[Fig. 72.—Electrolytic detector.]

[Fig. 73.—Electrolytic detector in circuit.]

[Fig. 74.—Potentiometer.]

[Fig. 75.—Diagram showing how potentiometer is connected in a circuit.]

[Fig. 76.—Analogy between swinging and tuning.]

[Fig. 77.—Receiving a message in a Marconi transatlantic station.]

[Fig. 78.—Tuning coil of the double slide type.]

[Fig. 79.—Diagram showing fixed condenser in circuit.]

[Fig. 80.–Fixed condenser.]

[Fig. 81.—Rotary variable condenser.]