446 THE POPULAR SCIENCE MONTHLY 



after ten additions of electricity, the plate C' communicated to the electrometer 

 a sufSciently 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 C 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 tlie 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 accord- 

 ing 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 



^Cavallo used his own form of multiplier in which the plates were not 

 named as in Bennett's figure. 



