36 Mr. Joseph W. Swan [March 10, 



of iridio-platinum, it is not liigli enough to allow of its being heated 

 to a degree that woukl yield a sufficiently large return in light for 

 energy expended. Before an economical temperature is reached, 

 iridio-platinum wire slowly volatilises and breaks. This is a fatal 

 fault, because in obtaining liglit by incandescence there is the greatest 

 imaginable advantage in being able to heat the incandescing body to an 

 extremely high temperature. I will illustrate this by experiment. 



Here is a glass bulb containing a filament of carbon. When I 

 pass through the filament one unit of current, light equal to two 

 candles is produced. If now I increase the current by one half, 

 making it 07ie unit and a half, the light is increased to thirty candles, 

 or thereabout, so that for this one half increase of current (which 

 involves nearly a doubling of the energy expended), fifteen times more 

 light is j)roduced. 



It will readily be understood from what I have shown that it is 

 essential to economy that the incandescing material should be able to 

 bear an enormous temperature without fusion. We know of no metal 

 that fulfils this requirement ; but there is a non-metallic substance 

 which does so in an eminent degree, and which also possesses 

 another necessary quality, that of loio conductivity. The substance is 

 carbon. In attempting to utilise carbon for the purpose in question, 

 there are several serious practical difficulties to be overcome. There 

 is, in the first place, the mechanical difficulty arising from its intract- 

 ability. Carbon, as we commonly know it, is a brittle and non-elastic 

 substance, possessing neither ductility nor plasticity to favour its 

 being shaped suitably for use in an electric lamp. Yet, in order to 

 render it serviceable for this purpose, it is necessary to form it into a 

 slender filament, which must possess sufficient strength and elasticity 

 to allow of its being firmly attached to conducting wires, and to 

 prevent its breaking. If heated white hot in the air, carbon burns 

 away ; and therefore means must be found for preventing its com- 

 bustion. It must either be placed in an atmosphere of some inert gas 

 or in a vacuum. 



During the last forty years, spasmodic efibrts have from time to 

 time been made to grapple with the many difficulties which surround 

 the use of carbon as the wick of an electric lamp. It is only within 

 the last three or four years that these difficulties can be said to have 

 been surmounted. It is now found that carbon can be produced in 

 the form of straight or bent filaments of extreme thinness, and 

 possessing a great degree of elasticity and strength. Such filaments 

 can be produced in various ways — by the carbonisation of paper, 

 thread, and fibrous woods and grasses. Excellent carbon filaments 

 can be produced from the bamboo, and also from cotton thread treated 

 with sulphuric acid. The sulphuric acid treatment efiects a change in 

 the cotton thread similar to that which is effected in paper in the 

 process of making parchment paper. In carbonising these materials, 

 it is of course necessary to preserve them from contact with the air. 

 This is done by surrounding them with charcoal. 



