Magnetic Tractive Force. 167 



a current of 40 amperes. The total calculated field is thus 

 51,200 C.G.S., which agrees well with the value found above. 

 In this experiment the area of the isthmus was about 

 5 of the area of the base of the conical pole-piece. 



200>000 



This induction (B = 74,200) exists also by continuity in the 

 narrow air-gap at the contact, and here it is to be regarded 

 as " external " field. This will not be much altered at the 

 moment of separation as long as the gap is narrow in com- 

 parison with its transverse dimension. It was then observed 

 with a magnifying glass that, unless the apparatus was very 

 carefully cleaned, small microscopical iron filings flew into the 

 gap when separation took place and formed new " isthmuses " 

 across it, whose thickness was very small in comparison with 

 that of the \ millim. iron wire, so that their self-demagnetizing 

 effect could be neglected. The induction B' in these is thus 



B'= 74200 +4^.1800=96,800 C.G.S. 



We may therefore conclude that inductions of nearly 

 100,000 C.G.S. can exist in iron without any special occur- 

 rence. 



If the wire is pulled back so that its end is in the plane of 

 the edge of the opening, the field between the pole-faces is 

 then about 60,000 C.G.S. ; it only extends, however, over a 

 few tenths of a millimetre*. 



At the instant of separation a tension of (74200) 2 /8?r^ = 224 

 kilogs. weight per square centim. (3185 lb. wt. per sq. inch) 

 is transmitted across the air-gap ; this is about the limit 

 of elasticity of lead. In this connexion it is interesting to 

 remember that the tension between the plates of an air- 

 condenser at atmospheric pressure cannot far exceed 2 grms. 

 weight per square centim. without a spark appearing. An 

 absolute vacuum, however, or a body of high specific induc- 

 tive capacity can transmit much greater tensions. 



My best thanks are due to Dr. H. du Bois, in whose 

 laboratory the above experiments were carried out, for his 

 help and advice. 



Berlin, Christmas, 1895. 



* If the field extends over several millimetres an intensity of over 

 40,000 C.G.S. can scarcely he reached. See du Bois, Wied. Ann. 1. 

 p. 547 (1894) ; J. B. Henderson, Phil. Mag. Nov. 1894. 



