CHAP, vii.] ELECTRICITY AND MAGNETISM. 335 



than the arc itself, though they are not so hot. In the 

 arc the most infusible substances, such as flint and 

 diamond, melt ; and metals such as gold and platinum 

 are even vapourised readily in its intense heat. When 

 the arc is produced in the air the carbons slowly burn 

 away by oxidisation. It is observed, also, that particles 

 of carbon are torn away from the + electrode, which be- 

 comes hollowed out to a cup-shape, and some of these 

 are deposited on the electrode, which assumes a 

 pointed form, as shown in Fig. 137. The resistance of 

 the arc may vary, according to circumstances, from 0-5 

 ohm to nearly 100 ohms. To produce an electric light 

 satisfactorily a minimum electromotive -[force of 40-50 

 volts is necessary ; and, as the current must be at least 

 from 5 to i o or more webers, it is clear that the internal 

 resistance of the battery or generator must be kept 

 small. With weaker currents or smaller electromotive- 

 forces it is impracticable to maintain a steady arc. The 

 internal resistance of the ordinary Daniell's or Leclanchd's 

 cells (as used in telegraphy) is too great to render them 

 serviceable for producing electric lights. A battery of 

 40-60 Grove's cells (Art. 171) is efficient, but will not 

 last more than 2 or 3 hours. A dynamo-electric machine 

 (such as described in Art. 417 to 420), worked by a 

 steam - engine, is the best generator of currents for 

 practical electric lighting. 



372. Electric Lamps or Regulators. Davy 

 employed wood -charcoal for electrodes to obtain the 

 arc ; pencils of hard gas -carbon were later introduced 

 by Foucault. These consume away less rapidly, but 

 still some contrivance is necessary to push the points of 

 the carbons forward as fast as needed. Many pieces of 

 self-adjusting mechanism have been devised by Duboscq, 

 Foucault, Serrin, Siemens, Brush, and others, to meet 

 the difficulties of the case ; the best of them having the 

 carbon rods attached to a toothed-wheel apparatus which 

 pushes them forward at the proper rates, and which is 



