3 6 4 ELECTRICAL ENGINEERING 



which such heat can be generated, viz. by passing the current through a high resistance 

 wire in some enclosure from which heat does not readily escape, and also by using the 

 current to make an electric arc in some enclosed space. 



The first method, called "resistance heating," is alone used for domestic heating. The 

 simplest form of electric heater is a so-called radiator comprising an ornamental frame 

 carrying several large electric lamps or else some coils of wire through which the current 

 passes. If lamps are used it is called a luminous radiator, and if coils of wire a non-luminous 

 neater or convector. If one electric unit (costing generally id.) of electric energy is passed 

 through such a radiator it creates 3,400 British thermal units (B.Th.U.) of heat, or enough 

 heat to raise 1 8 pints of water from 60 F. to 212 F. The complete combustion of I Ib. of 

 good coal liberates 12,000 to 14,000 B.Th.U., but only a small fraction of the total calorific 

 energy of coal is sent out into a room by a coal fire. Hence on the mere basis of fuel cost 

 ejectric heating cannot compete with coal fires, yet in indirect advantages it can do so, espe- 

 cially if the electric energy is supplied at o.5d. per unit. In the case of electric cooking two 

 classes of apparatus are in use. In one, each utensil, say a kettle or saucepan, has a resistance 

 wire contained in a double bottom or embedded in the sides. By means of a flexible wire 

 and plug it is connected to the electric supply circuit, and the heat generated in the wire is 

 imparted to the contained water or other liquid. In the same way an electric oven of this 

 type has coils of resistance wire embedded in the sides and bottom. A good electric kettle 

 so made can impart to the water 90 or 95 per cent of the heat created by the electric energy 

 given to it. Thus about 60 watt-hours would be required to raise I pint of water from 60 F. 

 to boiling, costing at id. per unit j^th of a penny for the operation. Similar arrangements 

 on a larger scale, as in the Therol neater, are used for heating bath water, etc. 



In the alternative arrangement an insulated resistance wire is embedded in a closed 

 flat plate of iron, and the passage of a current through the wire raises the plate to a high 

 temperature. Ordinary kettles or saucepans with flat bottoms are then placed on this hot 

 plate and cooking done in them. The "hot plate system" is somewhat less economical in 

 energy consumption, but much more convenient in practice, as each cooking vessel does not 

 require an electric connection. In the Berry "Tricity" system the oven is a light polished 

 sheet metal box which has two hot plates applied to apertures at the top and bottom. Elec- 

 tric cooking by this method is cleanly and better than gas cooking in that the meat, poultry, 

 etc. do not lose nearly as much weight in cooking as by gas or coal and preserve a better 

 flavour. Everything that can be cooked by gas or coal can be cooked as well or better by 

 electricity. With electric energy at id. per unit the cost of electric cooking is not greater 

 than gas cooking with gas at 2s. 6d. per thousand cubic feet. The convenience and comfort 

 to the cook is, however, far greater. Where electric energy is supplied at O-5d. per unit 

 the advantage lies with electric cooking in every way. 



Turning to larger operations we find that electric furnaces have now come into use in 

 many branches of metallurgy, especially in steel and iron manufacture. There are several 

 types of such furnace. Sir W. Siemens was the first to construct in 1879 an arc furnace for 

 smelting iron, consisting of a crucible having one carbon electrode passing through its bottom 

 and another which passed through the lid. Nearly 20 years later the matter was again taken 

 up in 1898 by Major Stassano, who invented one of the earliest large electric arc furnaces, 

 consisting of an iron cupola lined with fire clay in the interior of which a large electric arc was 

 formed between carbon electrodes. Lumps of steel placed on the hearth were melted by the 

 radiant heat so that the steel itself was not contaminated by the carbon. 



Another form of arc furnace has been invented by Paul Heroult. It consists of an iron 

 basin on trunnions, having a lining of refractory material and a cover through which pass 

 two massive carbon electrodes. When the furnace is filled with a charge of pig or scrap iron, 

 lime, and oxide of tar, the carbons are brought down on to it and a powerful arc formed 

 between them which melts the charge and oxidizes the impurities, sulphur, phosphorus, 

 carbon, etc. and finally converts the mass into liquid steel which is drawn off. An arc 

 furnace of this type invented by Paul Girod has only one carbon electrode passing through 

 the lid, the other electrode being one or more water cooled steel plugs inserted in the bottom 

 of the furnace. An arc furnace for smelting iron ore and producing pig iron has been devised 

 by the French firm Keller Leleux & Cie, in which the reduction of haematite iron ore by coke 

 can be carried out electrically. In all these arc furnaces there is a considerable combustion 

 or loss of the carbon electrodes, and to some extent carbon is introduced into the charge. 

 Hence their chief use is in the preparation of a material from scrap or pig iron which is 

 approximately free from sulphur, phosphorus and carbon, and can be subsequently refined 

 into a high class of steel. 



A form of electric furnace without electrodes was originally devised by S. Z. Fcrranti, 

 called an induction furnace. It consists of a closed rectangular ring of laminated iron having 

 wound round it at one part a magnetising coil through which an alternating current is passed. 

 This ring is embraced by a circular annular trough formed of refractory material, in fact a 

 sort of groove in a mass of brick and fire clay. This trough is filled either with scrap iron 

 or with crude liquid steel. It forms the secondary circuit of a transformer of which the iron 



