444 TEE POPULAR SCIENCE MONTHLY 



undertakes his experiments on this adhesive affinity in the case of differ- 

 ent metals. 



At the beginning of Section YII., 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 spon- 

 taneous 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 employ- 

 ment 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. 



Experiment I 



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. 



Experiment II 



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 +, ironwire — ; lead ore +, 

 lead — ; lead — , iron wire + • l^ad ore -|-, iron wire — : tinfoil — , iron 

 wire -|- ; zinc — , iron wire +• 



He then tried charging the plate --l of his doubler by a single sub- 

 stance while B was earthed. He found A to take a positive charge from 



