PRINCIPLES OF ELECTRICAL DESIGN 



and B = the corresponding resistance of the smaller standard 

 size of wire. It is proposed to make up the resistance R by 

 connecting x feet of A ohms per foot in series with y feet of B 

 ohms per foot, as indicated in Fig. 17. Thus, 



xA + yB = R 



also 



whence 



and 



x + y 



or, y = 1 x 



xA + B - xB = R 



x = 



R- B 



(28) 



A - B 



If it is preferred to work with cross-sections in circular mils, 

 instead of resistances in ohms per foot, we can put the relation of 

 formula (28) in the form 



1 J_ 

 (m) (B) 



(29) 



(A) (B) 

 (A) (B) - (m) 

 ~ (m) X (B) - (A) 

 where (m) = calculated circular mils, 



(A) = circular mils of larger standard wire, 



(B) = circular mils of smaller standard wire. 



When winding with two sizes of wire in series, it is usual to put 

 the smaller wire on the outside where the heat will be most readily 

 dissipated. 



11. Heat Dissipation Temperature Rise. The winding, if 

 calculated as explained in the preceding article, will furnish the 

 required excitation; but it is possible that the estimated value for 

 the current density may result in a temperature so high as to 

 injure the insulation, or so low as to render the cost of the magnet 

 owing to excess . of copper commercially prohibitive. The 

 highest temperature will be attained somewhere inside the coil, 

 and it is not easily calculated; the temperature as measured by a 

 thermometer on the outside of the coil is only a rough guide to 

 that of the hottest part. The average temperature is also higher 

 than the outside temperature; it can be ascertained by meas- 

 uring the resistance of the coil hot and cold. The maximum 

 temperature can be measured only by burying thermometers or 





