ELECTRIC LIGHT. 



273 



which is placed above being about half an inch 

 in thickness, while the negative carbon A, 

 placed below, is only about a quarter of an 

 inch thick. These two slabs of carbon are in 

 contact only along one edge. As soon as the 

 electric current passes through them, it brings 

 into play an electro-magnet, which lifts the 

 positive carbon about one eighth of an inch 

 away from the negative carbon below, which 

 is fixed. Across the gap thus made the voltaic 

 arc is established, the light being produced at 

 the point of least resistance between the car- 

 bons. At the luminous focus the space between 

 the plates gradually widens in consequence of 

 the combustion of the carbons, thereby in- 

 creasing the resistance at that spot. A time is 

 soon reached when the current is unable to 

 overcome this resistance, and it then establish- 

 es itself at an adjacent point which offers less 

 resistance. In this way the light slowly trav- 

 els from end to end along the edges of the car- 

 bons; but when it reaches the extremity it 

 makes a turn and slowly marches from point 

 to point in the opposite direction. The dis- 

 tance between the two edges is kept constant 

 by appropriate mechanism, and the light is thus 

 maintained of uniform intensity. It is stated 

 that with this arrangement the light can be 

 kept going for one hundred hours without re- 

 quiring a change of carbons. 



Another form of lamp, invented by M. Jab- 

 lochkoff, a Russian officer, in 1876, and now 

 extensively used in Paris and London, dis- 

 penses entirely with a regulator ; indeed, its ex- 

 treme simplicity of construction, and the man- 

 ner in which it burns, have gained for it the 

 name of the electric candle. Fig. 6 shows a 

 single one of these candles, which consists of 

 two pencils of carbon, each socketed in a brass 

 tube, one of which tubes is connected with the 

 positive and the other with the negative pole of 

 the electric machine. Between the two cylin- 

 drical rods of carbon is interposed a layer of 

 insulating material which keeps them electri- 

 cally separate while mechanically united. At 

 first the inventor used for this purpose a com- 

 position containing kaolin or China clay, but 

 this was afterward displaced by common plas- 

 ter of Paris, and lately Mr. Wilde has shown 

 that the insulating material may be wholly 

 dispensed with, as he obtains the light by sim- 

 ply mounting the two rods side by side, the 

 carbons being merely coated with hydrate of 

 lime. A small bridge of carbon at the upper 

 extremity of the rods serves temporarily to 

 connect them with each other and offers a 

 passage for the electric current from pole to 

 pole ; and when once this passage has been es- 

 tablished the arc is afterward self-maintained. 



If the two carbons were consumed at an 

 equal rate, the distance between them would 

 be kept uniform, that is, they would burn down 

 evenly together. But, as a matter of fact, the 

 positive carbon is consumed twice as rapidly 

 as the negative carbon. It is therefore neces- 

 sary to resort to some device that will keep 

 VOL. xvni. 18 A 



their extremities constantly opposite each other. 

 This was first sought to be accomplished by 

 making the rapidly burning carbon propor- 

 tionally thick. If the sectional area of the 

 positive carbon were twice that of the nega- 



FIG. 6. JAB:LOCHKOFF'S ELECTR-C CANDLE (actual size). 

 C C, carbon points of gas-coke ; 1 1 1 1, insulating 

 substance ; T T, tubes holding the carbon points : A, 

 socket of asbestus holding the system together ; F F, 

 copper wires conyeying the electric current. 



tive~ it might be supposed that the ends of the 

 two rods would be kept constantly at the same 

 level. This was tried, but proved far from 

 satisfactory, though it greatly improved the 

 light. The difficulty was subsequently over- 



