158 PRINCIPLES OF ELECTRICAL DESIGN 



square centimeter in air) at a distance y cm. from a straight 

 conductor is : 



. 0.2 X current in amperes 

 ri = 



y 



and since the width W a sin a I or 7= ) of the zone of commutation 



is relatively small, it may be considered as being at an average 

 distance y from EF. The field intensity due to the ampere- 

 conductors in the space of width dy (see Fig. 61) is therefore: 



lief = 



ry 



and since the length of EF is -7=. y, the total flux in the zone 



V 2 



considered, set up by the ampere-conductors in the space EF of 

 width d is: 



The flux due to all the conductors in the space ABC will there- 

 fore be 



which becomes 



0.2TT, 



(70) 

 ~y \ v ^ / 



2V2 



(71) 



W 



This is not the total end flux in the commutating zone, because 

 the conductors parallel to AB (Fig. 60) in the space ADB will 

 produce the same amount of flux in the zone of length AB and 

 width W a sin a as the conductors in the space ABC to the 

 right of AB. Moreover since the flux to be cut by each 

 "active" conductor in the commutating zone to annul the 

 effect of end flux must be equal in amount to that cut by the 

 coil sides AB + BC, or AB + A'B', it follows that the total 

 end flux with which we are now concerned is four times the 

 amount given by the above formula. Again, since the assump- 

 tions made cannot in any case lead to an accurate formula, a 



