INCANDESCED T LAMPS. 2 1 5 



currents are well known, and the arrangement of the gene- 

 rator in any given case is indicated perfectly by Ohm's 

 formulae. As regards this matter, we do not think that 

 Edison can discover anything more than we know already : 

 and we believe that it is rather by the material arrangements 

 of his lamp that he will have any chance of arriving at the 

 solution he is seeking. Now Edison's lamp, which has many 

 very different forms, consists of the following : 



The principle of the lamp, which is represented in Fig. 61, 

 is the incandescence of a spiral of platinum wire a alloyed 

 with iridium. And in order to prevent the spiral from burn- 

 ing when its temperature exceeds a certain point, Edison 

 places within the spiral a metallic rod G, which by dilating 

 causes a contact to be made at z, by means of the lever s, 

 precisely at the moment when the heat is about to reach 

 that degree. The current is then derived through this con- 

 tact, and immediately lowers the temperature of the spiral, 

 which causes a disjunction of the derivation and arrests the 

 cooling. The spiral then begins again to be heated, and is 

 thus maintained at a temperature which can vary only be- 

 tween very narrow limits, which may be regulated by the 

 greater or less separation of the contact piece i of the deri- 

 vation, by the resistance of the derivation, or by a resistance 

 regulator variable with the pressure, based on the principle 

 which Edison has applied to telephonic transmitters. 



Can a platinum spiral by this means acquire a temperature 

 sufficiently high to produce light ? This appears the more 

 doubtful from the fact that De Changy did construct a very 

 ingenious regulator under similar conditions, and that, never- 

 theless, his spirals were volatilized when they were heated 

 to such a point as to become luminous. 



It is true that, according to the American journals, Edison 

 has discovered a new metallic alloy, having its fusing point 

 much higher than that of any known metal.* 



* According to Edison's last patent, the incandescent spiral is formed of 

 platinum and iridium, and is covered with a metallic oxide, either of cerium, 



