130 PROCEEDINGS OF THE AMERICAN ACADEMY 



One electrometer commutator, and some binding screws. 



A condenser, with a capacity of ^- farad, was also brought up, but 

 when required for use was found to be defective and of no value. 



The first observations were made on June 17, at nine a. m. 



The first step was to measure the difference between the potential 

 of the air, at a point a few feet out from the observatory walls and 

 about five feet above the ground, and the ground potential. Instead of 

 using the insulated water-dropper devised by Sir William Thomson, I 

 made use of the method employed with success at Cambridge; namely, 

 of dropping water on an insulated metallic plate, and allowing it to 

 fall iu drops therefrom. Making the electrometer connections in the 

 way adopted by Thomson and English writers generally, — that is, 

 connecting one set of quadrants with the insulated plate, the other set 

 with the ground, and bringing the needle to a high potential by con- 

 necting with the jilus pole of a battery of high electromotive force, — 

 the electrometer indicated no appreciable difference between the po- 

 tential of the air at that point and the potential of the ground. The 

 ground, it may be remarked, was not at the observatory itself, — for 

 the summit is a ledge of solid rock, — but a telephone ground, made 

 of a wire running down the hillside some distance, and connected at 

 different places with metallic plates buried in the earth. 



Using the method adopted by Mascart in making the electrometer 

 connections, — that is, connecting one set of quadrants to the plus pole 

 of the 100-cell battery, and the other set of quadrants to the negative 

 pole of the same battery, while the needle is connected with the insu- 

 lated plate or body whose potential is to be determined, — no appreci- 

 able deflection due to the difference between the potentials of the air and 

 ground could be noticed. The insulated plate has to be disconnected, 

 and in its stead the ground substituted in order to ascertain this differ- 

 ence of potential. The electrometer is designed to measure only where 

 differences of potential exist that are of considerable value. The needle 

 of the electrometer carries a fine aluminium pointer, allowing one to 

 read the deflections directly. 



In the mean while, the kite having been raised to an elevation of 

 200 feet, the wired kite-string was now connected with one set of 

 quadrants, and the other set connected with the ground. The needle 

 was connected with the plus pole of the 100-cell Beetz battery, the 

 other pole being grounded. Great care was taken to insulate very 

 thoroughly the kite-string, as with electricity of high tension ordinary 

 methods of insulation are not sufficient. Instantly on making connec- 

 tion, the needle was deflected with a considerable impulse beyond the 



