-1859] 



WRITINGS OF JOSEPH HENRY. 



317 



ducting matter, suspended by a silk thread, the excitement 

 will be communicated to the ball, and if the communication 

 be from the glass rod the electricity will be that denomi- 

 nated positive; if from the rod of sealing-wax or shellac, 

 it will be what is called negative. Since the phenomena 

 exhibited by balls charged negatively and positively are 

 very nearly the same, it is not of much consequence which 

 we call the positive or which the negative, provided we al- 

 ways apply the same name to the same kind of excitement. 

 In the early discovery of the two kinds of electrical excite- 

 ment, that which was produced by rubbing glass with a 

 woollen cloth was called vitreous, and that from the friction 

 of the same substance on sealing-wax or gum shellac was de- 

 nominated resinous, and these terms are still retained, par- 

 ticularly in foreign works on the subject. 



The simplest instrument for exhibiting the attraction and 

 repulsion of electrified bodies, and determining the intensity 

 and character of the excitement, is the gold leaf electrometer, 

 or electroscope, which any person with a little patience and 

 some mechanical skill may construct for himself. Different 

 forms of this instrument are exhibited in Figures 2, 3, and 8. 



A brass wire, surmounted by a 

 ball of the same metal, is passed 

 through the cork of a small glass 

 jar, or a large-sized vial, from which 

 the bottom has been removed and 

 its place supplied by a disc of wood ; 

 and to the lower end of the wire, 

 which may be slightly flattened, is 

 attached, by means of any adher- 

 ing substance, two narrow strips of 

 gold leaf so as to hang freely, and 

 when un-excited parallel to each 

 other without touching. Two small pith balls suspended 

 close together by threads a few inches in length may be em- 

 ployed, in place of the gold-leaf strips. 



When we wish to ascertain if a body is electrified, or 

 whether different parts of it are charged positively to the 



Fig. 



