100 MR. S. BUTTERWORTH: EDDY-CURRENT LOSSES IN CYLINDRICAL CONDUCTORS. 
Thus the gain in time-constant by using stranded wire is 1 /s *; but, in addition, a larger 
number of turns, and therefore an increased inductance, may be obtained with stranded 
wire, while maintaining the best conditions. 
(28) Design of Coils of Large Inductance .—If a coil of large inductance is required 
to have minimum effective resistance at a specified wave-length, the conditions 
ajb = 1*6, 
N = 2’8 x 10~ 4 Xa x^s/d 3 , .(A) 
together with the formula for the inductance 
L = 25'5N 2 «,.(B) 
determine the radius, shape and number of turns for a given diameter of wire. 
Usually these coils are required to resonate with a condenser of given capacity. 
In this case, if C is the resonating capacity in micro-microfarads, 
X 2 = 3'55 x 10 -3 LC.(C) 
Eliminating L and X 2 between (A), (B), (C), we find 
a 3 = 1‘4 x 10 8 cZ 6 $C, 
a relation independent of the number of turns. Thus, whatever inductance is used, 
the coils must all have the same radius if wound with the same type of wire. In 
illustration, let the wire consist of nine strands, each of diameter 0-2 mm., and let the 
resonating capacity be 1,000 pp. F. 
Then 
s — 9, d — \/sS = 0*6, 
from which 
a == 9 cm. 
Thus, if L = 20 mh, N = 297. As the winding length b — 5*6 cm., this could be 
arranged by having 6 layers of 50 turns each. To avoid large self-capacities the 
winding should be “ sliced.” 
PRESENTS* 
2 6 SEE 
