1 7 6 ELECTRIC LIGHTING. 



In Gaiffe's Lamp the two carbon-holders H, H' are movable, 

 as in Foucault's and in Serrin's arrangements, and they are 

 so arranged as to keep the luminous point stationary. For 

 this purpose their motion is controlled by two racks K, u, 

 which engage two wheels M', o, of unequal diameter, and 

 moved by a simple spring barrel on the axle of which they 

 are fixed. This barrel is wound up by the mere separation 

 of the carbon-holders, which are, however, perfectly balanced 

 and turn between sets of rollers. The lower carbon-holder is 

 terminated by an iron rod K, to which is attached a rack, and 

 this rod is placed within an electro-magnetic bobbin L, with 

 a coil increasing in diameter from its upper to its middle 

 part in order to compensate for the unequal action of the 

 spring barrel through the whole range of this movement. 

 Finally, a small wheel R connected by the wheels M', o, and 

 another to the wheel M gives the means of simultaneously act- 

 ing on the two racks so as to raise or lower the luminous point. 



In the normal state the carbons touch each other, and 

 when the current traversing them excites the coil, the lower 

 carbon-holder is lowered at the same time that the upper one 

 HVI is raised, and this effect is continued until the attractive 

 force of the coil balances the resistance of the spring barrel, 

 and thus a voltaic arc is produced. Of course the length of 

 the arc depends upon the tension of the spring in the barrel, 

 and this can be regulated by a screw. So long as the arc 

 remains under the same conditions of resistance, the effect 

 is maintained ; but the moment the resistance increases on 

 account of the consumption of the carbons, the power of the 

 spring prevails over the electro-magnetic action, and the car- 

 bons are brought nearer together, until a condition of equili- 

 brium is again attained, and the same thing goes on until the 

 carbons are entirely consumed. 



The small mechanism fitted to the wheels of the racks 

 allows the luminous point to be raised or lowered, by means 

 of a key, without the lamp going out, and this is necessary 

 in optical experiments in order to properly centre the light. 



