456 PROCEEDINGS OF THE AMERICAN ACADEMY 



advantage over the metallic vessel of being more convenient to arrange 

 and more easily insulated when the humidity of the air is high. The 

 electricity produced by the impact of the water upon the plate is too 

 slight to be measured. 



Mechanical Collector. 



Continuous records cannot be obtained in this climate with the 

 water-dropper, because in winter water will freeze l)efore it has been 

 dropping long. At the suggestion of Professor Trowbridge several 

 mechanical collectors were tested to see if any could be found that 

 would be superior to the water-dropper. 



1. The lirst consisted of a wheel thirty centimeters in diameter, 

 with strips of tinfoil fastened to its circumference in such a manner 

 that when the wheel revolved on a horizontal axis the pieces of tin- 

 foil touched successively two metal knobs, A and B. A was con- 

 nected to the electrometer, and B to the ground. When the wlieel 

 revolved, each strip of tinfoil carried off a part of the charge of A, 

 and discharged it to the ground through B. Now A, losing con- 

 tinuously a part of its original charge, would approximate nearer and 

 nearer to the potential of the surrounding air. 



2. The second consisted of a pendulum attached to a framework. 

 A non-conducting fibre with a metal ball at its lower end passed up 

 tlirough a hole in the framework and was attached to the bob of the 

 pendulum. A piece of metal, G, connected to the ground, was placed 

 in such a position that, when the pendulum hung vertically, the ball 

 rested lightly on G. Another piece of metal, E, connected to the 

 electrometer, was so placed that, when the pendulum swung to either 

 side, the ball touched E. "When the pendulum was in motion, the 

 action was the same as in the first experiment. The proper facilities 

 were not at hand for carrj'ing out these experiments to the best 

 advantage, so that the results were not satisfactory. With the ap- 

 paratus used, the electricity produced by friction was too great to be 

 neglected. 



3. Fine platinum wires were attached at equal intervals to the 

 dial-plate of a minute clock, in such a manner that the seconds hand 

 could strike them. The clockwork was of sufficient strength not to 

 be stopped when the hand came against a wire. Eacii alternate wire 

 was connected to the ground. The others were connected to a metallic 

 plate in the air, and to the electrometer. The seconds hand was in- 

 sulated from each set. The principle is the same as in the first and 

 second examples. 



