VIII. MEASUREMENT. 2S-5 



1417. Attracted Disc Heterostatic Station-Electro- 

 meter on same electric principle as latest portable electrometers, 

 but with mechanism inverted. Sir William Thomson. 



This instrument is of convenient dimensions and general plan for stationary 

 observations of atmospheric electricity and various electrostatic measurements. 

 Made by James White, Glasgow. 



1418. Large Portable Electrometer of same general plan 

 as No. 10, altered to measure distance between two metallic con- 

 ductors giving sparks with electro-motive force measured by another 

 electrometer, in continuation of Smith and Ferguson's measure- 

 ments. (Proceedings of the Royal Society, 1860, and Thomson's 

 Reprint of Paper's on Electrostatics and Magnetism, XIX., 320.) 



Sir William Thomson. 



Numerous accurate experiments were made many years ago by this piece of 

 apparatus, but the results have not hitherto been published. Made by James 

 White, Glasgow. 



1420. Station Electrometer. Sir William Thomson. 



This electrometer Avas used by Professor Everett in his two years series of 

 observations on atmospheric electricity at Windsor, Nova Scotia, described in 

 the Transactions of the Royal Society of London for 1868. Its electric prin- 

 ciple is the same as that of No. 8 of the perfected form of portable electro- 

 meter of the first kind. (See Thomson's Reprint, xvi. 777.) 



1421. First divided Ring (semi-circular) Electrometer. 



Sir William Thomson. 



This was used for several years in the University of Glasgow, and described 

 in the Accademia Pontificia dei Nuovo Lincei, February 1857, and in Thom- 

 son's Reprint, xviii. 311. 



The moveable electrified body projects from one side of the bearing wire 

 far enough to travel over the flat semi-circular rings and experience their 

 electric force. It is kept electrified by a fine platinum wire dipping in sul- 

 phuric acid, which forms the outside coating of the Leyden jar below it. 



1422. Attracted Disc Electrometer, with double micro- 

 meter screw, arranged, to give the same period of free oscillation 

 with different forces at different distances. Sir William Thomson. 



The lower disc is insulated, the upper connected with the metal work of the 

 case of the instrument and of the micrometer screws by a spiral spring by 

 which it is hung. By turning the torsion head the upper end of the spring 

 and the sight marks with movable stops for the lower end of the spring are 

 moved through different distances, of which the former is 1| times the latter. 

 The instrument exhibited was made 15 or 20 years ago. The present condi- 

 tion of the spiral spring frhows that it has become elongated through time, 

 without stress, because the hook at its lower end, bearing the disc, rests firmly 

 against the lower stop, with the stop in its lowest position that the micrometer 

 screws allow. 



