high surrounding temperature due to keeping up a large fire for an hour or two before the oven ia required in order to attain the necessary degree of heat; and there is no risk of finding that one side of the oven has been nearly red-hot, and has charred the eatables on one side, while the other is unbaked; or that the fire, not having been regulated with sufficient nicety, has either burnt the eatables or left them quite uncooked; for, by the peculiarities of construction above referred to, not only can we surround an electric oven with heating-surfaces on all sides, top or lx>ttom, but, by merely turning the handle of a switch, we can regulate the current passing through these heating-surfaces, and consequently the degree of heat which any perl of such an oven may attain, as shown in Fig. 556. Thus, in an oven with six switches, all can be turned on to hake a certain article, and if this is not immediately required, all Dot may be turned off, the remaining one being capable of keeping it warm. When we consider also that such an oven can attain its fullest heat in about 10 or 15 minutes after being first switched on, and can be turned out immediately after use, we can at once see that the efficiency of electricity used in this direction is very high, and the cost consequently low; on the other hand, as all unnecessary heat is waste, the burning down of a coal-fire after use in cooking is dead loss. Nor is the evil of loss up the chimney only confined to the householder bearing it, for, as anyone living in a large town may see, it is these small contributions, and not those from mills and workshops, which make the atmosphere in all great centres what it is, ruinous alike to health and property.
Fig. 555 . - An Electric Kitchen.
Now as to cost. The energy will probably be obtained from an Electric Supply Co., for as electricity can be generated more cheaply on a large scale than on a small one, this is, at any rate with small consumers, the cheapest method of obtaining it. In Great Britain such an electrical supply is usually dear, as it has to be generated from steam, being usually about 6d. per unit.1 In places where generating power can be had more economically, electrical energy is much cheaper; thus in some parts of America, where electricity can be produced from large flows of water, we are assured that power will soon be distributed from house to house at a charge of 1/8d. per unit. Nor is this report exaggerated, as we might at first suppose, for in some parts of Europe to-day, such power, also derived from water, is being sold for ¾d. per unit. We will assume that Ad. per unit is a fair standard on which to base our calculations, this being, I understand, the actual amount now charged by one of the corporate bodies in London, though it is much higher than that at which an ordinary large private installation could supply it. We find that in an electric kettle 1 lb. of cold
Fig. 556. - An Electric Oven.
1 By an "unit" is meant 1000 "watt hours", a watt being the standard of electrical energy, obtained by multiplying the pressure (or "voltage") of the circuit by the current (or "amperage") absorbed by any particular apparatus; in other words, when the electrical energy, multiplied by the time in hours, equals 1000, an unit of electricity has been expended. The definitions of the terms used in connection with electricity will be more fully given in the chapters on Lighting by Electricity.
VOU II water can be boiled in approximately three minutes, with a current of 10 amperes at a pressure of 100 volts, which works out into units as follows: -
100 volts x 10 amperes x , 3/40 hour = 50 watt hours; this, at 4d. an unit, works out thus: -
1000 : 50 : 4 : ½ of a penny.
An oven like that illustrated in Fig. 556 will, with a pressure of 100 volts, take a current of approximately 25 amperes for about ¼ hour, by which time full cooking temperature of 325° to 400° Fahrenheit will attained. After this first quarter of an hour a current of only 10 to 15 amperes will be sufficient to maintain this degree of heat.
Care should be taken in the purchase of cooking or heating goods that they are built to suit the pressure of the consumer's circuit, for such pressures vary, and it will readily be Been that applying current at too great a pres-ure results in more current being forced through than it is capable of receiving without damage to it through overheating.