THE ELECTROMAGNET. IN.“PETROGRAPHY 4% 
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!? 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. 
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. 
' See Mikroskopische Physiographie der petrographisch wichtigen Mineralien, 
vol. i. part 1. p. 431, 1904. 
* This apparatus, including an eight-volt secondary battery, an electromagnet, 
and a switch, is supplied for the modest sum of about £3 Ios. by the General 
Electric Co., 71, Queen Victoria Street, E.C. The electromagnet alone costs 
about £1 Ios. 
