MAGNETIC INDUCTION. 711 



electro-magnet an iron -wire is used, 6 cm long and 4 mm thick. A 

 piece of paper is first smoothly fixed round it with paste, leaving 

 only the ends free ; the paper protects the covering of the copper 

 wire against contact with iron rust, which is gradually formed on 

 the bar, and which would destroy the covering. For the electro- 

 magnet only one spiral is required ; the last turns at each end are 

 tied with thread, and bent into the shape shown, in fig. 355, leaving 

 one end to stand out just from the middle of the electro-magnet and 

 the other 6 nim from it. At a and b the ends are secured with 

 thread. A frame, c c, made of wire, serves for suspending the electro- 

 magnet ; it has a ring in the middle, and at the ends suitable hooks, 

 into which the bar is placed. 



The second electro -magnet required is made of the same size, but 

 the free ends of the spiral are left a little longer, and they need not 

 be bent towards the middle. 



The action of a magnetic piece of iron to produce 

 parallelism of the Amperian currents in another piece 

 of iron, and to effect the magnetisation of the latter, is 

 termed magnetic induction. In fig. 358 A, a may re- 

 present an electro-magnet with its currents, and b an 

 ordinary bar of iron ; the end ?? 2 of the latter, which is 

 directed towards the south pole Si of the electro-magnet, 

 must evidently become a north pole if the magnetic bar 

 a causes parallelism of the currents in &, or generally, 

 that end of a bar of iron which is nearest to one of the 

 poles of a magnet becomes by magnetic induction a pole of 

 contrary name. 



An electro-magnet which is U-shaped, or is angular 

 (l_j), a so-called horse-shoe magnet, is capable of sup- 

 porting a piece of soft iron which connects its poles 

 with considerable force. A piece of iron of this kind 

 is called a keeper or ' armature.' Such a keeper be- 

 comes magnetised under the influence of both poles. 

 Thus if the pole of the electro-magnet, fig. 361 -4, which 

 is on the left side, is a south pole, the adjoining end of 



