320 



POPULAR SCIENCE MONTHLY. 



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. 7. Fig. 8. 



Figs. 6, 7, 8. — Diagrams ilhstratino the Phin 

 ciri.Es OF Electko-Magnets. 



made of steel and is hardened it will retain the magnetism, and 

 become what is called a permanent magnet; such a magnet, in fact, 

 as we have considered in all the previous figures. If the bar is 

 made of iron it will not retain the magnetism, but will only be a 



magnet as long as the elec- 

 tric current flows through 

 the coil W. A magnet of 

 the latter type is called 

 an electro-magnet. If the 

 iron is of poor quality — 

 that is, from an electrical 

 standpoint — it will require 

 an appreciable time to lose 

 its magnetism, but if it is 

 soft and high grade, elec- 

 trically considered, it will 

 lose its magnetism instantly, or nearly so. If we take two bars of 

 soft iron and arrange them side by side, as in Fig. 7, and w^ind coils 

 around them as indicated each one will become magnetized when 

 the ends p n of the coils are connected with an electric circuit. If 

 the lower ends of the two bars are joined by a piece, as shown at 

 M, we will have a horseshoe electro-magnet. If we take a round 

 disk of iron, as in Fig. 8, and wind a coil around it, it will also 

 become a magnet when an electric current traverses the coil. Thus 

 it will be seen that it makes little difference what the shape of the 

 iron may be, providing it is surrounded by a coil of wire and an 

 electric current is passed through the latter. This being the case, 

 it is evident that either of the processes explained in connection 

 with Figs. 4 and 5 can be made available for the production of a 

 continuous rotation by the aid of electro-magnets. Suppose we 

 make a drum, as shown in Fig. 9, and wind a wire coil around it in 

 the direction indicated, then when a current passes through the 

 wire the drum will be magnetized, with poles at top and bottom. 

 If the electric current passes through the wire from end p to end n 

 the drum will be magnetized positively at the top and negatively at 

 the bottom, and if the direction of the current through the wire 

 is reversed the polarity of the drum will be reversed. If we con- 

 struct a horseshoe magnet of the slia])(^ shown in Fig. 10, and place 

 within the circular opening between its ends the drum of Fig. 9, 

 we will have a device that is capable of developing a continuous 

 rotation, providing we have suitable means for reversing the direc- 

 tion of the electric current through the wire coil; and this machine 

 constitutes an electric motor in its simplest form. 



In an electric motor the horseshoe magnet is calh'd the field 



