﻿Electricity on Insulators and Metals. 269 



causes. If the surface of the metal had become tarnished 

 by oxide, the charge generated was low and in some cases 

 reversed in sign. This reversal was noticed in the case of 

 zinc and iron. If the surfaces of these metals were rubbed 

 when tarnished, the charge was positive for moderate amounts 

 of rubbing and negative for greater amounts, whereas the 

 surfaces when previously cleaned with fine emery-paper 

 always showed a negative charge. Apparently the rubbing 

 gradually wore off the film of oxide and ultimately produced 

 a charge similar to that obtained on the pure metal. A 

 reversal of the sign of electrification also appeared when the 

 silk rubber had been used for some time, and was probably 

 due to the silk having become smeared with oxide. A fresh 

 silk band always gave an electrification of the proper sign 

 when the cleaned specimens were rubbed. 



When the metals were all cleaned with very fine emery- 

 paper, all the specimens except thallium, lead, and bismuth 

 (which have the highest atomic weight) gave a negative 

 charge at the first rub. Continued rubbing, however, pro- 

 duced a polish on the metals, and then it was found that the 

 negative charges on aluminium, iron, copper, zinc, and 

 antimony became less and less and changed to positive, 

 though the other metals, on continued rubbing, did not show 

 this effect. 



Owing to these reversals and to the difficulty of preventing 

 a roughened surface becoming smoothed out and polished by 

 the friction, the specimens were, as far as possible, finally 

 tested all with their surfaces at about the same degree of 

 polish and with fresh silk bands. 



The metals rubbed were lithium, boron, aluminium, cal- 

 cium, iron, copper, zinc, silver, tin, antimony, platinum, gold, 

 thallium, lead, and bismuth. The metals which could be 

 obtained in the form of thin disks 1*2 cm. in diameter were 

 soldered on brass cylinders 3 mm. thick, and second specimens 

 were in some cases cemented on ebonite cylinders of the 

 same thickness. Metals such as iron were made up to the 

 same dimensions as the brass cylinders. All the above 

 metals were rubbed for a maximum charge, and measure- 

 ments of the charges produced by various amounts of work 

 were also taken for a few typical specimens. In the case of 

 the metals that readily tarnish in air, a few rubs were rapidly 

 given immediately after cleaning their surfaces. The boron, 

 supplied amorphous, had to be made up into a thick paste 

 and allowed to set hard before it could be tested. Some 

 of the specimens employed were of the ordinary commercial 

 grade of purity, those of the rarer metals being refined. 



