HISTORY AND PROGRESS. 



77 



thereby inducing in the coil, if the circuit is closed, thirteen 

 positive and thirteen negative currents of equal magnetic 

 effect. 



When the coil is turned half round on its vertical axis, the 

 polarity of the armature is also changed, and a magneto- 

 electric current induced in the coil w in one direction, and 



Fig. 40. 



on turning it half a revolution farther round, so that it takes 

 up its former position, a current in the opposite direction is 

 induced. 



The ends of the coil of wire, which are attached to the 

 brass caps, are connected, one to an earth plate, the other 

 to the coils of the electro-magnet of the indicator, and thence 

 to the line. 



In employing a series of separate magnets, arranged at a 

 distance from each other, a greater inductive effect is produced 

 than if only a single permanent magnet were used, or if the 



