1878.] Scale Value of a Thomsons Quadrant Electrometer. 357 



showed that the greatest potential which could be obtained with them 

 was very inadequate for the purpose. 



Having named this fact to Dr. De La Rue, he very generously 

 placed his large chloride of silver battery at the disposal of the Ob- 

 servatory, and by its means we have been able to test the value of the 

 deflections of the instrument at different points of the scale throughout 

 its entire range. 



Owing to the difficulty of transporting the large battery the experi- 

 ments were all made in Dr. De La Rue's laboratory, Charlotte-street, 

 Portland-place, London. 



The electrometer (Thomson's Quadrant, No. 19, White, Glasgow, 

 maker) was dismounted from its position in the Observatory on the 

 3rd December, 1877, careful measurements having first been made of 

 the distance of a mark on the instrument from the source of light and 

 from the point of incidence of the reflected spot of light on the circum- 

 ference of the cylinder of the registering apparatus. 



The acid was then removed from the jar, and the needle fixed for 

 transit. 



Before moving the instrument the exact position of the quadrants, 

 which are kept separated to some distance in ordinary use, was marked 

 by lines drawn on the cover, so as to ensure as far as possible that the 

 same inductive power should be acting on the needle during the expe- 

 riments as there had been whilst the electrometer was in daily use at 

 the Observatory. 



The time of vibration of the needle was also carefully determined 

 and noted, with the view of detecting any change that might occur 

 in its sensibility from derangement of its supporting fibres during 

 transit. 



The apparatus was then conveyed to Dr. De La Rue's laboratory, 

 and there fitted up on a bed prepared for the purpose, on which the 

 distances as mentioned above were marked. 



The jar having been refilled with acid the needle was liberated, and 

 its time of vibration being found unaltered, it was charged positively, 

 and the instrument left for several days in order that its interior might 

 become thoroughly dried. 



In the first experiment readings were taken on a scale divided into 

 fortieths of an inch, which was placed in the exact position occupied 

 by the front face of the cylinder of the registering apparatus of the 

 electrograph. 



The battery terminals were then attached to the electrodes of the 

 instrument, and the cells joined up in series as required. The deflec- 

 tions produced by the different potentials were read off on the scale 

 with the following results, the quadrants being put to earth at the 

 beginning of each experiment with the view of obtaining a correct 

 value of the zero. 



