CHAP. II.] ELECTRICITY AND MAGNETISM. 83 



method, known as double touch, differs slightly from 

 that last described. A piece of wood or cork is inter- 

 posed between the ends of the two bar magnets employed, 

 and they are then both moved backwards and forwards 

 along the bar that is to be magnetised. By none of 

 these methods, however, can a steel bar be magnetised 

 beyond a certain degree of intensity. 



95. Laminated Magnets. It is found that long 

 thin steel magnets are more powerful in proportion to 

 their weight than thicker ones. Hence it was proposed 

 by Scoresby 1 to construct compound magnets, consisting 

 of thin laminae of steel separately magnetised, and after- 

 wards bound together in bundles. These laminated 

 magnets are more powerful than simple bars of steel. 



96. Magnetisation derived from the Earth. 

 The magnetism of the earth may be utilised, where no 

 other permanent magnet is available, to magnetise a bar 

 of steel. Gilbert states that iron bars set upright for 

 a long time, acquire magnetism from the earth. If a 

 steel poker be held in the magnetic meridian, with the 

 north end dipping down, and in this position be struck 

 with a wooden mallet, it will be found to have acquired 

 magnetic properties. Wires of steel subjected to torsion, 

 while in the magnetic meridian, are also found to be 

 thereby magnetised. 



97. Magnetisation after Heating. Gilbert dis- 

 covered also that if a bar of steel be heated to redness, 

 and cooled, either slowly or suddenly, while lying in the 

 magnetic meridian, it acquires magnetic polarity. No 

 such property is acquired if it is cooled while lying east- 

 and-west. It has been proposed to make powerful 

 magnets by placing hot bars of steel to cool between the 

 poles of very powerful electro-magnets ; and Carre* has 

 recently produced strong magnets of iron cast in moulds 

 lying in an intense magnetic field. 



1 It is said that a similar suggestion was made by Geuns of Venlo, in 1768, 

 Similar magnets have been constructed recently by Jamin. 



