OF ARTS AND SCIENCES. 457 



4. We next made use of the principle, that, if a metallic sphei-e 

 is carried out in the air and connected by a fine wire to the ground, 

 and then insulated, it will have the potential of the air.* Two 

 wires connected together were placed on the dial of the clock de- 

 scribed above, and. carefully insulated from the clockwork. They 

 were connected to a metal plate in the air, and to the electrometer. 

 The hands of the clock were connected to the ground. The clock 

 was placed inside of a box lined with tinfoil and hung upon the wall 

 of the room. The tinfoil was connected to the ground. In this way 

 the clock was shielded from the influence of the electricity of the 

 room, and also from the effects of the weather without. A rubber 

 hose ending in a glass tube was placed near the jilate, and air drawn 

 in over the plate through the hose by an aspirator. By this means 

 the air around the plate was continually renewed. "When the clock 

 was running, the plate was connected to the ground every half-minute 

 for a moment, and then insulated, and therefore took the potential of 

 the adjacent air. When the plate was connected to the ground, the 

 needle would tend to swing toward the zero, but the plate being 

 immediately insulated, the needle would return to the proper deflection. 

 The needle used had suflTicient inertia to prevent much swinging. 

 There was only a slight oscillation. In using this collector, if the 

 inertia of the needle is small, the clock can be stopped long enough 

 to take an observation after it has once been in operation and con- 

 nected the plate to the ground. 



With all these collectors, including the water-dropper, we do not 

 measure the difference of potential between the ground and the air, 

 but between the ground and some combination of the ground and air. 

 If the ground is zero, the results will be correct; but if the ground has 

 a local charge, the results will be a combination. For instance, when 

 the mechanical collector is grounded, it takes the potential of the 

 ground. Being then insulated, it combines this potential with the 

 potential of the air, and the electrometer measures the difTercnce 

 between the ground and this combination. In the case of the water- 

 dropper, water is drawn from the pipes in contact with the ground, 

 so that the electrometer measures the difference between the potential 

 of the ground and the combination formed by the charge of the water 

 and the charge of the air. It is probable that this effect is generally 

 very small, and is soon neutralized ; but under certain circumstances, 

 (e. g. when an electrified cloud is near the place of observation,) it 



* Maxwell's Elect, and Mag., sect. 221. 



