284 ELEMENTARY LESSONS ON [CHAP. v. 



hollow of which is placed a bar of iron or steel, which is 

 thereby magnetised. The separate turns of the coil 

 must not touch one another or the central bar, other- 

 wise the current will take the shortest road open to it 

 and will not traverse the whole of the coils. To pre- 

 vent such short-circuiting by contact the wire of the coil 

 should be overspun with silk or cotton (in the latter case 

 insulation is improved by steeping the cotton covering in 

 melted paraffin wax) or covered with a layer of gutta- 

 percha. If the bar be of iron it will be a magnet only 

 so long as the current flows ; and an iron bar thus sur- 

 rounded with a coil of wire for the purpose of magnetising 

 it by an electric current is called an Electromagnet. 

 Sturgeon, who gave this name, applied the discoveries 

 of Davy and Arago to the construction of electromagnets 

 far more powerful than any magnets previously made. 



By applying Ampere's Rule (Art. 186), we can find 

 which end of an electromagnet will be the N. -seeking 

 pole ; for, imagining ourselves to be swimming in the 

 current (Fig. 1 1 4), and to face towards the centre where 

 the iron bar is, the Ni-seeking pole will be on the left. 

 It is convenient to remember this relation by the fol- 

 lowing rules : Looking at \tJie S.- seeking pole of an 

 electromagnet, the magnetising currents are circulating 

 round it in the same cyclic direction as the hands of a 



. : .^.--X. clock move j and, looking at 



the N.- seeking pole of an 

 electromagnet the magnetis- 

 ing currents are circulating 

 round it in the opposite cyclic 

 - "5- direction to that of the hands 



of a clock. Fig. 115 shows this graphically. These 

 rules are true, no matter whether the beginning of the 

 coils is at the end near the observer, or at the farther 

 end from him, i.e., whether the spiral be a right-handed 

 screw, or (as in Fig. 114) a left-handed screw. It will 

 be just the same thing, so far as the magnetising power 



