FIGURE 15. INDUCTANCE FACTOR (TURNS).

(b) *Coil Diameter*. The second factor is the coil diameter.

Figure 1.6 on the following page shows a coil. (B) which has

twice the diameter of coil (A). Physically, it. requires more

wire to construct a coil of large diameter than one of small

diameter with an equal number of turns. Therefore, more lines

of force exist to induce a counter emf in the coil with the

larger diameter. Actually, the inductance of a coil increases

directly as the crosssectional area of the core increases.

Doubling the radius of a coil increases the inductance by a

factor of four.

(c) *Length of the Coil*. The third factor that affects the

inductance of a coil is the length of the coil. Figure 17 on

page 26 shows two examples of coil spacings. Coil (A) has three

turns, rather widely spaced, making a relatively long coil. A

coil of this type has few flux linkages due to the greater

distance between each turn. Therefore, coil (A) has a

relatively low inductance. Coil (B) has closely spaced turns,

making a relatively short coil. This close spacing increases

the flux linkage, increasing the inductance of the coil.

Doubling the length of a coil while keeping the number of turns

the same halves the inductance.