Winding of Alternate- Current Electromagnets. 571 



Coil 163 turns ; resistance (hot) 0*15 ohm. 



Air-gap 

 (millimetres). 



Virtual Volts. 



Virtual 

 Amperes. 



Coefficient of 

 self-induction. 



Self-induction 

 of one turn. 



952 

 6-35 



50 

 50 



149 

 12 



000574 

 000714 



0-216 Xl0 _t5 

 0-268 xl0~ 6 



Coil 274 turns ; resistance (hot) 0'6 ohm. 



9-52 

 6-35 

 3-17 



50 

 50 



50 



5 



44 

 35 



0-01712 



00194 



00244 



0-228 X 10 ~ 6 

 0-259 XlO -6 

 0326 Xl0~ 6 



6. An advantage of the alternate-current electromagnet is 

 the greater constancy in the pull which it exerts over a given 

 range. If supplied at constant voltage a fairly constant pull 

 on the armature can he obtained over a considerable range. 

 The reaction set up by the core tends to set up a back-electro- 

 motive force the limiting value of which is equal to the 

 impressed voltage. If the resistance of the coil were actually 

 zero and the magnetic leakage also negligible, this limiting 

 value would be attained. ISow suppose that when acting 

 upon its armature at a distance the self-inductive reaction 

 chokes the current down to a certain value, the magnetization 

 of the core going round cycles between definite maxima. 

 Suppose the core to be drawn in nearer to the pole, the 

 reluctance of the magnetic circuit is lessened, and fewer 

 ampere-turns will suffice to produce an equal magnetization ; 

 but because the reluctance is lessened the coefficient of self- 

 induction is correspondingly increased, and the current choked 

 in corresponding proportion. These two effects counter- 

 balance one another, the magnetization going round cycles of 

 practically the same amplitude as before. Hence the pull 

 will be practically unchanged, save in so far as any change 

 in the magnetic leakage at the different positions of the 

 armature comes in to affect the question by weakening the 

 field between the pole-faces and the armature. Were the 

 pole-faces made relatively large so as to obviate the tendency 



