THE PROGRESS OF THE ELECTRIC LIGHT 7 



We thus see that during the last ten years the invention 

 first of the Nernst lamp, then of the tantalum lamp, finally of 

 the tungsten lamp, has completely revolutionised the position 

 of interior electric lighting. The remarkable progress is per- 

 haps more clearly seen if the relative merits of the different 

 types of lamp are set out in tabular form : 



Cost of Lighting per i,ooo Candle Hours 



A word may be said in reference to the question of life. 

 There are three ways of regarding the life of an incandescent 

 electric lamp, the simplest being to consider its life as the 

 number of hours it runs before the filament actually fails. All 

 the different types of lamp, however, deteriorate in use, the 

 candle power steadily falling and the consumption in watts 

 at best remaining steady and often rising. For a given lamp 

 there is therefore a point, generally known as the " smashing 

 point," at which it is most economical to discard the lamp and 

 substitute a new one ; this is the economical life. Unfortunately 

 the economical life is very indefinite, depending on the price of 

 the lamp, the cost of energy and the rate at which the watts per 

 candle increase during use ; moreover, as there is no obvious 

 indication when a lamp has reached the smashing point, the 

 economical life is not of any practical value. The third method, 

 in many ways the most rational, is to say the lamp has reached 

 the limit of its life when the candle power has fallen to a certain 

 percentage of its original value : this is known as the " useful 

 life " and 80 per cent, of the initial value is taken as the limit. 

 This principle of determining life is based on the assumption 

 that when the candle power is only 80 per cent, of the initial 

 value it is no longer sufficient for the purpose for which the 

 lamp was intended ; it has the advantage that it is a principle 

 which the user actually recognises, in a rough and ready way, 



