288 ELEMENTARY LESSONS ON [CHAP. v. 



(d) The strength of an electromagnet is independent of 

 the diameter of the coils. Whether the coils are bigger 

 than the core they enclose, or enwrap it quite closely, 

 makes no matter, provided that there are enough of them 

 to make the electromagnet at least twice as long as it is 

 broad, and that the iron core protrudes beyond the ends 

 of the coils. It is also found that a hollow tube of iron 

 answers as well as a solid core, provided there be plenty 

 of iron in the substance of the tube. 



(e) A ctirrent requires time to magnetise an iron core 

 to the full extent of its power. This is partly owing to 

 the fact that a current, when circuit is first made, does 

 not suddenly attain its full strength ; but it is chiefly 

 owing to the solid iron itself taking time to magnetise. 

 Faraday's great electromagnet at the Royal Institution 

 takes about two seconds to attain its maximum strength. 

 Beetz made the observation, that, though the strength of 

 current be the same in each case, the magnetism of the 

 core of an electromagnet is more rapidly established by 

 a current of great electromotive-force working through a 

 great resistance, than by one of small electromotive-force 

 working through a small resistance. This would seem 

 to show that the apparent slowness of iron to magnetise 



is due to the presence of tran- 

 sient reverse induction cur- 

 rents (Art. 393) in the iron 

 itself, which, while they last, 

 set up a magnetic induction 

 of their own opposed to that 

 due to the external current. 



329. Construction of 

 Electromagnets. The 

 most useful form of electro- 

 magnet is that in which the 

 iron core is bent into the 

 form of a horse-shoe, so that both poles may be applied 

 to one iron armature. In this case it is usual to divide 



