In 1888 Hallwachs made the following observations. A negatively charged zinc plate was connected to an electroscope.

The plate and electroscope were well insulated from the surroundings and remained charged for a long time.

However, when the zinc plate was illuminated with ultra-violet radiation, the charge rapidly disappeared.

Other researchers subsequently showed that electrons were liberated from the metal under the influence of the radiation. This process is now referred to as the photo-electric effect and has been observed in other metals. Some metals exhibit the same behaviour when illuminated by visible light.

• Investigating the Photo-Electric Effect
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