286 ELEMENTARY LESSONS ON [CHAP. v. 



through the solenoid will be n times as great as the 

 number due to one single turn. The use of the iron 

 core is by its greater magnetic induction to concentrate 

 and increase the available number of lines of force at 

 definite poles. The student has been told (Art. 191) 

 that the lines of force due to a current flowing in a wire 

 are closed curves, approximately circles (see Fig. 85), 

 round the wire. If there were no iron core many of 

 these little circular lines of force would simply remain as 

 small closed curves around their own wire ; but, since 

 iron has a high coefficient of magnetic induction, where 

 the wire passes near an iron core the lines of force alter 

 their shape, and instead of being little circles around the 

 separate wires, run through the iron core from end to 

 end, and round outside from one pole back to the 

 other, as in a steel magnet. A few of the lines of force 

 do this when there is no iron ; almost all of them do this 

 when there is iron. Hence the electromagnet with its 

 iron core has enormously stronger poles than the spiral 

 coils of the circuit would have alone. 



328. Laws of Electromagnets. The following 

 are the principal laws of electromagnets : 



(a) The strength l of an electromagnet is proportional 

 to the strength of the magnetising cttrrent (i.e. to the 

 quantity of electricity that circulates round it). This is, 

 however, only true when the iron core is still far from 

 being "saturated" with its maximum intensity of mag- 

 netisation. If the iron is already strongly magnetised 

 by a current, a current twice as strong will not make the 

 iron into a magnet of double strength. According to 

 Jenkin it is no use to make the current stronger than will 

 give the " field "135 units of intensity. Miiller gave for 

 the relation between the strength of the magnetising cur- 



l The word "strength" means here "magnetic strength," as defined in 

 Art. 102, and must on no account be confused with " lifting power" or 

 "sustaining power," which depends both on the magnetic strength and on 

 the form of the magnet and of its poles. 



