< Page:Popular Science Monthly Volume 83.djvu
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446

THE POPULAR SCIENCE MONTHLY

after ten additions of electricity, the plate [1] communicated to the electrometer a sufficiently sensible quantity of positive electricity, which shows that the tin plate had been electrified negatively by the contact of the zinc. This experiment was repeated four times within the space of half an hour, and was constantly attended with the like effect; but on the following day the effect was found to be less conspicuous, for three times twenty additions just enabled the plate to communicate a sensible degree of positive electricity to the electrometer. In short, the different states of the atmosphere seem to be much concerned in the result of this experiment, and yet the whole effect can not be attributed to it; but of this further on. Before, however, I proceed to the narration of other experiments, it will be necessary to dwell a little longer on the above-mentioned operation, not only to render it more intelligible, but likewise to avoid repetitions. The tin plate used in the preceding as well as in many of the subsequent experiments, measures eight inches in diameter; and is fastened to a small piece of wood about three inches in length. Two glass sticks covered with sealing-wax are cemented into this piece of wood, and their other extremities are cemented into a larger piece of wood, which forms the stand or basis of the instrument. The operation is as follows: I hold this apparatus by the last mentioned piece of wood in my left hand, and keeping the plate in an horizontal situation, let the piece of zinc or other metallic body, fall upon it from my right hand, which I hold a few inches above the plate; then by inclining or shaking the plate, the piece of metal is caused to fall upon the table or upon a chair; from whence I take it up, and again let it fall upon the tin plate, and so on.

Cavallo repeated this experiment with a considerable number of metals, and with great precautions to guard against any other source of electrification. He found that repeated touching with an insulated body gave no greater effect than a single contact. He tried lifting his pieces of metal with iron tongs, or in a metal spoon, and found that in some cases this changed the sign of the electrification. He then performed a long series of experiments on the effect of heating the metals, and found a change in their electric properties due to temperature. In the case of bismuth, he was able to change the sign of the electrification produced on the tin plate by heating the bismuth very hot. His experiments upon the temperature change in contact electrification were almost the only ones made for a hundred years, and were probably the most important ones that have yet been published. At the end of his experiments he stated the following conclusions:

1. The contact of one metallic substance with another generally produces electricity.

2. The quantity and quality of the electricity so produced, is various according to many circumstances which seem to concur in the production of it, or in great measure to influence it.

3. Those circumstances are, the various nature of the metallic substances, their various degrees of heat, the state of the atmosphere, and the other body concerned in the experiment, viz. the hand of the operator, etc. Each of those causes has a share in the result of the experiment; for the variations of any one of them, when everything else remains unaltered, produce different effects. Thus in different states of the atmosphere, the very same metallic substances treated

  1. ↑ Cavallo used his own form of multiplier in which the plates were not named as in Bennett's figure.
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