Light Bulb Wheatstone Bridge
Safety Warnings
Danger: High Voltage Warning
Warning: Hot Bulbs
Purpose
Light bulbs serve as resistive elements in a Wheatstone bridge, making bridge balance and imbalance immediately visible. With four identical bulbs the center indicator stays dark; replacing one 60 W bulb with a 25 W bulb breaks the balance and the indicator glows.
Figure 1:
The Wheatstone bridge board unpowered. Four corner sockets hold 60 W bulbs (labeled R1–R4), the center indicator socket has a small bulb, and a knife switch controls the bridge branch. A spare 25 W bulb is on the bench.
Figure 2:
Unbalanced bridge: one 60 W bulb has been replaced with a 25 W bulb. The center indicator glows, showing current through the bridge when the resistance ratios are unequal.
Figure 3:
Balanced bridge: four identical 60 W bulbs glow equally and the center indicator remains dark, demonstrating zero current through the bridge when $R_1/R_2 = R_3/R_4$.