ELECTRIC LIGHT 205 



arrives at the other pole of the pile by the upper cross piece of the apparatus and the 

 metal column, to which one of the conducting wires is attached. 



So long as the currrent is passing and producing light, the bobbin reacts upon the 

 iron of the electro-magnet rod, which carries the lower carbon and attracts it on 

 account of the magnetic reaction that solenoids exercise over a moveable iron in their in- 

 terior. It is this which gives to the carbons a separation sufficient for the luminous effect. 

 But immediately the current ceases to pass, or is weakened, in consequence of the 

 consumption of the carbons, this attraction ceases, and the moveable carbon, acted on 

 by the counterpoise, is found to be drawn on and raised until the current passes 

 again ; equilibrium is again established between the two forces, and the carbons may 

 be employed again. Thus, in proportion as the light tends to decrease, the coun- 

 terpoise reacts ; and this it is that always maintains the intensity of the light equal. 



M. Breton has an apparatus which differs somewhat from the above, and M. 

 Poucault has also devised a very ingenious modification. 



M. Duboscq has made by far the most successful arrangement (fig. 790), for a descrip- 

 tion of which we are indebted to De la Rive's ' Treatise on Electricity/ translated by 

 C. V. Walker. 



The two carbons, between which the light is developed, burn in contact with the 

 air, and shorten at each instant ; a mechanism is consequently necessary, which 

 brings them near to each other, proportionally to the progress of the combustion ; 

 and since the positive carbon suffers a more rapid combustion than the negative, it 

 must travel more rapidly in face of this latter ; and this in a relation which varies 

 with the thickness and the nature of the carbon. The mechanism must satisfy all 

 these exigencies. The two carbons are unceasingly solicited towards each other, 

 the lower carbon by a spiral spring, that causes it to rise, and the upper carbon by 

 its weight, which causes it to descend. The same axis is common to them. 



The galvanic current is produced by a Bunsen's pile of from 40 to 50 elements : 

 it arrives at the two carbons, as in apparatus already known, passing through a hollow 

 electro -magnet, concealed in the column of the instrument. When the two carbons 

 are in contact, the circuit is closed, the electro-magnet attracts a soft iron, placed at 

 the extremity of a lever, which is in gear with an endless screw. An antagonist 

 spring tends always to unwind the screw as soon as a separation is produced between 

 the two carbons ; if it is a little considerable, the current no longer passes, the action 

 of the spring becomes predominant, the screw is unwound and the carbons approach 

 each other until the current, again commencing to pass between the two carbons, the 

 motion that drew them towards each other is relaxed in proportion to the return of 

 the predominance of the electricity over the spring ; the combustion of the carbons 

 again increases their distance, and with it the superior action of the spring ; hence 

 follows again the predominance of the spring, and so on. These are alternatives of 

 action and reaction, in which at one time the spring, at another time the electricity, 

 has the predominance. On an axis, common to the two carbons, are two pulleys : 

 one. the diameter of which may be varied at pleasure, communicates by a cord with 

 the rod that carries the lower carbon, which corresponds with the positive pole of the 

 pile ; the other, of invariable diameter, is in connection with the upper or negative 

 carbon. The diameter of the pulley, capable of varying proportionately to the using 

 of the carbon, with which it is in communication, may be increased from three t 

 five. The object of this arrangement is to preserve the luminous point at a con- 

 venient level, whatever may be the thickness or the nature of the carbons. It is only 

 necessary to know that at each change of kind or volume of the carbon, the diameter 

 of the pulley must be made to vary. This variation results from that of a moveable 

 drum, communicating with six levers, articulated near the centre of the sphere ; the 

 moveable extremity of the six arms of the lever carries a small pin, which slides in 

 cylindrical slits. These slits are oblique in respect of the sphere ; they form inclined 

 planes. A spiral spring always rests upon the extremity of the levers ; so that if 

 the inclined planes are turned towards the right, the six levers bend towards the 

 centre, and diminish the diameter. If, on the contrary, they are turned towards the 

 left, the diameter increases, and with it the velocity of the translation of the carbon, 

 which communicates with the pulley. We may notice, in passing, that this apparatus 

 is well adapted to the production of all the experiments of optics, even the most 

 delicate ; and that, in this respect, it advantageously supplies the place of solar light. 

 Mr. (now Justice Sir William) Grove calculated, some years ago, that for acid, zinc, 

 wear and tear, &c. of batteries, a light equal to 1,444 wax candles could be obtained 

 for about 3s. 6d. per hour. The cost of the light employed for about five minutes at Her 

 Majesty's Theatre, as an incident in the ballet, which was obtained by employing 75 

 cells of Callan's battery of the largest size, was said to be 21. per night. In this 

 calculation we expect we have not a fair representation of all the conditions. To ob- 

 tain a light for ten minutes, a battery as large must be used as if it were required to 



