42 PRINCIPLES OF ELECTRICAL DESIGN 



must be taken into account. A little practice will enable the 

 designer to form a rough idea as to the size of wire that will 

 be required, this being of small diameter for high voltages and of 

 large diameter for low voltages. He can select a probable value 

 of the space factor from the curves of Fig. 15. The cross-section 

 of the coil can now be calculated because, 



Any convenient relation between I and t may be chosen, but 

 the value of one of these dimensions is usually decided upon in 

 the first instance. It is well to avoid making the depth of wind- 

 ing, t, more than 3 in., even in large magnets, because the internal 

 temperature is then liable to become excessive. 



The size of the wire will depend upon the length of the mean 

 turn; and, with a known value for t, and a core of circular cross- 

 section, we have: 



Mean length per turn = ir(D + t) in. (See Fig. 16.) 



Applying formula (21) for the resistance of a copper wire at a 

 temperature of about 60C., we may write, 



length in inches E 

 - 



resistance = 

 whence 



(m) I 



and 



t)S E 



8i (26) 



E 



In this manner the size of the wire can be determined. It 

 should be noted that, for a given excitation, its cross-section 

 depends only upon the applied potential difference and the 

 average length per turn; it is quite independent of the number 

 of turns of wire, S. That this must necessarily be the case is 

 seen when it is realized that for every increase in S, the resist- 

 ance increases in like manner, causing the current I to decrease 

 by a proportional amount. 



By referring to a wire table such as those on pages 34 and 35, 

 the standard gage size nearest to the calculated cross-section 

 can be chosen. If it does not seem close enough to the required 

 size for practical purposes, the coil can be wound with two sizes 





