A mercury lamp emits some visible light in the blue end of the spectrum as well as ultraviolet light. Here a mercury lamp is placed so that its emission strikes a zinc-coated metal sheet connected to an electroscope. An electroscope is a device for detecting electric charges.

When the mercury lamp is turned on and illuminates the zinc-coated metal sheet attached to the post of the electroscope, the leaves of the electroscope move apart. This indicates that light from the mercury lamp has created a charge on the electroscope and metal sheet. Bringing a positively charged glass rod near the electroscope makes the leaves move farther apart; bringing a negatively charged hard rubber rod near makes the leaves collapse. This demonstrates that the charge on the electroscope is positive.