-1859] WRITINGS OF JOSEPH HENRY. 319 



Conduction and Insulation of Electricity. — By means of a 

 simple electroscope of the kind we have just described we 

 may at once determine whether a body is a conductor or 

 non-conductor of electricit}^ If a slight charge be given to 

 the electroscope, (which may be effect-ed by touching the 

 knob with a rod which has been rubbed by woollen cloth,) 

 the charge will remain with but little diminution for several 

 hours, provided the air is perfectly dry; while if the air is 

 moist, the charge is soon dissipated. These facts show that 

 the former is a non-conductor, and the latter a partial con- 

 ductor. Dry air would be a perfect insulator of electricity, 

 provided it were motionless; the atoms which impinge 

 against a charged body however become electrified with the 

 same kind of excitement, and are consequently repelled, 

 their place being supplied by others and so on until the 

 charge is gradually diminished and finally dissipated. 



If, when the electrometer is charged in dry air, we touch 

 the knob with a glass rod, the leaves will be but little affected; 

 but if we breathe on the surface of the rod, the glass will be- 

 come a partial conductor and the leaves will slowly converge. 

 If the ball be touched with one end of a metallic wire, the 

 electricity will instantly be conducted off. If we make a simi- 

 lar experiment with a piece of dry wood, the charge will be 

 gradually dissipated, a fact which indicates that wood is a 

 partial conductor. By increasing the length of an imperfect 

 conductor we shall find that the time of drawing off the 

 charge is increased, and in this way it may be shown that 

 there are very few bodies which are perfect conductors or 

 non-conductors; that every body offers some resistance to 

 the passage of an electrical current, provided we increase 

 the length sufficiently to make it perceptible. By experi- 

 menting on various bodies in the way we have described, 

 we may form an approximate table of the degrees in 

 which different substances are conductors or non-conduc- 

 tors of electricity. The human body is a very perfect 

 conductor of ordinary electricity, since if we touch the knob 

 of the electroscope with the fmger, the leaves instantly col- 

 lapse, provided we are standing on the ground at the time. If 



