[ 128 ] 

 XVII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 68.] 

 April 28. — Dr. William Allen Miller, Treasurer and Vice- 

 President, in the Chair. 



r PHE following communication was read : — 



-*- '* On a Cause of Error in Electroscopic Experiments." By Sir 

 Charles Wheatstone, F.R.S. 



To arrive at accurate conclusions from the indications of an elec- 

 troscope or electrometer, it is necessary to be aware of all the sources 

 of error which may occasion these indications to be misinterpreted. 



In the course of some experiments on electrical conduction and in- 

 duction which I have recently resumed, I was frequently delayed by 

 what at first appeared to be very puzzling results. Occasionally I 

 found that I could not discharge the electrometer with my finger, or 

 only to a certain degree, and that it was necessary, before commen- 

 cing another experiment, to place myself in communication with a 

 gas-pipe which entered the room. How I became charged I could 

 not at that time explain ; the following chain of observations and ex- 

 periments, however, soon led me to the true solution. 



I was sitting at a table not far from the fireplace, with the electro- 

 meter (one of Peltier's construction) before me, and was engaged in 

 experimenting with disks of various substances. To ensure that the 

 one I had in hand, which was of tortoiseshell, should be perfectly 

 dry, I rose and held it for a minute before the fire ; returning and 

 placing it on the plate of the electrometer, I was surprised to find 

 that it had apparently acquired a strong charge, deflecting the index 

 of the electrometer beyond 90°. I found that the same thing took 

 place with every disk I thus presented to the fire, whether of metal 

 or any other substance. My first impression was that the disk had 

 been rendered electrical by heat, though it would have been extra- 

 ordinary that, if so, such a result had not been observed before ; but 

 on placing it in contact with a vessel of boiling water, or heating it 

 by a gas-lamp, no such effect was produced. I next conjectured that 

 the phenomenon might arise from a difference in the electrical state 

 of the air in the room and that at the top of the chimney ; and to 

 put this to the proof, I adjourned to the adjacent room, where there 

 was no fire, and bringing my disk to the fireplace I obtained precisely 

 the same result. That this conjecture, however, was not tenable was 

 soon evident, because I was able to produce the same deviation of the 

 needle of the electrometer by bringing my disk near any part of the wall 

 of the room. This seemed to indicate that different parts of the room 

 were in different electrical states ; but this again was disproved by 

 finding that when the position of the electrometer and the place where 

 the disk was supposed to be charged were interchanged, the charge of 

 the electrometer was still always negative. The last resource was to 

 assume that my body had become charged by walking across the 

 carpeted room, though the effect was produced even by the most 



