CHAP, v.] ELECTRICITY AND MAGNETISM. 285 



is concerned, if the coils begin at one end and run to 

 the other and back to where they began ; or they may 

 begin half-way along the bar and run to one end and 

 then back to the other : the one important thing to know 

 is which way the current flows round the bar when you 

 look at it end-on. 



327. Solenoid. Without any central bar of iron or 

 steel a spiral coil of wire traversed by a current acts as 

 an electromagnet (though not so powerfully as when an 

 iron core is placed in it). Such a coil is sometimes 

 termed a solenoid. A solenoid has two poles and a 

 neutral equatorial 

 region. Ampere 

 found that it will 

 attract magnets and 

 be attracted by mag- 

 nets. It will attract 

 another solenoid ; it 

 has a magnetic field 

 resembling gene- FIg 



rally that of a bar 

 magnet. If so arranged that it can turn round a vertical 

 axis, it will set itself in a North and South direction 

 along the magnetic meridian. Fig. 1 1 6 shows a solenoid 

 arranged with pivots, by which it can be suspended to a 

 "table "like that shown in Fig. 121. 



Reference to Fig. 86 and to Art. 192, will recall how 

 a single loop of a circuit acts as a magnetic shell of 

 equivalent form and strength. A solenoid may be re- 

 garded as made up of a series of such magnetic shells 

 placed upon one another, all their N. -seeking faces being 

 turned the same way. Since the same quantity of 

 electricity flows round each loop of the spiral coil the 

 loops will be of equal magnetic strength, and the total 

 magnetic strength of the solenoid will be just in propor- 

 tion to the number of turns in the coil ; and if there be 

 n turns, the number of magnetic lines of force running 



