LABORATORY, 



FACULTY OF AFPUEI1 SCIENCE. 



PRACTICAL 

 ALTERNATING CURRENTS 



AND 



ALTERNATING CURRENT TESTING. 



CHAPTER I. 



ALTERNATING ELECTROMOTIVE FORCE AND CURRENT. 



General Laws. Two experimental laws may be said to form 

 the foundation of the actions which take place in electrical 

 generators and motors, whether for alternating or continuous 

 currents. 



Law of the Generator. When a conductor moves in such 

 a way as to cut magnetic lines of force, there will be an 

 electromotive force generated in the conductor. 



Law of the Motor. When a current flows along a conductor, 

 it produces a magnetic field round the conductor which is 

 capable of acting upon, or being acted upon by, other 

 magnetic fields. 



In order to make clear the application of these laws to 

 the case of alternating-current machinery it will be advisable 

 to consider them more fully. 



Production of Electromotive Force. The strength of a 

 magnetic field is expressed numerically by the pull, measured 

 in dynes*, which would be experienced by a unit magnetic 

 pole f if situated in the field. A magnetic field is usually 

 spoken of as consisting of a number of lines of force. The 

 density of these mathematical lines is, for convenience, so 



* A dyne is the unit of force in the absolute or C. G. S. system of units. It 

 is the force necessary to give unit acceleration (1 cm. per second per second) 

 to unit mass (1 gramme). A force of 1 dyne is roughly equal to the weight 

 of 1 milligramme. 



t Unit magnetic pole is defined to be a pole which, if placed 1 cm. from 

 an equal and similar pole, would be repelled with, a force of 1 dyne, the 

 medium being air. 



