CHAPTER II. 

 ARC IN AIR BETWEEN CARBON ELECTRODES. 



The arc shown in Fig. i was one between carbon ter- 

 minals. It is, however, possible to use any solid which is 

 an electrical conductor or any metal in the liquid state for 

 the electrodes. Electrolytes may even be used, if the 

 E.M.F. is sufficiently high. Any other gas or a vacuum 

 may take the place of air. However, only a few of the dif- 

 ferent forms of arc have been found to be of practical use, 

 and of these the arc in air between carbon electrodes has 

 been the most common. For the present we may give our 

 attention to this form of arc. 



The earliest carbon electrodes were pieces of charcoal 

 which had been heated and plunged into mercury to make 

 them better conductors, but in 1843 Foucault found that 

 better results were secured by employing pencils cut from 

 the hard graphitic carbon which is deposited on the in- 

 terior of gas retorts. The carbon electrodes which are 

 used to-day are usually made from retort or petroleum 

 coke which has but a small percentage of ash. Lampblack 

 has also been used for this purpose. Whichever is used, 

 it is heated to a high temperature for several hours, to drive 

 off any moisture or oil which may adhere to it. This 

 heating increases the conductivity of the mass. It is 

 then thoroughly mixed with some such binder as tar, and 

 formed into rods of the desired size while under a pres- 



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