SUBSTANCES OTHER THAN IRON AND STEEL 205 



The arrows show the direction in which the magnetic action 

 increases. 



Faraday was able to state the following General Law : Para- 

 magnetic substances tend to move from positions where the 

 magnetic action is weaker to .where it is stronger, i.e. from weaker 

 to stronger parts of the field ; while diamagnetic substances tend to 

 move from stronger to weaker parts of the field. The axial setting 

 of paramagnetic bars and the equatorial setting of diamagnetic 

 bars at once come under this general law. The field in Fig. 157 is 

 strongest along and close around the axis N S, and a paramagnetic 

 body tends to stay there. A bar is in the strongest part of the 

 field when it lies along the axis. The field rapidly weakens as we 

 travel along the line er either way from the axis. A diamagnetic 

 bar therefore gets, on the whole, into a weaker part of the field 

 when it sets equatorially. 



The law is further illustrated by an experiment devised by 

 Pliicker, which shows whether a liquid is paramagnetic or diamag- 

 netic. A watch glass containing some liquid is placed on the 

 poles ; the liquid if paramagnetic is attracted to, and if diamagnetic 



is repelled from, points where the intensity of the field is greatest 

 and so the level of the surface is disturbed, its shape depending on 

 the nature and distance apart of the poles. Thus when the poles 

 are very close together, say y 1 ^ inch apart, a paramagnetic liquid has 

 the section shown in Fig. 158 (), while a diamagnetic liquid has the 

 section shown in Fig. 158 (6), the liquid retreating from the central 

 strongest field. When the poles are further apart the strongest 

 parts of the field are near to the pole pieces and the centre is 

 weaker and the effects are nearly reversed. 



Effect of the medium. Faraday also made experiments on 

 the effect of the medium, and he showed that a paramagnetic 

 solution enclosed in a glass tube behaves as a paramagnetic if 

 Mispended in a weaker solution of the same kind, and as a diamag- 

 netic if suspended in a stronger solution. For example (Eocp. Res. 

 vol. iii. p. 58), a clear solution of protosulphate of iron was prepared 

 containing 74 grains of the hydrated crystals to 1 ounce of 

 water. Another weaker solution was prepared by adding to one 

 volume of this three volumes of water. Suspended in air in tubes 

 both were paramagnetic, the former the more strongly so. If the 

 tube containing the first solution was suspended between the poles 

 in a vessel containing the same solution no directive force acted upon 



