Page:Popular Science Monthly Volume 83.djvu/448

444 undertakes his experiments on this adhesive affinity in the case of different metals.

At the beginning of Section VII., page 91, he says:

Having fully proved by a frequent repetition of experiments, that the positive or negative spontaneous charge of the doubler depended upon the absorption or repulsion of the electrical fluid by the approximation of its parallel plates, and that by applying larger plates covered with minium or flour its electricity might be changed at pleasure, it easily occurred, that if the spontaneous electricity in the beginning of the process was sufficiently weak, the mere contact of metals or other substances having a different adhesive affinity with the electrical fluid might also change it, and a new and interesting employment for the doubler be discovered.

This supposed effect of contact was confirmed by the following experiments, in which the doubler and electrometer were deprived of electricity, and used with the precautions and improvements mentioned in the last section.

The spontaneous charge of the doubler having been negative, and being deprived of this charge by the usual method, the plate $$B$$ was placed parallel to the plate $$A$$, but so that $$B$$ was not connected with the earth. The plate $$A$$ was then touched with the blade of a knife, and the plate B at the same time touched with the point of a soften'd iron wire. With sixteen revolutions the gold leaf diverged about one third of an inch positively.

The doubler being deprived of electricity as before, and the plate $$B$$ placed as in the last experiment, the knife was applied to $$B$$ instead of $$A$$, and the soft iron wire to $$A$$ instead of $$B$$, which opened the gold leaf negatively at 15 revolutions.

These experiments were repeated very often, and the electricity changed each time, being always positive in the plate touched by the knife.

To distinguish so minute a difference of adhesive electricity, as that which might be supposed between two metals so nearly alike as hardened steel and soft iron, wou'd appear incredible had not the frequent repetition of experiments confirmed it.

Being now well convinced of this fact I tried many other substances with various success, sometimes the charge wou'd change regularly for a long time together, by applying the opposed substances to A and B alternately, as in the above experiments; and sometimes with other substances the charge wou'd be quite uncertain.

Bennett gives his experiments with six pairs of substances, each pair being tried about ten times. The charges given by contact to the plate A of the doubler were as follows: steel, $$+$$ iron wire $$-$$; lead ore, $$+$$ lead $$-$$; lead, $$-$$ iron wire $$+$$; lead ore $$+$$, ironwire $$-$$; tinfoil $$-$$, iron wire $$+$$; zinc $$-$$, iron wire $$+$$.

He then tried charging the plate $$A$$ of his doubler by a single substance while $$B$$ was earthed. He found $$A$$ to take a positive charge from