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# Long description

At the left is an advertisement for Osram Lamps dating from about 1910. It is a cartoon of a working-class family sitting around a dining table, with the father at the left, the mother at the right and two children in the foreground. They are all looking upwards to an electric light bulb in a green shade about 1 metre above the table. This lights the table, the diners and the floor, but the rest of the room is in darkness. On the floor a cat is shielding its eyes from the bright light. The words ‘the light for all’ is written immediately below the light bulb and the words ‘Wonderful – 20 hours for one penny’ across the bottom. At the right is a diagram showing the inverse square law. An area of space is shown as a pyramid with an electric light shining down from the top. The light from the lamp is labelled ‘luminous flux, ‘F’, from lamp’. Two horizontal square areas are shown in yellow. The larger one forms the floor of the pyramid. Halfway up the pyramid, at a distance ‘d’ from the floor and ‘d’ from the top is a horizontal square. This is labelled ‘area ‘A’ square metres, illuminance ‘E’ equals ‘F’ over ‘A’’. The floor of the pyramid is shown with dotted lines as having an area of 4 ‘A’ square metres. It is labelled ‘illuminance ‘E’ equals ‘F’ over 4 ‘A’’.