"* T \ f 



iX-JL-t 



FIG. 135. 



iH 



FIG. 136. 



328 Dynamic Theory. 



is produced. And so it is when the ring is transferred from a dense 

 field to one less dense, the current in these cases taking the positive direc- 

 tion ; that is, opposite to the direction of the hands of a clock ( when 

 you look from the point toward which the lines of force run ). If the 

 transfer is made to a field in which the lines run in a contrary direc- 

 tion, the current is generated, as in fig. 138. (See S. P. Thompson.) 



>> v x x It thus appears that a part of the 



energy of an electric current exists 

 in the form of whirls in the space 

 surrounding the conductor, and it 

 is through the process of the exci- 

 tation of these whirls that the in- 

 duction currents are developed, 

 which excitation is caused by the 

 motion of the wire in a mass ; which 

 motion requires the expenditure of 

 work or energy. We here see the 

 conversion of visible energy of mo- 

 tion into electricity. The more 

 rapid the motion the stronger the 

 currents will be. Approach of the 

 coil causes a current in an opposite 

 direction to that caused by its re- 

 cession. The stronger the mag- 

 netic field into which the coil is in- 

 troduced the stronger will be the 

 induced current. In order to 

 produce a continuous current it is 

 obviously necessary to move back- 

 """" ^' *" wards and forwards in the field of 



FIG. 138. force so as to keep up continuous 



motion of the coil within the field ; hence, currents obtained that way 

 are alternating in direction. In some forms of the electric dynamo, for 

 lighting, &c. , these alternating currents are .all made to flow in one di- 

 rection by the use of "commutators. " * In generating the currents the 

 longer the conductor used to move in the field of force, the stronger the 

 current, whether it be a bar or a coil ; and if the latter, the more turns 

 the better, and the less of the conductor that remains outside of the 

 field, the better. The magnet furnishing the field may be either per- 

 manent steel magnet, or electro magnet. The latter is commonly used in 

 the ordinary dynamo. 



The control which the earth has over the magnetic needle is supposed 



* This is the case with the Brush dynamo, and all dynamos used in electrolysis. The 

 Westinghouse, and others, are alternating current machines, no commutators being used. 



li 



