ELECTRIC LIGHT, EDISON'S. 



337 



exposed to tho atmosphere to a temperature near 

 tlu-ir melting-point, by a current of electricity 

 passing through them for a number of hours to- 

 gether, crack and break in innumerable places. 

 Those fissures are found under the microscope 

 all over the surface of tho metal, running in 

 every direction, and sometimes penetrating to 

 the center of the rod or wire. Holding plati- 

 num and alloys of platinum and iridium in the 

 heat of a candle, he observed a loss of weight; 

 and even when they are exposed to heated air 

 there is a diminution of weight. The consump- 

 tion is sufficient to cause a hydrogen jet to take 

 on a greenish hue. The metal after a while 

 becomes so fractured that it falls to pieces. He 

 thus perceived that the ordinary platinum or 

 platinum and iridium, as sold in the market, is 

 useless for his purpose, and also that the metal 

 can not be employed for illumination by incan- 

 descence, as the cracks cause it gradually to de- 

 teriorate and eventually destroy it, while they 

 greatly lessen the degree of incandescence of 

 which its surface is capable. The knowledge of 

 the cause of the disintegration of platinum sug- 

 gested the remedy. Lodyguine, the Russian 

 physicist, invented a carbon-lamp in 1873, in 

 which tho cracking and wasting away of the car- 

 bon under incandescence, by the action of the 

 oxygen of theatmosphere,wasobviated by inclos- 

 ing the burner in a glass globe from which the air 

 was exhausted. It was necessary to purify the 

 platinum and inclose it in a vacuum to prevent 

 its deterioration when heated to incandescence. 

 Edison devised a method of producing a more 

 perfect vacuum, and at the same time cleansing 

 the platinum humor of all the air and other 

 gases which it contains. A glass globe is con- 

 nected by an aperture with a mercury air-pump, 

 and the air exhausted. The wires connecting 

 the spiral or other form of burner with the 

 battery pass through holes in the glass which 

 are fused together and hermetically sealed. 

 After the air is exhausted from the glass the 

 current is turned on, heating the platinum to 

 a temperature of about 150 F., at which point 

 it is kept for from ten to fifteen minutes. The 

 gases which issue from the platinum are car- 

 ried away by the air-pump. The current is 

 then increased until the temperature rises to 

 800, at which point it is kept again ten or fif- 

 teen minutes. It is thus raised by successive 

 stages until the platinum attains a brilliant in- 

 candescence, and the glass about the aperture 

 connecting with the mercury pump melts with 

 the heat and fuses together, hermetically seal- 

 ing the vacuum. The wires purified by this 

 process are found to have a gloss and bright- 

 ness greater than that of silver. Their light- 

 giving power is increased in a remarkable ratio. 

 The same burner which will give when new a 

 light of only three candles, emits in the vacuum 

 a light of twenty-five. Testing spirals which 

 had been prepared and sealed in a glass vacuum 

 in this manner by subjecting them to sudden 

 currents of electricity which raised them to in- 

 candescence a great number of times, no cracks 

 VOL. xix. 22 A 



or breaks were discoverable, nor the slightest 

 loss of weight. Wires of chemically pare iron 

 and nickel were found to give a light in tho 

 vacuum equal to that of platinum exposed to 

 tho atmosphere ; and carbon-sticks, freed from 

 air and inclosed in a vacuum in the same man- 

 ner, may be heated until they become soft and 

 plastic, and then regain their former consis- 

 tency when cool again. Edison next tried tho 

 combination of platinum and iridium alloy with 

 magnesia in the vacuum. He found that the 

 oxide will unite with the metal, hardening it 

 and rendering it more refractory to such a de- 

 gree that a spiral so fine that it would melt 

 without the coating of magnesia could be raised 

 to a dazzling incandescence and remain quite 

 elastic. Such a spiral, with a surface of only 

 three-sixteenths of an inch, will give a light of 

 forty candles. He next turned his attention to 

 securing the greatest possible amount of resis- 

 tance in the conductor. Instead of using lamps 

 of only one or two ohms of resistance, he 

 reached the conoluson that the light could be 

 more economically produced from conductors 

 having two hundred ohms of resistance or 

 more. 



The perfected form of tho platinum lamp 

 consists of a long coil of wire coated with 

 magnesia, supported in a glass vacuum tube by 

 a rod of platinum, the tube resting upon a 

 metallic frame containing a regulating ap- 

 paratus in a chamber within. The conducting 

 wires pass through the bottom of the globe 

 and into this chamber, where the circuit can 

 be instantaneously broken and closed again by 

 the regulator. Around the vacuum tube is a 

 glass globe resting upon the frame, with open- 

 ings into an aneroid chamber below, whose 

 bottom is a diaphragm which distends suffi- 

 ciently when the air within the globe is heated 

 to a certain degree to press a pin in its center 

 downward against a straight spring, which 

 rests with an upward pressure upon a metallic 

 block, through which the current is transmitted 

 through the spring to the wire which leads it 

 to the incandescent spiral. When the contact 

 between the spring and the block is broken, 

 the flow of electricity is interrupted, to be re- 

 stored again by the immediate cooling and con- 

 traction of the air in the globe and aneroid 

 chamber, which is so instantaneous that no varia- 

 tion in the intensity of the light is perceptible. 

 While bringing the platinum lamp to this high 

 state of perfection, Edison set on foot inquiries 

 regarding a larger supply of platinum ; and the 

 miners of the gold regions, incited by his ad- 

 vertisements, discovered such frequent indica- 

 tions of its presence that this exceedingly 

 valuable metal may be expected to bo produced 

 in much larger quantities than the present 

 supplies. The vacuum which Edison's method 

 produced was much nearer perfect than had 

 been before attained. One of the reasons for 

 the want of success of lamps in which the 

 light was produced by tho incandescence of 

 carbon in a vacuum was the impossibility of 



