9 9 Jo 



Magnetic Tractive Force. 165 



force. The agreement is, however, on the whole such that 

 the equation (3) 



B 2 - 



Sirg 



can be regarded as verified. 



According to Maxwell's theory (I. c. § 643) the first term 



B 2 



rr — is the total electromagnetic tension existing in the narrow 



air-gap between the ends of the two bars ; similarly, H 2 /S7rg 

 is the tension in the gap, depending only on the " external " 

 field H. The correctness of this interpretation of the term 

 H 2 /87r# follows from the well-known experiments of Quincke 

 and others. 



This latter part of the stress is not included in the tractive 

 force in the present arrangement : it merely serves to com- 

 press the brass pillars S l5 S 2 , S 3 . 



If the bar C 2 were firmly fixed in its pole-piece (instead of 

 sliding in it), and if the two halves of the electromagnet were 

 pulled from each other, the pull in grammes weight in this 

 case, corrected for the tension due to all those tubes of in- 

 duction which do not pass along the isthmus, would be equal to 



B 2 



■5 — X area of contact. 



%Trg 



This case corresponds theoretically to the arrangement of 

 my former experiments, but cannot of course be practically 

 carried out. 



Measurement of High Inductions. 



After Maxwell's law of tractive force, or electromagnetic 

 stress, had been verified as explained above for inductions up 

 to 40,000 C.Gr.S., its general truth could be assumed. And 

 just as various physicists have conversely used the tractive 

 force for the measurement of lower inductions, it now seemed 

 feasible to apply a similar process to the measurement of the 

 highest attainable inductions, all other known methods failing 

 in this case. 



I therefore arranged a few more experiments with this 

 object. Two pole-pieces of 120° vertical angle were made of 

 the very best annealed Swedish iron. The point of one was 

 slightly flattened, ground plane, and polished, so that it pre- 

 sented a small circular pole-face whose diameter was only a 

 fraction of a millimetre. Through the opposite pole-piece a 

 slightly conical hole was bored, through which a long wire of 



