194 MAGNETISM 



are turned round so as to be parallel to the magnetising force. 

 The existence of such a limit is borne out by experiment. The 

 curve BC in Fig. 130, p. 183, tends to become nearly parallel to 

 the axis when the magnetising force is increased to a very great 

 value. An interesting experiment by Beetz * also shows that 

 there is probably a limit. A varnished silver wire with a very 

 narrow scratch through the varnish was made the cathode in an 

 electrolytic cell containing a salt of iron in solution. This was 

 arranged in a weak magnetic field parallel to the scratch, and the 

 iron molecules as they were laid down on the silver one by one 

 were free to turn in the direction of the field. The magnetisation 

 of the iron deposited was very great for so small a mass, and was 

 found to be only very slightly increased when the wire was placed 

 in a very powerful field. 



When a bar undergoing magnetisation is vibrated, say, by 

 hammering, it is magnetised more strongly than if quiet, and we 

 may perhaps assume that the vibration frees the moleculei from 

 their constraint, so that they yield more easily to the magnet is 

 ing force. 



A modification of the theory was made by Wiedemann + and 

 by Hughes.J If a number of bars of iron are made up into a 

 bundle, magnetised along their lengths, and then partially 



demagnetised by, say, hammering, 

 on taking the bundle apart it is 

 found that some of the bars have 



their polarity reversed. The ap- 

 parent demagnetisation is partly 

 (llR , to the com pletion of the mag- 

 netic circuit of the stronger bars 

 through the weaker ones, which 

 S they reverse as represented in 

 Fig. 141. Carrying this idea down 

 S to molecular regions, Wiedemann 

 and Hughes suggested that in an 

 FIG. 141. un magnetised bar the molecular 



magnets are arranged in neutral- 

 ising circuits of two or more molecules, as in Fig. 142. 



As originally described, Weber's theory gives no account of 

 hysteresis. Wiedemann sought to explain it by supposing that 

 there is some sort of friction to be overcome in turning the 

 molecules round, friction which has to be overcome again when the 

 molecules seek to return after the magnetising force is removed. 

 Maxwell introduced the idea that the constraining force on each 

 molecule is analogous to an elastic force, that for small displace- 



* Pogg. Ann. cxi. (1860), p. 107. 



t Cfalvani*mu9, 2nd ed. vol. ii. [1], p. 373. 



$ Proo. A.S., xxxv. (18*3), p. 178. 



Electricity and Maynffitm, 3rd ed. vot. H. p. 85. 



