288 



ADVANCED ELECTRICITY AND MAGNETISM. 



ionized vapor between the carbon tips is called an electric arc, 

 and the intense heating of the two carbon tips is due to their 

 bombardment by the ions which move across the arc and carry 

 the electric current. The electric arc may be easily maintained 

 between a hot carbon cathode and a rapidly rotating disk (a cold 



anode), but not between a cold 

 cathode and a hot anode. This 

 shows that the emission of nega- 

 tive ions (electrons) by the hot 

 cathode is essential to the forma- 

 tion of the electric arc. The ap- 

 pearance of the arc between carbon 

 electrodes is shown in Fig. 210.* 



146. Chemical effect of the dis- 

 charge through gases. The dis- 

 charge of electricity through gases 

 is accomplished by the ionization 

 of the gas as above explained. 

 This ionization means not only 

 the breaking down of the molecules 

 of a compound gas but also the 

 separation of electrons from indi- 

 vidual atoms. The ionization of 

 mixed gases promotes chemical 

 combination. Thus the nitrogen and oxygen of the air combine 

 slowly under the action of the electric spark. 



When oxygen (or air) is ionized, the recombination of the 

 oxygen ions results in the production of ozone. Thus the 

 peculiar odor which is given off by a Toepler-Holtz machine or a 

 Wimshurst machine is due to the ozone which is formed. The 

 action which takes place in the formation of ozone from oxygen 



* The properties of the electric arc are discussed in great detail in a paper by 

 C. P. Steinmetz, Transactions International Electrical Congress, Vol. II, pages 710- 

 730, St. Louis, 1904; in a paper by W. R. Whitney, Transactions American Electro- 

 chemical Society, Vol. 7, pages 291-299, 1905; and in J. J. Thomson's Conduction of 

 Electricity Through Gases, pages 604-620. 



Fig. 210. 



