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PHOTOELECTRICITY consists
of a sensitive gold foil electroscope, a zinc plate, a power supply,
an ultraviolet lamp and a glass plate which blocks the lamp. Light
shining on the zinc plate liberates electrons with considerable
energy of motion, demonstrating the particle theory of light. The
visitor is able to charge the electroscope with a probe which is
attached to the power supply, but since the zinc plate is attached
to the electroscope, and since the ultraviolet light liberates electrons
from the zinc plate the electroscope slowly discharges. If the visitor
blocks the light from the ultraviolet lamp the electroscope stays
charged. If the electroscope receives a positive charge, (electrons
are taken away) then the electroscope stays charged regardless of
the ultraviolet light. This occurs because the lamp acts to reinforce
the positive charge by removing electrons. .
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