This Photocell camera is enclosed in a light-tight box with a camera shutter. It should be noted that this instrument is referred to as a camera, not because it takes a picture, but because it uses this shutter to control the light that enters the device. When the shutter is opened light falls onto the large photocell below. This photocell is comprised of a concave cathode (the metal plate) and a grid anode (the wire mesh suspended above the plate), in an evacuated glass bulb.
Light can be modelled as both a wave or a particle. A light carrying particle is called a photon. Incoming photons can be absorbed by some metals, and the energy carried by these photons can be great enough to eject electrons from the surface of the metal. This is called the photoelectric effect and was first observed in 1839 by A.E. Bequerel and was explained in 1905 by Albert Einstein using the quantum nature of light.
In the case of this photocell, photons fall on the metal cathode at the bottom of the bulb and electrons are ejected. These electrons are then collected by the anode grid above, creating a current. This current may then be measured by attaching a galvanometer or oscillograph to the photocell. Photocells like this are the first forms of electronic light detectors. All the other objects on the ‘Before Photonics’ tour used visual or heating methods of measuring and quantifying light.
This device was one of many produced by the Cambridge Scientific Instrument Company. When the University of Cambridge first introduced its Science Tripos (Cambridge’s way of denoting the Science degree) it did not have a facility for the manufacture of scientific instruments. In order to supply the students and faculty with equipment, Horace Darwin, the ninth and youngest son of Charles Darwin, formed the Cambridge Scientific Instrument Company in 1881.
This item is part of the UQ Physics Museum ‘Before Photonics’ Tour
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