674 



SCIENCE. 



[Vol. II., No. 42, 



the circuit. The small number of parts used 

 in these fixtures, tlieir correct proportious, the 

 adaptation of tl\eir forms to machine-tool manu- 

 facture, and their beauty of design, excite the 

 admiration of the mechanic and the artist. 



^. 



The lamps are of thin glass, pear-shaped, 

 containing a thread of bamboo carbon al30ut 

 the thickness of a horse-hair. The small end 

 of the lamp contains glass of sufficient thick- 

 ness (fig. 13) to make a tight joint on the 



platinum-wire conductors which carrj- the cm- 

 rent to the carbon. The atmosphere is ex- 

 hausted from the lamp by Edison's modifica- 

 tion of the Sprengel pump, through a tube at 

 the upper end, and the tube is then fused and 

 broken otl'. Platinum wire is used because 

 its index of expansion is the same as glass, 

 thus preventing any leakage or breakage from 

 unequal expansion from the heat. The bam- 

 boo carbon and platinum wire are soldered 

 together by electrically deposited copper. One 

 wire, passing through the glass, is soldered to 

 a small brass disk, which is centred on the 

 bottom of the lamp, while the other wire is 

 soldered to the spun brass screw-thread which 

 surrounds the cylindrical part of the bottom of 

 the lamp ; and, when the lamp is screwed into 

 the socket (figs. 14 and 12), the circuit may 

 be completed or broken b}- the switch or kej* 

 already- descri,bed. When the circuit is closed, 

 the carbon thread becomes heated, from its 

 high resistance, to incandescence, and contin- 

 ues to glow, in vacuum, without burning, so 

 long as the current continues to flow. The 

 wires having a larger sectional area and higher 

 conductivity carrj- the current without percep- 

 tibly warming. By varying the length or sec- 

 tional area of the carbon thread, keeping the 

 electromotive force constant, Edison has varied 

 the eandle-^)ower of his lamps. 



For example : let the electrical resistance be 

 represented by R, the sectional area of the 

 wire or carbon bj' S, the length by i, and the 

 constant, dependent on the material of which 

 the conductor is made, by a; theu S li =^ a L, 

 from which simple equation the relative sizes 

 of carbons and wires may be determined, and 

 proportioned to - the tension in the circuit. 

 Mr. Edison employs a number of ditferent- 

 sized dj'uamos, which he designates by letters ; 

 but he winds them for but two tensions, i.e., 

 the A and B circuits. The A lamp belongs 

 to the A circuit, as its carbon thread is of such 

 resistance that the B circuit would heat it to 

 onlj- a cherry red. A B lamp, however, in the 

 A circuit, would acquire an intense brightness, 

 but its duration would be verv limited. Two 

 B lamps in series, in the A circuit, would, by 

 their augmented resistance, glow at about their 

 normal incandescence. 



The average life of a lamp is said to be 

 about 1,000 hours, when kept up to its normal 

 incandescence ; but they will last much longer 

 if their brightness is a little suppressed. This 

 may be effected either b}' throwing iu resist- 

 ance, or by slowing the engine. On board 

 ship, however, about as many lamps are broken 

 by accident as from natural deterioration. 



