ELECTRICAL ENGINEERING 363 



4. ELECTRIC LAMPS AND ELECTRIC LIGHTING 



Notable improvements have recently been made in the metallic filament electric 

 lamps, in the flame arc lamp, and in the application of electric lamps to street lighting. 



The metallic filament glow lamp made with drawn tungsten wire is now rapidly re- 

 placing the carbon filament lamp. In 1907 this lamp had hardly begun to affect the sales 

 of carbon lamps, but in the United States in 1912 the sales of carbon lamps had fallen off 

 to 52 per cent of the total, and that of metallic filament lamps correspondingly increased. 

 Of these latter 25 per cent were tungsten, only about 3 per cent tantalum, and about 

 19 per cent the gem or metallised carbon filament lamps. The original process of 

 manufacture of tungsten lamps was to provide the metal in a finely divided condition, 

 bind it with starch, sugar or camphor, squirt into a thread and then volatilise and burn 

 out the binder, leaving a metallic filament composed of tungsten granules sintered to- 

 gether. The modern process is to draw down through diamond dies a wire of tungsten 

 alloyed with some other metal as fine as .001 in^h diameter and to wind this on a frame 

 which is inserted in the bulb. Lamps are thus made taking only i watt per candle. 



The metallic filament lamp requires a more perfect vacuum than the carbon lamp or else 

 blackening of the bulb soon takes place. A considerably larger percentage of the total power 

 supplied is converted into light by the metallic filament. The carbon filament radiates 

 2.9 per cent of its total energy consumption as light, the osmium 5.2 per cent, the tantalum 

 6.5 per cent and the tungsten 7 5 per cent. This improved efficiency is due to higher tem- 

 perature, but a greater percentage of ultra violet radiation which affects some eyes un- 

 pleasantly. The metallic filament lamp has, however, by its reduced energy consumption 

 increased the demand for electric lighting. It has also given economy in street lighting by 

 incandescent lamps, as it has, in addition to increased efficiency, an excellent life and 

 therefore renewals are not large. The wire lamp has destroyed the domestic use of lamps 

 such as the Nernst, which are not instantly illuminated on switching on. 



In addition to improvements in metallic filament or wire lamps a good deal of work 

 has been done in endeavouring to utilise vapour lamps. 



The mercury vapour lamp consists of a glass or quartz tube provided with two electrodes 

 highly exhausted but containing a little mercury. The lamp is started by tilting it so that 

 the two electrodes are connected together for an instant by liquid mercury and the current 

 then flashes through it and continues to travel in the mercury vapour, when the tube is 

 again tilted back into an inclined position. The vapour is rendered incandescent and glows 

 with a greenish light giving light at about 4 candles (mean spherical) per watt. The colour 

 of the light is, however, a great disadvantage, and efforts to improve it by alloying the 

 mercury with other metals have not yet been very successful. Another vacuum tube lamp 

 is the Moore Lamp consisting of a long glass tube containing rarefied air or COi. The 

 particular improvement consists in a device by which the increase in the vacuum due to 

 working is made to admit a little more air or gas. This automatic valve is essential to suc- 

 cess. The use of neon as a gas for filling vacuum tube lamps is recommended on account 

 of the high luminosity and low dielectric strength of the gas when traversed by a current. 



In out-door lighting the use of flame arc lamps has now become very general. The 

 carbons are impregnated with certain salts such as fluoride of calcium and sodium, 

 which give great luminosity and a reddish yellow colour to the light. The efficiency 

 is increased to about 4 to 5 candles (mean hemispherical illumination) per watt. 



The question of effective street lighting and its cost has given rise to great discussion. 

 The open plain carbon arc lamp is unable to hold its own with high pressure gas illumina- 

 tion, but the flame street arc lamp taking 360 watts and giving a mean hemispherical 

 illumination of 2,200 candles costs about 14 per annum for energy and maintenance, 

 and its colour gives an effective illumination for main streets. 



The proper use of electric light and of illuminants generally has recently attracted much 

 attention, and "Illuminating Engineering" has almost become a separate branch of elec- 

 trotechnics. This is more necessary now that artificial light of greatly increased intensity is 

 demanded and its safe and economical use has to be considered. 



5. ELECTRIC HEATING APPLIANCES AND FURNACES 



The power of an electric current to produce heat is now utilised technically in electric 

 heating for domestic purposes, e.g. cooking, house warming and water boiling, and also in 

 manufacturing processes in electric furnaces of various kinds. There are two ways in 



