THE ELECTROMAGNET IN PETROGRAPHY 45 



The adjacent portions of the pole pieces should be bent down- 

 wards, but not at right angles, narrowing slightly and thinning 

 towards the tips, somewhat after the manner suggested by 

 Doelter. The practice of having the adjacent ends of the pole 

 pieces bent downwards at right angles in the way recommended 

 by Rosenbusch 1 is very faulty. It is clear that with such an 

 arrangement the lines of force must be distributed over all 

 the space between the vertical plates, and as only the tips are 

 used, most of the magnetic force is thereby wasted. 



The magnet may be conveniently put into action by an 

 eight-volt secondary battery of the portable Lithanode type. It 

 is an additional convenience if, as is usually the case, the battery 

 be arranged so that two, four, six, or eight volts can be obtained, 

 as this affords an additional means of varying the intensity of 

 the magnetic field. 



Instead of a secondary battery an ordinary current, such as 

 is used for lighting purposes, may be used, in combination with 

 some form of resistance apparatus. A dynamo or a Bunsen 

 battery may also be used as a substitute. 



The wire connecting the battery with the electromagnet 

 should be provided with a simple lever-switch arrangement, by 

 which the current can be switched on or off as required. 



The simple apparatus thus outlined will do all that is neces- 

 sary in the way of isolation and separation, with such small 

 amounts as one usually uses in the examination of sands and 

 rocks generally. 2 



It may be cheaper, but not so convenient, to use a compound 

 and permanent horseshoe magnet, as suggested by Delesse. 

 Moreover, such a permanent magnet may be provided with 

 adjustable pole pieces, like those already described for the 

 electromagnet. But, in the absence of some mechanical device 

 for suspending the action of the magnet on the grains, and 

 thus permitting them to fall, such an arrangement is scarcely 

 suitable for making separations. 



1 See Mikroskopische Physiographie der petrographisch ivichtigen Mweralien, 

 vol. i. part i. p. 431, 1904. 



2 This apparatus, including an eight-volt secondary battery, an electromagnet, 

 and a switch, is supplied for the modest sum of about £2 10s. by the General 

 Electric Co., 71, Queen Victoria Street, E.C. The electromagnet alone costs 

 about £1 10s. 



