THE MAGNETIC CIRCUIT ELECTROMAGNETS 43 



of standard wire, as will be explained shortly, but it is generally 

 possible to modify the average length per turn and so obtain the 

 desired result. The formula (26) can be written, 



E(m) 



+ I) = 



whence 



irXSI 



Now, if (A) 'is the area in circular mils of the standard size of 

 wire it is proposed to use instead of the previously calculated 

 cross-section (m) the required ampere-turns can be obtained 

 by making the depth of winding, 



E(A) 



- D 



(27) 



*XSI 



Now estimate (by using the space factor curves, or by calculation) 

 the number of turns required to fill the spool to the required 

 depth, and calculate the total resistance, R, and the current, 



E 



I = 



R 



A. convenient rule, which usually provides sufficient winding 

 space to prevent excessive temperature rise, is to allow 1 sq. 

 in. of winding space cross-section for every 500 ampere-turns 

 required on the coil. This simply means that the product 

 A X sf of formula (25) is taken as 500. 



Winding Shunt Coils With Two Sizes of Wire. For a definite 

 mean length per turn, the exact ampere-turns required on a 

 magnet can always be obtained with standard sizes of wire by 



H x Feet of A Ohms H* V Feet of B Ohms H 



One Foot = R Ohms 



FIG. 17. Two sizes of wire in series. 



using, if necessary, two wires of different diameter connected in 

 series or in parallel. The series connection is most usual for 

 magnets or field coils to be connected across a definite voltage. 

 Let R stand for the ohms per foot length of wire to give the 

 required excitation at the proper temperature; A = ohms per 

 foot, at the same temperature, of the standard wire of larger size; 



