﻿of finding the Forces acting in Magnetic Circuits. 91 



referred to above (§ 1), the stresses in the medium in the air- 

 gap are not at all the same as the stresses in the medium in 

 tho iron. In our entire ignorance of the connexion existing 

 between the medium and matter, it is not to me at all clear 

 that, in calculating the magnetic forces tending to close the 

 ring, we ought to consider the stresses in aether in air, and 

 those alone. It is at all events conceivable that the nature 

 of the connexion between the medium and the iron may be 

 modified in some manner by the internal stresses in the iron. 

 Also the ordinary laws of magnetic and electromagnetic 

 action leceived their experimental demonstration at low in- 

 ductions ; and we have no right to say, without experimental 

 evidence, that some terms not contemplated by Maxwell 

 might not begin to produce effects on the stresses in air at 

 high inductions. In the parallel case in iron such stresses do 

 in fact occur. 



I therefore attribute great importance to the experimental 

 verification of the results deduced from Maxwell's theory as 

 applied to the traction between iron bars in general, and 

 especially at high inductions. 



§ 5. Experimental position. — The simplest case is that of 

 the traction between two plane faces of iron, the faces being 

 either the terminals of otherwise closed iron circuits, or of 

 very long bars. 



The case of the ring has been implicitly investigated by 

 Bidwell (Phil. Trans. 1886) ; and the case of short bars by 

 Bosanquet explicitly (Phil. Mag. 1886, vol. xxii.). The 

 latter is the only investigation 1 know of in which simul- 

 taneous observations of Induction and Tractive force were 

 made. In both cases Maxwell's theory leads to the expression 



87T 



for the force in air between two opposing plane faces of iron, 

 infinitesimally separated, A being the area of the faces and 

 B the (uniform) induction-density. 



The net result of Bosanquet's work was to show : — 



(1) When B is below 5000 the Tractions observed are 

 generally much too large. 



(2) The formula does not hold when the air-gap is appre- 

 ciable. 



(3) It holds within about 5 per cent, up to very high 

 inductions. 



It is obvious, therefore, that there is room for more work 

 on the subject. 



§ 6. For the reasons given I felt very strongly that it was 



