DIMENSIONS OP RINGS AND BODS OP IRON AND OTHER METALS. 217 



pressing against each other with a certain force per unit of area, which can be 

 measured by the weight necessary to tear one half from the other. The same pressure 

 will exist between any two portions of the rod separated by any possible cross-section, 

 and a certain longitudinal contraction of the rod will be the consequence. If now the 

 rod, having been first demagnetised, be placed in a vertical position upon a fixed base, 

 and loaded at the upper end with a weight equal to the greatest it could support 

 when magnetised, it will undergo the same contraction as before, the compressional 

 stresses being equal in the two cases. The amount of contraction occurring in this 

 latter case is well known, and does not differ greatly in different kinds of iron. Does 

 then this mechanical contraction completely account for the magnetic retraction which 

 is observed in high fields ? 



With the view of answering this question, I carried out a series of experiments 

 upon the lifting powers of electromagnets in the form both of rods and of divided 

 rings, the results of which have been communicated to the Royal Society.* It was 

 found that the lifting power did not, as was generally believed, reach a practical limit 

 with a comparatively small magnetising force, such as 135 or even 250 C.G.S. units.t 

 It is true that after this latter point was passed the lifting power increased much 

 more slowly than at first, but with a force of (500 units it was nevertheless upwards 

 of 10 per cent, greater than with a force of 250, the weight supported being in the 

 two cases 13,500 and 15,000 grammes per square centimetre, allowance having been 

 made for the electromagnetic action between the iron and the coil.J Between these 

 limits the increments of lifting power and magnetic force appear to be approximately 

 proportional, the curve expressing their relation being a sensibly straight line. 



Here, then, we have the means of comparing the mechanical with the magnetic 

 effect. Taking 2040 X 10 6 as the value of YOUNG'S modulus for wrought iron, in 

 grammes weight per square centimetre, the contraction produced by loading the 

 upper end with a weight of P grammes per centimetre is expressed as a fraction of the 

 original length by P/2040 X 10 8 . 



If P = 13,500 grms. (the weight per centimetre supported in u field of 250 units), 

 the contraction will be 



13500/2040 X 10 6 = 0'0000066, 



or 66 ten-millionths of the length of the rod. 



Again, if the rod be compressed by a load of 15,000 grms. (the weight supported 

 in a field of 600 units), the contraction will be 



15000/2040 X 10" = 0-0000074 



* ' Roy. Soc. Proc.,' vol. 40, 1886, p. 486. 



t See JEHKIN'S ' Electricity,' 7th edition, 1883, p. 124. 



J It is, perhaps, questionable whether any deduction should be made on this account. The figures 

 without such deduction would be respectively about 14,000 and 15,000 grms. But the difference is not 

 sufficiently great to affect the argument. 



MDCCCLXXXV1II. A. 2 F 



