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XLII. On the Effect of small Variations of Temperature on Steel 

 Magnets, By J. E. H. Gordon and William Newall, 



lately Students in the Laboratories of King's College, London*. 



THE Astronomer Royal, in speaking of the effect of tempe- 

 rature on magnets (Treatise on Magnetism, p. 68), says: — 

 "The ratio of change .... is very different with different mag- 

 nets; it probably depends on the quality of the steel, possibly 

 on the mode of magnetization ; but on this point nothing is 

 known with certainty." We have been for some time engaged 

 in investigating this subject, and have arrived at a result which 

 we venture to hope will not be considered altogether devoid of 

 interest. Our plan of proceeding was as follows. We took five 

 steel bars procured from different makers, each bar about 5 mil- 

 limetres in diameter and 80 millims. in length. We observed, 



1st. The fall in magnet power for 1° C. between 20° and 50°. 



2nd. The specific gravity of each bar at two temperatures ; 

 and from this we deduced the expansion rate. 



3rd. The chemical composition, i. e. the percentages of iron 

 and of impurity. 



To ascertain the variations in the magnetic power, we used a 

 magnetometer of Mr. Becker's make. The scale is about 2 

 metres long ; it is curved to a radius of 2 metres, and is placed 

 at that distance from the suspended magnet. The scale is di- 

 vided into millimetres, each of which corresponds to 51*5 seconds 

 of arc. It is illuminated either by a lamp or by sunlight re- 

 flected from a mirror. The magnetometer was firmly clamped 

 down upon a table of brickwork. The scale stood on the boarded 

 floor. Its three legs were firmly screwed to the floor ; and one 

 end was wedged against the wall. A curtain of black linen 

 with a hole for the telescope to come through was suspended 

 behind the scale so as to form a background. On the base of 

 the magnetometer, near the opposite end to that at which the 

 mirror is suspended, is screwed a plate of ivory about 12 cen- 

 tims. square by 6 millims. thick. On this lies the box which 

 contains the disturbing magnet. The ivory is to prevent the 

 heat of the box expanding the base and altering the distances. 

 The box (which is of brass) is about 85 millims. long, 50 

 high, and 100 broad as the instrument lies, At each end is a 

 circular hole 20 millims. in diameter. In these holes are in- 

 serted disks of cork with holes in them, in which the magnet is 

 fixed. When the magnet is fixed in position the corks are coated 

 with shellac varnish ; and while it is still wet, slips of glass 

 warmed over a spirit-lamp are placed over them. This makes 

 a perfectly water-tight joint. In the top of the box are fixed 

 * Communicated by the Authors. 



