Lecture-experiments in Electricity. 231 



These results show very plainly the nature and importance of 

 the electrical changes which take place in the lower plate of the 

 electrophorus while the apparatus is being used. Their mean- 

 ing is too obvious to require further comment. 



Equally decisive results are obtained if the lower plate is left 

 constantly in connexion with the earth through the galvano- 

 meter, and the cover is repeatedly put on, touched, raised, dis- 

 charged, and replaced, as in the common way of taking a series 

 of charges from the electrophorus. On putting on the insu- 

 lated cover, the galvanometer is not affected ; but on afterwards 

 touching the cover, a strong deflection is obtained in the direc- 

 tion indicating a downward positive current (that is, a current 

 through the galvanometer into the ground). When the cover is 

 raised, there is a deflection to the opposite side, indicating an 

 upward positive current, which is again inverted if the cover be 

 replaced without having been discharged; but, if it be touched be- 

 fore being replaced, no deflection is caused on putting it on again. 



The importance of free electrical communication between the 

 lower plate of the electrophorus and the earth is still further 

 illustrated by the following experiments. First, the lower plate 

 was insulated, both during the rubbing and afterwards, and the 

 cover was connected through the galvanometer with the earth- 

 wire : on now putting the cover on or taking it off by means of 

 the glass handle, a deflection of from 5 to 10 divisions was ob- 

 tained alternately on the two sides of zero. Next, the experi- 

 ment was repeated, the india-rubber being rubbed the same 

 number of times, in the same manner as before, but during the 

 rubbing the lower plate was uninsulated ; this time the deflec- 

 tion caused by putting on the cover amounted to 130 divisions, 

 and on taking it off to 127. 



A Thomson's galvanometer also serves very conveniently 

 for proving the movement of electricity which takes place when 

 a conductor is charged by statical induction. For example, one 

 terminal of the galvanometer being connected to earth and the 

 other with an insulated brass cylinder 2 inches in diameter and 

 17*5 inches long, a deflection of 10 or 12 divisions was obtained 

 on bringing the slightly charged cover of the electrophorus near 

 to the cylinder, and an equal deflection on the opposite side on 

 removing it. These deflections, which might easily have been 

 increased by using a body more strongly electrified, could be 

 reproduced an indefinite number of times by simply moving the 

 electrophorus-cover towards or away from the brass cylinder. 



2. Comparative Measurement of the Electrical Capacity of Con- 

 ductors. — The quantity Q of electricity which passes into or out 

 of any insulated conductor, when put into electrical communi- 

 cation with any source of constant electrical potential, is pro- 



