On the Self-Demagnetizing Factor of Bar Magnets. 729 



indicates that it is an inert gas, and similar in that respect 

 to the group of monatomic gases. Taking the view that it 

 is monatomic, the emanation is the heaviest gas known with 

 a density 111 times that of hydrogen. 



For purposes of comparison, the atomic weight, boiling- 

 point, and density o£ liquid of the heavier monatomic gases 

 are given below. 



Radium 

 Ai-eon. Krypton. Xenon. Emanation. 

 Atomic Weights 39-9 82 128 222 



Absolute Boiling-point... 86°'9 121°'3 163°-9 208° 



"bSOS*"}- 1213 2 ' 155 3 ' 52 6? 



It is seen from the above table that the boiling-point of 

 xenon is about a mean between that of krypton and the 

 emanation. From the increase of density of the liquid with 

 atomic weight, it might reasonably be expected that the 

 density of liquid emanation should be about 6 — a result, as 

 we have seen, not inconsistent with experiment. In a similar 

 way, it is possible to form some idea of the probable critical 

 pressure and temperature of the emanation. 



I desire to express my thanks to the Radium Commission 

 of the Vienna Academy of Sciences for the loan of the 

 radium preparation which has made this and other work on 

 the emanation possible. 



LXVIII. On the Self-Demagnetizing Factor of Bar Magnets. 

 By Silvanus P. Thompson, D.Sc./F.R.S., and E.W. Moss*. 



[Plate XV.] 



THIS paper consists of three parts : — (i.) A discussion of 

 the significance and definition of the self-demagnetizing 

 factor of magnets in general, and of bar-magnets in particular; 

 (ii.) a redetermination of the values of the self-demagnetizing 

 factor for bar-magnets of circular section ; (iii.) determination 

 of the values of the self-doma<metizino; factor for bar-magnets 

 of rectangular cross-sections of various proportions. 



Part I. — Preliminary. On the Significance and 

 Definition of the Self-Demagnetizing Factor. 



Between any two magnet-poles, whether they are regarded 

 as points, or as regions over which there is a surface- 

 distribution of magnetism, there are magnetic forces. In 



* Communicated by the Physical Society : read February 26, 1909. 



