Winding of Alternate- Current Electromagnets. 565 



is proportional to the square of the intensity of the magnetic 

 flux, then, if this flux were proportional to the number of 

 ampere-turns of excitation (as would be the case if the per- 

 meability were constant), the pull would be proportional to 

 the square of the number of ampere- turns. Now the ampere- 

 meters for alternate currents operate upon principles that 

 cause them to indicate the square root of the mean square 

 values of the current. Hence it follows that, if permeability 

 were constant, the pull should be proportional to the square 

 of the current, whether the current be continuous or alterna- 

 ting. But since, with an alternating current of the same 

 nominal value as a continuous current, the magnetism is 

 carried at each period to a point considerably higher than 

 with the continuous current, and since also at the higher 

 degrees of magnetization the permeability is lower, it follows 

 that the pull of the electromagnet when excited by alternate 

 currents ought to be slightly less if the excitation is so 

 great as to carry the magnetization 

 into the regions commonly spoken of as 

 approaching saturation. The pull ought 

 to be independent of the frequency of the 

 alternation provided this exceeds a certain 

 minimum. 



Experiments made show that these 

 views are justified in practice. 



An electromagnet of the form shown 

 in fig. 1 was made up of iron stampings 

 about 0*5 millim. thick. 





Fig. 1. 

















The principal dimensions were as follows : — 



Mean length of limbs and yoke 

 Breadth of limb . 

 Gross thickness of limb 

 Nett thickness of limb . 

 Nett cross section of limb 

 Width between limbs . 

 Total length of armature 



38 cms. 



3*1 cms. 



2*54 cms. 



2*1 cms. 



6*5 sq. cms. 



6*3 cms. 



12*7 cms. 



The stampings of the armature were of the same thickness, 

 number, and breadth as those of the electromagnet. In the 

 succeeding experiments the electromagnet was wound with 

 163 turns of copper wire 2'03 millim. in diameter, and the 

 armature was separated from the pole- faces by a distance- 

 piece of wood 0*952 centim. thick. 



