290 Prof. Max Mason on the Flow oj 



Summary. 



1. A supply o£ negative ions from without to a positively 

 electrified point lowers the ionizing field at its surface. 



2. Positive ions supplied to a negative point are without 

 effect when the point is new. 



These two facts are shown to be consistent with accepted 

 theory. 



3. A negative point may become aged with use, but 

 temporarily acquires the properties of a new one when 

 bombarded with positive ions. 



4. The minimum ionizing field for fully formed negative 

 ions is about half, and that for corpuscles about one-seventh 

 of the field in which ordinary positive point discharge takes 

 place. In each case the field is measured at the surface of 

 the metal. 



XXIII. The Flow of Energy in an Interference Field. By 

 Max Mason, Ph.D., Professor of Mathematical Physics y 

 University of Wisconsin*, 



THE following investigation may answer some of the 

 questions recently raised by Professor R. W. Wood |, 

 regarding the lines of energy flow in a field produced by two 

 similar light sources. 



The discussion will be limited, for simplicity, to the 

 following case : Two points A l5 A 2 are centres of electro- 

 magnetic radiation, produced by the isochronous vibration of 

 equal point charges. The direction of vibration will be 

 taken at right angles to the line A 1 A. 2 . Those lines of energy 

 flow will be studied which lie in the plane containing the 

 line AxA 2 and perpendicular to the direction of vibration 



§ 1. The differential equation of the lines of mean 

 energy flow. 



Let r x and r 2 be the distances from A 2 and A 2 to the 

 point P (fig. 1). The electric and magnetic vectors at P 

 due to the radiation from A 1 and from A 2 will be denoted 

 by E l5 H x ; E 2 , H 2 . The vectors k x and k 2 are of unit 

 lengthi and have the directions from A 1 to P and from 

 A 2 to P respectively; j is a unit vector in the direction of 

 vibration. 



* Communicated by the Author. 

 t Phil. Mag. 1009, xviii. p. 250. 



