Fig. 1,272.—Experiment showing effect of inductive and non-inductive coils in alternating current circuit. The apparatus is connected up as shown; by means of the switch, the lamp may be placed in parallel with either the inductive or non-inductive coil. These coils should have the same resistance. Pass an alternating current through the lamp and non-inductive coil, of such strength that the lamp will be dimly lighted. Now turn the switch so as to put the lamp and inductive coil in parallel and the lamp will burn with increased brilliancy. The reason for this is because of the opposition offered by the inductive coil to the current, less current is shunted from the lamp when the inductive coil is in the circuit than when the non-inductive coil is in the circuit. That is, each coil has the same ohmic resistance, but the inductive coil has in addition the spurious resistance due to inductance, hence it shunts less current from the lamp than does the non-inductive coil.
Ques. Why is the iron core of an inductive coil made with a number of small wires instead of one large rod?
Ans. It is laminated in order to reduce eddy currents and consequent loss of energy, and to prevent excessive heating of the core.
Ques. How does the number of turns of a coil affect the inductance?
Ans. The inductance varies as the square of the turns.
That is, if the turns be doubled, the inductance becomes four times as great.
The inductance of a coil is easily calculated from the following formulæ:
L = 4π2r2n2 ÷ (l × 109) (1)
for a thin coil with air core, and