ELECTRO-MAGNETISM. 



439 



Tig. 370. 



gions it dips vertically do'WTi in order that its currents may be parallel with, 

 those of the earth ; for in those regions the sun performs his daUy motion in 

 circles parallel to the horizon. At the equator, the course of the sun ia 

 nearly at right angles to the horizon, and the needle maintains a horizontal 

 position. 



What is Mag- 816. As au elcctfic current passing round 

 neto-eiectricity? ^|^g exteiior of 8. bar of soft iron induces mag- 

 netism in it, so on the contrary, a magnetized bar is able 

 to generate an electric current in a conducting Avire sur- 

 rounding it. 



Electricity thus produced by the agency of a magnet is 

 called Magneto-electricity. 



This may be shown by introducing one of tha 

 poles of a powerful bar magnet within a helix of 

 fine insulated wire (see Fig. 370), tbe ends of 

 which are connected with a dehcate galvan- 

 ometer. The deflection of the needle will indi- 

 cate the flow of an electric current every time the 

 magnet enters or leaves the coil — ^the direction 

 of the current changing with the poles entered- 

 The same results will be obtained, if instead 

 of introducing and removing a permanent steel 

 magnet, we continually change the polarity of a 

 soft; iron bar. Thus, in Fig. 371, let a 6 be a bar 

 of soft iron surrounding a heUx, and N E S a 

 horee-shoe magnet so aiT^mged that it can revolve 

 freely on a pivot at c. the poles in their revolution just passmg by the termina- 

 tions" of the bar a b. On causing the magnet to re- 

 volva, the polarity of the bar a 6 will be reversed 

 with every half revolution the magnet makes, and 

 this revei4al of polarity will generate electric cur- 

 rents in the wire. 



To instruments constructed on 

 these principles the name of mag- 

 neto-electric machines is given. 



, SIT. Whenever an 



Can one pIpo wj. i . 



trie current in- eigctric currcut flows 



due* another ? v-i^'-<-i ^ ^ , • 



tl3rou<^-ha wire it excites another current m an 

 opposite direction, in a second wire held near to and 

 parallel with it. Its duration, however, is only momentary. 

 On stopping the primary current, induction again takes 



