46 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
39. 
In case of formed coils, which are separately completed by 
means of formers and placed upon the exterior surface of 
the armature, the total length of the winding is found 
from formula (24) or (25), in which then the constant 
k has the following approximate values: 
For coils spanning 4 circumference ( 4 poles), k = 1.2 
cé a3 “ce t ‘é ( 6 6é ), co is 
“é “é (73 4 ‘ec (8 6“ ) be as 6 
’ ° 
‘ce ‘é 66 qa ‘ec (10 (73 ye co es 
79 cé 6é ts ‘ce (12 a3 ), co — A 
‘é c¢ cf ds 77 (16 6é ); ce 3 
ce 6é 6é st 73 (24 “ce ), co 2 
Armature Resistance.—The electrical. resistance of 
the armature winding is determined by the total length 
of wire wound on the armature and by the sectional area 
of the conductor. If R, denotes the total resistance of 
the armature wire, all in one continuous length, and if 
there are n, bifurcations in the armature, and, therefore, 
2n, electrically parallel armature circuits, then the arma- 
ture forms the combination of 2n, parallel branches of 
4 ohms resistance each; their joint resistance, 7,, which 
is the actual armature resistance, will consequently be: 
1 Bf Meare: 
2p 2p AM” 
The total resistance R, of the armature conductor is the 
product of the total length 7, and the resistivity, p:, in 
ohms per foot of the conductor, hence the armature © 
resistance: 
me = 
js Om EP See (27) 
In case that the armature conductor is subdivided into two 
or more wires, the resistivity of that wire must be divided 
by the number of wires used in parallel. The resistivi- 
ties, Pr, of the various sizes of armature wire at 20° C. 
(68° Fahr.) are given in column 5 of Table 13. When 
