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Spark Sign
European, mid-19th century
Electricity travels along a metal strip. If we fasten that metal strip to a glass sheet, the electricity will travel through the metal just as nicely. If we make a small gap in the metal strip, and the voltage is high enough, a visible spark will leap across that gap. That's the principle of the spark sign. A strip of foil is zigzagged back-and-forth on a glass plate. A word is written, or a picture drawn, on paper; and the glass plate is put down over the paper. Everywhere a line of the word or picture crosses a line of the foil, a small gap is cut. Then it's all cleaned up, and another sheet of glass put over it to protect the foil. When a spark from a leyden jar is sent through the spark sign, each little gap gives a spark of light. Together the sparks spell out the word or draw the picture. The drawing below (from Adams' 1792 Essay on Electricity) shows a spark sign arranged to spell "LIGHT". If you examine the actual spark sign above, you can see that it spells "FINIS". The odds are almost overwhelming that this spark sign was used as the finale for an electrical demonstration, back in the days when such demonstrations were popular.
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