Notices respecting New Books. 623 



Tlie Electric Arc. By Heetha Ayrton, Member of the Institution 

 of Electrical Engineers. London: " The Electrician " Printing 

 and Publishing Company, Limited, 1902. Pp. xxvi-{-479. 



The Electric Arc is one of those physical phenomena which, 

 although generally known for well-nigh a century, have defied the 

 efforts of the experimenter, and resisted most of the attempts to 

 lift the veil of mystery surrounding them. It is only within recent 

 years that our notions regarding the processes which take place in 

 the electric arc have beguu to clear ; and this result is in no small 

 measure due to the distinguished writer of the book before us, who 

 by a series of experiments requiring an amount of care and patience 

 that few could command, has succeeded not only in establishing 

 definite laws regarding the phenomenon of the arc, but has also 

 thrown a flood of light on the labours of previous experimenters, 

 and put forward a hypothesis regarding the mechanism of the arc 

 which is in striking agreement with the experimental knowledge 

 of the present day. 



Notwithstanding the striking nature of the phenomenon, and 

 the fascination which it seems at all times to have exerted upon 

 the experimenter, the origin of its discovery seems to be hidden in 

 obscurity. Davy is generally credited with the discovery of the 

 electric arc, but, as Mrs. Ayrton clearly points out in the interesting 

 chapter on the History of the Arc, his claims are by no means as 

 substantial as is generally assumed to be the case. The difference 

 between an ordinary electric spark and an arc does not appear to 

 have been realized in the early days of the arc, this latter being 

 regarded simply as a species of spark. 



It will be useful here to give a brief outline of the contents of 

 Mrs. Ayrton's book. Chapter I. contains a very clear description, 

 illustrated by numerous fine diagrams and sketches, of the appear- 

 ance of the arc, and the effects produced by changes in the current 

 and length of arc, and by coring the carbons. In Chapter II. we 

 have a brief history of the arc, which includes a most useful 

 summary of the researches of Continental physicists. This chapter 

 must have involved an immense amount of labour, and English 

 readers will feel grateful to Mrs. Ayrton for having brought w T ithin 

 their reach much information which has up to the present remained 

 inaccessible. Chapters IV., V. and VI. deal with the relations 

 connecting P.D., current, and length of arc. The distribution of 

 potential in the arc, and the drop of potential in passing from the 

 arc to either carbon, are considered in Chapter VII. The next 

 two Chapters, VIII. and IX., deal with the steadying resistance 

 included in the arc circuit, and the power efficiency of the arc. 

 Chapter X. contains an account of Mrs. Ayrton's highly interesting 

 experiments on hissing arcs, and her theory regarding the cause 

 of hissing. Chapters XI. and XII . are concerned with the luminous 

 efficiency of the arc, and include an account of Mrs. Ayrton's 

 theory regarding the mechanism of the arc, such questions as the 



