IV PREFACE 



Ayrton's book. A rather full account has, accordingly, 

 been given to such experiments as those on the mercury 

 arc, and to a discussion of the more recent theories. 



The use of the arc in commercial ways has already been 

 ably discussed in such books as those on Electric Lighting, 

 Photometry and Wireless Telephony. Accordingly, these 

 topics have received less attention here. 



A few pages, 'however, are given to photometry and to 

 the whistling arc, since these are of interest from a scien- 

 tific as well as from an industrial standpoint. No account 

 has been given concerning the use of the arc in chemical 

 and metallurgical processes, since its function there ap- 

 pears to be merely to produce a high temperature and a 

 study of these phenomena would give us no knowledge 

 concerning the arc itself. 



I have endeavored to keep in mind the needs of those 

 who may wish to make investigations in the future. An 

 effort has, therefore, been made to give references to all the 

 important articles on this subject, excepting those which 

 relate only to the commercial side or to those concerning 

 investigations in which the arc was merely a means for 

 studying some other phenomenon, as when used to pro- 

 duce the spectrum of a metal. 



It would often have made much simpler and more satis- 

 factory reading, if I could have given a brief and definite 

 statement of the laws governing the action of the arc, in- 

 stead of producing so extended a review of what different 

 experimenters have thought about these laws, but in the 

 majority of cases it is not yet known what the laws are 

 and the only available method is to discuss the results of 

 those who have endeavored to find them. Not only is 

 there this uncertainty concerning the laws, but the expla- 



