MAGNETISM AND ELECTRICITY 395 



a magnet is held over the centre of the trough, will be pro- 

 jected on the screen. 



Magnetic torsion may be projected in the same way, with 

 any form of torsion balance whose graduated dial is of glass. 



234. Magnetic Induction. Clamping an annealed iron 

 rod 3 inches long by the centre in a small Bunsen holder 

 horizontally across the field of the condensers, it will be 

 readily shown that it supports a key, or one or more pieces of 

 iron, at one end, when the other is touched by the pole of a 

 strong magnet. And using iron filings or small bits of wire, 

 it can be shown that induction takes place, though the 

 magnet does not actually touch. 



Induction by the earth's magnetism is projected by the 

 usual experiment of attracting or repelling the pole of a com- 

 pass needle (placed on the vertical attachment) by the lower 

 end of an iron bar held approximately in the magnetic meridian. 

 Or a dipping-needle may be projected direct, and similarly 

 affected by the end of an iron bar. 



235. Magnetic Curves. These project in the vertical 

 attachment, simply laying a plain sheet of glass over the 

 magnet laid on the face of the condenser, sifting filings upon 

 the glass, and if needful tapping the plate a little. Using a 

 horseshoe magnet, the modifications caused by different 

 armatures between the poles may be shown. Elevating the 

 glass plate sufficiently, duplicate similar poles may be brought 

 up endways under it and the different character of the curves 

 projected : or the lines of force as modified in the neighbour- 

 hood of a space screened by sheets or plates of soft iron may 

 be shown. 



236. Dia-magnetism. Only an electro-magnet will give 

 sufficient power for this class of experiment. A sufficient 

 size for the magnet will be a cross section about J inch square, 

 built up of bent pieces of well-annealed hoop-iron, the total 

 length of each limb about 4 to 6 inches ; or it may be more, 

 as it is quite unnecessary for the lower part of the magnet to 



