Winding of Alternate- Current Electromagnets. 567 



Brush alternate-current arc - lamp. This plunger with 

 attached mechanism weighed about 820 grammes, and was 

 arranged to be drawn up into position by a current of 10 

 amperes. This plunger was loaded with various weights and 

 the current required to raise it was measured, with the 

 following results : — 



Total weight. 



Continuous Current. 

 Amperes. 



Alternate Current. 

 Virtual Amperes. 



820 



10 



10 



933 



172 



18 



1039 



182 



19-5 



1276 



25 



265 



1601 



31 



33 



1728 



36 



40 



3. When alternate currents are supplied at a given voltage 

 to an electromagnet having a given configuration of the mag- 

 netic circuit, and it is desired to produce by a winding any given 

 number of ampere-turns of excitation, it is obvious that there 

 are many ways of attaining the result with coils of various 

 sizes and numbers of turns. If the resistance of the coil is 

 so small relatively to the inductance as to be negligible, then 

 there will only be one possible solution as to the number of 

 turns. If, however, the resistance of the coil is of a magni- 

 tude comparable with that of its inductance, then there are 

 various possible solutions, for an equal number of ampere- 

 turns may be produced by using currents of different strengths 

 and coils of different numbers of turns. In the cases of 

 different windings the lag of phase of the currents will be 

 different. If the resistance is large relatively to the inductance, 

 the difference of phase between the current and the impressed 

 volts will be small, and the wasted watts large. If the in- 

 ductance is large relatively to the resistance, the difference of 

 phase will be nearly 90° ; and the current will be nearly 

 wattless, the efficiency being relatively high. 



It is in accordance with good constructive practice to use 

 such frequencies in the supply, and coils of such relatively 

 high coefficients of self-induction as to justify the assumption, 

 for the purpose of design, that the inductance is the important, 

 and the resistance of the coil the unimportant matter. How- 

 ever, to bring the calculations to bear upon all cases that may 

 occur, both resistance and inductance have been taken into 

 consideration. 



