L = 4π2r2n2μ ÷ (l × 109) (2)
for a coil having an iron core. In the above formulæ:
- L = inductance in henrys;
- π = 3.1416;
- r = average radius of coil in centimeters;
- n = number of turns of wire in coil;
- μ = permeability of iron core;
- l = length of coil in centimeters.
EXAMPLE.—An air core coil has an average radius of 10 centimeters and is 20 centimeters long, there being 500 turns, what is the inductance?
Substituting these values in formula (1)
L = 4 × (3.1416)2 × 102 × 5002 ÷ (20 × 109) = .00494 henry
Ques. Is the answer in the above example in the customary form?
Ans. No; the henry being a very large unit, it is usual to express inductance in thousandths of a henry, that is, in milli-henrys. The answer then would be .04935 × 1,000 = 49.35 milli-henrys.
