104 Electric Lighting. 



from twenty-five to eighty candle-power, absorbing, of 

 course, a larger, but not proportionately larger, amount of 

 current. The incandescent lamp, whilst not being nearly so 

 economical as the arc light, possesses advantages that would 

 more than compensate for this deficiency. It may be 

 accepted, I think, as a fair average that about 200 candle- 

 power per horse-power can be obtained from incandescent 

 lamps, everything being in its highest state of efficiency; but it 

 probably will be found with respect to endurance that from 

 120 to 150 would produce the most economical results. 

 Probably one of the most peculiar features about the carbon 

 loop is the alteration in its resistance when heated. It is of 

 course well known that increase of temperature in metals 

 increases their resistance ; with the carbon loop it is different, 

 increase of temperature reducing the resistance to a remark- 

 able degree, in some cases as much as 60 per cent. A 

 Swan lamp with a resistance of 65 ohms when cold, will only 

 have a resistance of about 37 when heated to incandescence. 

 The current necessary to maintain a Swan lamp of about 

 fifteen candle-power is 1*25 amperes, with an electro-motive 

 force of forty volts. It will thus be seen that certain 

 arrangements have to be made in placing a number of 

 incandescent lamps in circuit. They are arranged usually as 

 parallel, compound parallel, and in parallel series circuit, 

 according to the resistance of the lamps and the machine 

 supplying the current. No difficulty need therefore be ex- 

 perienced in working any number of incandescent lamps 

 from very large dynamo-machines of low tension, 



For purposes of regulation, two or three methods present 

 themselves as being efficient — by interposing a high resist- 

 ance in the leading circuit, by alteration of the position of 

 the brushes on the commutator of the machine, or by inter- 

 posing a resistance in the circuit of the field-magnets when 

 a separate exciter is used. By this means less power 

 is absorbed, and, consequently, it is the most economical. 



Incandescent lamps can be supported by either con- 

 tinuous or alternate currents, by either high or low tension, 

 so that almost any kind of machine can be adapted to 

 them. 



That the successful introduction of incandescent lighting 

 will cause a revolution in ordinary systems of illumination 

 is now, I think, quite evident. Even in its present stage 

 incandescent lighting has commenced to play an important 

 part, and is being rapidly introduced for lighting places of 



