Apparatus Configuration

The receiver in Hertz's apparatus consisted of a coil with a spark gap, where sparks appeared upon detecting electromagnetic waves. He placed the apparatus in a darkened box to see the spark better, but noticed that the maximum spark length was reduced inside the box. A glass panel between the electromagnetic-wave source and the receiver absorbed ultraviolet radiation that assisted electrons in jumping across the gap; when removed, spark length increased. Replacing the glass with quartz, which does not absorb UV radiation, produced no decrease in spark length. The use of clean metal surfaces in evacuated tubes is a general feature of most practical photoelectric experiments and devices, not a documented feature of Hertz's spark-gap receiver.