Edison Effect

DCS Code: 5D40.42 Last Updated: March 27, 2026
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Safety Warnings

Purpose

Demonstrates thermionic emission (the Edison effect): a heated filament emits electrons that flow toward a positively charged external electrode, but current does not flow when polarity is reversed. Students observe one-way conduction, the operating principle of vacuum tube diodes.

Heathkit regulated power supply with projection meter on top, incandescent bulb wrapped in aluminum foil on a small base, and variac on a lecture bench
Figure 1: Edison effect apparatus: Heathkit regulated power supply with projection meter on top (left), light bulb with aluminum foil electrode on base (center), and variac (right).