The capacitance versus voltage relationship (the "C-V" characteristic) of a varicap is determined, and the device is used for tuning an "LC" circuit and for building a voltage-controlled oscillator. With a data-acquisition system, the "C-V" characteristic can be demonstrated in a short time. The necessary equipment includes a function generator, an oscilloscope, a 9 V battery, capacitors, resistors, coils, and a multimeter. The varicap provides good opportunities for experimentation with electrical circuits. The experiments may be offered as student activities. (Contains 4 figures.)