cl 2 



Electromagnetic Stress. 257 



found, however, that the stress agreed better with an ex- 

 pression of the form 



where I is the magnetic moment per unit volume. Wass- 

 rnuth's experiments are subject to the same objections as 

 Siemens', viz., imperfect contact of magnet and keeper due to 

 bending &c, and, in addition, to the difficulty of ensuring that 

 the magnets separate at the two places of contact exactly 

 simultaneously. If separation takes place at one place first, 

 there will be an immediate diminution of the induction, and 

 the limiting weights will generally be too small. 



Wassmuth further deduced from Siemens' numbers an 

 expression for the lifting-power of the form a + frl 2 + cl 4 , 

 which represented the latter' s results fairly well. 



Neither Siemens nor Wassmuth appears to have thought 

 of comparing his results with the theory given in sections 

 641-644 of Maxwell's ' Electricity and Magnetism/ which 

 had been published several years before. Maxwell there 

 arrives at the expression WA/Sir for the electromagnetic 

 traction in air between two opposing, plane, infinitely near, 

 and uniformly and normally magnetized pole-faces each of 

 area A ; where B is the induction. 



In 1886 Bosanquet* experimented with two straight iron 

 electromagnets whose ends were ground together. One 

 electromagnet was fixed vertically, and the other supported 

 beneath it on the beam of a balance by which its weight was 

 compensated, this allowing measurements with very small 

 currents to be made. Weights were placed in a scale-pan 

 suspended from the lower electromagnet, and the induction 

 was measured by a small secondary coil near the surface of 

 contact. 



For low and medium currents the weights supported were 

 much greater than those given by Maxwell's theory, the 

 values being better represented by an expression of the form 

 aB + 6B 2 ; while with high currents the results appear to be 

 very uncertain, most of the readings widely differing from 

 the theoretical values. The mean results, however, agree 

 to within about 5 per cent. 



In the same year Bid well f made a series of measurements 

 of the tractive force between two bar-magnets, and of the 

 magnetizing current, but not of the induction, his object 



* Phil. Mag. xxii. p. 535 (1886). 

 t Proc. Roy. Soc. xl. p. 486 (1886). 



