1882.] on Electric Lighting hij Incandescence. 37 



Here is an example of a carbon filament produced from parch- 

 mentised cotton thread. The filament is not more than the '01 of an 

 inch in diameter, and yet a length of three inches, having therefore a 

 surface of nearly the one-tenth of an inch, gives a light of twenty 

 candles when made incandescent to a moderate degree. 



I have said, that, in order to preserve these slender carbon filaraents 

 from combustion, they must be placed in a vacuum ; and experience 

 has shown that if the filaments are to be durable, the vacuum must be 

 exceptionally good. One of the chief causes of failure of the earlier 

 attempts to utilise the incandescence of carbon, was the imperfection 

 of the vacua in which the white-hot filaments were placed ; and the 

 success which has recently been obtained is in great measure due to 

 the production of a better vacuum in the lamps. 



In the primitive lamps, the glass shade or globe which enclosed 

 the carbon filament w^as large, and usually had screw joints, with 

 leather or indiarubber washers. The vacuum was made either by 

 filling the lamp with mercury, and then running the mercury out so 

 as to leave a vacuum like that at the upper end of a barometer, or the 

 air was exhausted by a common air pump. The invention of the 

 mercury pump by Dr. Sprengel, and the publication of the delicate 

 and beautiful experiments of Mr. Crookes in connection with the radio- 

 meter, revealed the conditions under which a really high vacuum could 

 be produced, and in fact gave quite a new meaning to the word vacuum. 

 It was evident that the old incandescent lamp experiments had not 

 been made under suitable conditions as to vacuum ; and that before 

 condemning the use of carbon, its durability in a really high vacuum 

 required still to be tested. This idea having occurred to me, I com- 

 municated it to Mr. Stearn, who was working on the subject of high 

 vacua, and asked his co-operation in a course of experiments having 

 for their object to ascertain whether a carbon filament produced by 

 the carbonisation of paper, and made incandescent in a high vacuum 

 was durable. After much experimenting we arrived at the conclusion 

 that ivhen a loell-formed carbon filament is firmly connected ivith con- 

 ducting wires, and placed in a hermetically sealed glass hall, perfectly 

 exhausted, the filament suffers no apparent change even, ichen heated to 

 an extreme degree of tvhiteness. This result was reached in 1878. It 

 has since then become clearly evident that Mr. Edison had the same 

 idea and reached the same conclusion as Mr. Stearn and myself. 



A necessary condition of the higher vacuum was the simplification 

 of the lamp. In its construction there must be as little as possible of 

 any material, and there must be none of such material as could occlude 

 gas, which being eventually given out would spoil the vacuum. There 

 must besides be no joints except those made by the glass-blower. 



Therefore, naturally and per force of circumstances, the incan- 

 descent carbon lamp took the most elementary form, resolving itself 

 into a simple hulh, pierced hy two platinum wires supporting a filament 

 of carbon. Probably the first lamp, having this elementary character, 

 ever publicly exhibited, was shown in operation at a meeting of the 



