METALS AND SOME OF THEIR COMPOUNDS 269 



Aght for internal illumination. Several conditions were neces- 

 sary ; first electric current was wanted at a moderate price, and 

 next the means of producing a high vacuum pretty easily was 

 also indispensable, especially as the filaments to be made luminous 

 by the current were all at that time made of carbon. Sprengel's 

 mercury pump, invented in 1864, provided the means of getting 

 a vacuum, but there were great difficulties about the production 

 of threads of carbon. The late Sir Joseph Wilson Swan after 

 experiments made so long ago as 1860 exhibited the first electric 

 glow lamp in February, 1879, at a meeting of the Newcastle 

 Chemical Society, and in November, 1880, gave a demonstration 

 at the Institution of Electrical Engineers in London. His 

 carbon filaments were first made by heating parchmentised 

 thread, and later by squirting collodion through a die. These 

 were exhibited at the Inventions Exhibition in 1885. 1 Mr. T. A. 

 Edison, the well-known American inventor, had in the meantime 

 begun working on the subject, but rival claims disappeared 

 under a prudent and amicable arrangement, and in the end the 

 Edi-Swan lamp became familiar to everyone. 



In more recent years, however, the filament to be heated, and 

 so made luminous by the current, is more usually made of some 

 metal of low conducting power. Platinum was the first in which 

 the phenomenon of luminosity produced by the current was 

 studied, but platinum, though its melting point is high, is too 

 fusible for use in the lamp. It has one property which has made 

 it useful in the lamps and that is its low coefficient of expansion. 

 As it expands and contracts with change of temperature to 

 nearly the same extent as glass a wire of this metal can be melted 

 into glass and on cooling the glass does not crack. Hence 

 platinum may be used for making the connections between the 

 fittings outside the lamp and the filament which gives the light 

 within. 



Many metals and alloys have been tried for the production of 

 lamp filaments, the object being to obtain the maximum of light 

 with the minimum expenditure of current. Tungsten and tan- 

 talum have found the most success, but the details of the pro- 

 cesses by which these metals are obtained in the form of suffi- 

 ciently thin but yet strong wires have so far been kept secret. 



Tungsten (p. 266) when quite pure is tough and ductile, 



1 A collection of apparatus used in Swan's early experiments is now exhibited 

 in the Science Museum, South Kensington. 



