20 ELECTRIC LIGHTING. 



the carbon, whence they dart across to the opposite electrode. 

 These globules are due to mineral substances, and, among 

 others, to the alkaline carbonates usually contained in gas- 

 retort carbons, which fuse under the excessively high tem- 

 perature of the voltaic arc. These globules, which are not 

 formed with very pure carbon, disturb the fixedness and con- 

 stancy of the light. 



When the voltaic arc is produced in the air the two carbons 

 are soon consumed by burning, but on account of the dif- 

 ferent physical conditions of the two electrodes, and the 

 transport of carbonized particles by the current, one of the 

 electrodes the positive burns much more quickly than the 

 other ; and that in the proportion of two to one. This un- 

 equal consumption occasions several inconveniences : in the 

 first place, a displacement of the luminous point, and, be- 

 sides this, a disfigurement of the polar extremities of the two 

 electrodes, one of which becomes more pointed, while the 

 other is hollowed out in the form of a crater, thus surround- 

 ing the luminous point with a more or less prominent rim, 

 which acts as a shade. The polarization effects induced on 

 the electrodes themselves give rise to somewhat complex 

 effects, which result in an elevation of temperature, and 

 ought to be taken into consideration. With currents alter- 

 nately reversed, like those supplied by the induction machines 

 of the Alliance Company, these inconveniences do not exist; 

 the consumption of the carbons is equal and regular, and 

 their points, always remaining perfect in shape, give out the 

 light in its entirety. With regard to this matter, what at first 

 was supposed to be a defect in the magneto-electric machines, 

 and a defect which must be corrected by means of reversing 

 commutators, is, on the contrary, an advantage, not only by 

 avoiding the loss of current which takes place through these 

 commutators, but by the better conditions under which the 

 work is produced. 



These effects, however, have not been regarded in England 

 in the same way as they have been by us, and in a report drawn 



