Light Bulbs in Series

DCS Code: 5F20.52 Last Updated: March 27, 2026
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Safety Warnings

Purpose

Students observe how voltage divides across identical resistors in series and how brightness changes with the number of bulbs. The dramatic dimming when adding bulbs challenges the common misconception that batteries are constant-current sources.

Wooden board with eighteen small incandescent bulbs in sockets wired in series, connected to a DC power supply displaying 30.0 V via red and black leads with alligator clips
Figure 1: Eighteen bulbs wired in series on a wooden board. Alligator clips can connect to any portion of the string to vary the number of bulbs in the circuit.
Demo Apparatus