690 INDUSTRIAL APPLICATIONS. 



apparatus do not work continuously, which is the most frequent 

 case for instance, in lighting. In practice, much denser currents 

 are used, attaining 3 to 4 amperes per square millimetre. 



1245. LIGHTING BY INCANDESCENCE. In incandescent lighting, 

 the calorific energy of the current is utilised in raising to a very high 

 temperature a filament of carbon placed in a vacuum. 



The luminosity rapidly increases with the temperature, but in 

 practice it is necessary to keep the heating considerably below 

 the temperature at which the wire breaks or rapidly deteriorates. 



When the temperature is in equilibrium, the energy of the 

 current is compensated by radiation. The ratio of the quantity of 

 light emitted by the lamp to the energy expended is a function of 

 the temperature alone, and does not depend on the shape of the 

 carbon threads, provided they have the same emissive power. 



Experiment shows, for example, that good conditions of lustre 

 and duration are obtained with incandescent lamps, equal to about 

 171 carcel, or 12-5 candles, when they are traversed by a current 

 of 0*8 ampere with a difference of potential of 100 volts at the 

 terminals. 



The resistance of each lamp is then - =125 ohms, and the 



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energy necessary for maintaining it is 100x0-8 = 80 units of work 

 per second, or 8*155 kgm. ; the electrical energy equivalent to a 

 horse-power will then be capable of producing an illumination of 



- =1573 or 1 6 carcel lamps. Experiments at the Electrical 

 8<I 55 

 Exhibition in 1881 gave 12 to 22 carcels. 



If the lighting were done by Bunsen's elements with a yield 

 of 80 per cent., the price per electrical horse-power would be 



0*92 

 -=1-15 francs, and that of a carcel 0-072 francs. 



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The lamps are usually arranged as shunts on the circuit. 

 Supposing there are m identical lamps; let r be the resistance of 

 each, / the current which traverses it, E the electromotive force 

 of the electromotor, R its resistance with the connections. The 



resistance of the whole of the lamps is x = , and we have 



ni 



E = (R + x) mi = (;//R + r) i , 

 x r 



u = 



