DIRECT-CURRENT DYNAMOS AND MOTORS. 93 
well as a shunt winding, assisting each other in the 
former, and opposing each other in the latter; the total ex- 
citation, therefore, is the swm of the series and shunt am- 
pere-turns in one case, and their difference in the other, 
The methods employed for determining the number of turns 
and the size of wire for the various kinds of field wind- 
ings are given in the following paragraphs 72 to 74. 
72. Calculation of Series Winding.—The field winding 
of a series machine being always fed by the total current 
generated or supplied, which has a definite value, the re- 
quired number of turns, 7’, is immediately found by divid- 
ing the ampere-turns, AT, by the given value, C, of the 
current; thus: 
- The size of wire must be sufficient to carry the given cur- 
rent C without overheating and without causing an undue 
loss of power in the magnets. This condition is usually 
fulfilled by providing a conductor area of from 1,000 to 
4,000 circular mils per ampere. Fcr series dynamos fur- 
nishing a constant current of 10 amperes, the usual cur- 
rent output of series arc-lighting machines, the size of the 
magnet wire has been found by experience to be between 
7s and 4 inch in diameter—that is, No. 10, No. 9, or No. 
& B. & S. gauge, depending upon the depth of the wind- 
ing and the temperature limit prescribed. For the usual 
range of temperature rise employed, which is from 20° to 
50° C., the specific cooling surface of the magnet coils 
should be from 4 to 14 square inches per watt, respec- 
tively, as follows: 
For a rise of 20° C. allow about 4 square inches per watt; 
“ce ce 666 (a4 95° 6é ee 6< 34 ce ¢é éé ee 
ce cc 6 a4 30° cc 6¢ (a4 3 ‘<é ce ce cc 
as ES" 66 ‘< 40° ce cé ‘< 9 ‘< é<é¢ ‘é ce 
66 €6€» 66 “é 50° ce “é ce 14 ‘eé ce ‘é t< 
43. Calculation of Shunt Winding.—In the determina- 
tion of the best size of wire for a shunt winding, the 
