ELECTRIC LIGHT. 



275 



for this mode of lighting was invented by Mr. 

 E. A. King, of London, in 1845. In it a narrow 

 strip of platinum-leaf was held vertically be- 

 tween suitable conductors and rendered lumi- 

 nous by a properly regulated current ; the 

 whole arrangement being protected by a glass 

 globe which screened the incandescent metal 

 from currents of air. Four years later Petrie 

 invented a lamp in which iridium in the form 

 of small rods was used in the place of plati- 

 num ; and it is rumored that Mr. Edison gets 

 his light by the incandescence of an alloy of 

 platinum and iridium. 



In 1873 M. Lodyguine, a Russian physicist, 

 again called attention to the subject by the in- 

 vention of a lamp in which the light was pro- 

 duced by the incandescence of carbon. The 

 rod of carbon through which the current pass- 

 es is cut thin in one portion of its length, and, 

 as the electric resistance is therefore greater 

 here than elsewhere, it is this part alone that 

 becomes incandescent. In order to avoid the 

 waste of oxidation, the rod is inclosed in a her- 

 metically sealed glass chamber from which the 

 air has been exhausted ; but even in a vacuum 

 the carbon is slowly destroyed, and the pencil 

 in Lodyguine's lamp was soon burned out. To 

 overcome this difficulty, a lamp similar in prin- 

 ciple but of more ingenious construction was 

 patented in 1875, by M. Kohn, of St. Peters- 

 burg, in which several carbon rods are placed 

 side by side in such a relation that as soon as 

 one is used up another is automatically brought 

 into the circuit. In this way the light may be 

 maintained without in- 

 terruption for several 

 hours; and this lamp has 

 been used for the illu- 

 mination of warehouses 

 and other large buildings 

 with very good results. 



The Sawyer-Man lamp, 

 which has attracted con- 

 siderable attention in this 

 country, also employs car- 

 bon for the production of 

 the light. The apparatus 

 is shown in Fig. 8. 



The light - giving ar- 

 rangement is separated 

 from the lower part of the 

 lamp by three diaphragms 

 which cut off downward 

 heat radiation. The cop- 

 per standards below are 

 so shaped as to present 

 a great radiating surface, 

 whereby the conduction 

 of heat downward to the 

 mechanism at the base is wholly prevented. 

 The electric current enters from below, fol- 

 lows the metallic conductor to the burner, and 

 thence downward on the other side to the re- 

 turn circuit. The light-producing portion is 

 completely insulated, and also sealed at the 

 base gas-tight. The glass vessel is charged 



with pure nitrogen, and the crumbling of the 

 carbon due to sudden heating when the lamp 

 is lighted is provided against by the use of a 

 switch so contrived that it is impossible to 

 turn the current on or off abruptly. 



So much for the lamps ; now for the machines 

 which supply the power to run them. Little 

 progress toward the extension of electric light- 

 ing could be made as long as the voltaic battery 

 was the only source of electricity, the cost of 

 generating it by this means being very great. 

 But after Faraday's discovery of magneto-elec- 

 tricity in 1831, machines were constructed for 

 the production of electricity by the rotation of 

 an induction coil in front of the poles of a 

 magnet. Here the mechanical force expended 

 in the rotation of the coil or armature, as it is 

 called, is transformed into electricity, while in 

 the battery it is chemical force that gives rise 

 to the electric energy. Machines in which 

 permanent magnets are thus used are gener- 

 ally known as magneto-electric machines, and 

 among those first employed the devices of 

 Pixii, Clarke, and Saxton were long conspicuous. 

 But perhaps the best known apparatus of this 

 class, and the one still used to some extent for 

 lighthouse purposes abroad, is the "Alliance 

 Machine," invented by Nollet and Van Mal- 

 deren, of Brussels. This machine (Fig. 9) has 

 eight rows of compound horseshoe magnets 

 fixed symmetrically round a cast-iron frame. 

 They are so arranged that opposite poles al- 

 ways succeed each other, both in each row and 

 in each circular set. There are seven of these 



FIG. 9. 



circular sets, with six intervening spaces. Six 

 bronze wheels, mounted on one central axis, 

 revolve in these intervals, the axis being driven 

 by stearn -power transmitted by a pulley and 

 belt. The speed of rotation is usually about 

 350 revolutions of the axis per minute. Each 

 of the six bronze wheels carries at its cir- 



