1882.] on Electric Lighting by Incandescence. 35 



the ten times longer piece, ten times the electro-motive force or, if I 

 may be allowed the expression, electrical pressnre, is required, and 

 exactly ten times the amount of energy would be expended in pro- 

 ducing this increased electro-motive force. 



Considering, therefore, the proportion between power applied and 

 light produced, there is neither gain nor loss in heating these dinerent 

 lengths of wire. In the case of the longer wire, as it had ten times 

 the extent of surface, ten times more light was radiated from it than 

 from the shorter wire, and that is exactly equivalent to the propor- 

 tional amount of power absorbed. It is therefore evident that wltether 

 a short piece of wire or a long piece is electrically heated, the amount 

 of light produced is exactly proportional to the power expended in 

 producing it. 



This is extremely important ; for not only does it make it possible 

 to produce a small light where a small light is required, without 

 having to pay for it at a higher rate than for a larger light, but it 

 gives also the great advantage of obtaining equal distribution of 

 light. As the illuminating effect of light is inversely as the square 

 of the distance of its source, it follows that where a large space is to 

 be lighted, if the lighting is accomplished by means of centres of 

 light of great power, a much larger total quantity of light has to be 

 employed, in order to make the spaces remotest from these centres 

 sufficiently light, than would be required if the illumination of the 

 space were obtiiined by numerous smaller lights equally distributed. 



In order to practically apply the principle of producing light by 

 the incandescence of an electricallv heated continuous solid conductor, 

 it is necessary to select for the light-giving body a material which 

 offers a considerable resistance to the passage of the electric current, 

 and which is also capable of bearing an exceedingly high temperature 

 without undergoing fusion or other change. 



As an illustration of the difference that exists amoncr different 

 substances in respect of resistance to the flow of an electric current, 

 and consequent tendency to become heated in the act of electrical 

 transmission, here is a wire formed in alternate sections of platinum 

 and silver ; the wire is perfectly uniform in diameter, and when I 

 pass an electric current through it, although the current is uniform in 

 every part, yet, as you see, the wire is not uniformly hot, but white- 

 hot only in parts. The white-hot sections are platinum, the dark 

 sections are silver. Platinum offers a hisher decrree of resistance to 

 the passage of the electric current than silver, and in consequence of 

 this, more heat is developed in the platinum than in the silver sections. 



The high electrical resistance of platinum, and its high melting- 

 point, mark it out as one of the most likely of the metals to be useful 

 in the construction of incandescent lamps. When platinum is mixed 

 with 10 or 20 per cent, of iridium, an alloy is formed, which has a 

 much higher melting-point than platinum ^ and many attempts have 

 been made to employ this alloy in electric lamps. But these attempts 

 have not been successful, chiefly because high as is the melting-point 



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