2 6o ELEMENTARY LESSONS ON [CHAP. iv. 



kite, or (as Becquerel did) to shoot into the air an arrow 

 communicating with an electroscope by a fine wire, which 

 was removed before it fell. Gay Lussac and Biot lowered 

 a wire from a balloon, and found a difference of potential 

 between the upper and lower strata of the air. None 

 of these methods is quite satisfactory, for they do not 

 indicate the potential at any one point. To bring the 

 tip of a rod to the same potential as the surrounding air, 

 it is necessary that material particles should be discharged 

 from that point for a short time, each particle as it 

 breaks away carrying with it a + or a charge until 

 the potentials are equalised between the rod and the 

 air at that point. Volta did this by means of a small 

 flame at the end of an exploring rod. Sir W. Thomson 

 has employed a " water -dropper," an insulated cistern 

 provided with a nozzle protruding into the air, from 

 which drops issue to equalise the potentials : in winter 

 he uses a small roll of smouldering touch-paper. Dell- 

 mann adopted another method, exposing a sphere to 

 induction by the air, and then insulating it, and bringing 

 it within doors to examine its charge. Peltier adopted 

 the kindred expedient of placing, on or near the ground, 

 an electrometer of the form shown in Fig. in, which 

 during exposure was connected to the ground, then 

 insulated, then removed in-doors for examination. This 

 process really amounted to charging the electrometer 

 by induction with electricity of opposite sign to that of 

 the air. The principle of this particular electrometer 

 was explained in Art. 260. Of recent years the more 

 exact electrometers of Sir W. Thomson, particularly the 

 " quadrant " electrometer, described in Art. 262, the 

 " divided-ring " electrometer, and a " portable " electro- 

 meter on the same general principle, have been used 

 for observations on atmospheric electricity. These 

 electrometers have the double advantage of giving 

 quantitative readings, and of being readily adapted to 

 automatic registration, by recording photographically the 



