88 



TREATISE ON ALTERNATING CURRENTS. 



With a current of 5 amperes, the magnetizing force is 

 4?r x 100 X 5 628-32 



101 



I 



where / is the length of the magnetic circuit, i.e. the average 

 length of the closed lines of magnetic induction. 

 The induction is therefore 



823 x 



Therefore 



whence 



628-32 



823 x ~- = 14,000 



= 37 centimetres 



11 a 



so that the core could be built up of iron stampings of section 

 shown in Fig. 35 to a thickness of 9'8 centimetres. 



The wire to be used should be 

 equivalent to No. 14 S.W.G., since 

 this will carry 5 amperes at a cur- 

 rent density of 1000 amperes per 

 square inch. 



We may notice that the self- 

 induction of the choking coil is 

 given by 



pLi = v 

 =.91-6 



therefore 



L = 



91-6 



FIG. 35. 



2;r X 50 X 5 

 = 0-0583 henry 



The foregoing calculation is not quite correct, since we have 

 omitted to take into account the iron losses of the core of the 

 choking coil. These could not be allowed for until the dimen- 

 sions of the core were fixed. 



The following table gives the hysteresis loss in ergs per cycle 

 per cubic centimetre of good soft annealed iron : 



