ON THE KEW OBSERVATORY. 125 



The discharger (fig. 9), also our " safety valve," is perhaps an improve- 

 ment upon Lane's electrometer. 



The length of the spark is measured by means of a long index R, which 

 exhibits the distance of two balls, S and S', from each other on a multiplying 

 scale T, S being attached to a rod V, which is raised and lowered by means 

 of a glass lever W, forked piece X, &c. ; V slides accurately through the base Y 

 and the piece A. The bolt, &c. (Z), which is in intimate metallic connec- 

 tion with the safety conductor C, clamps the whole down to the table in 

 the same manner as that in which the voltaic electrometers are fixed. 



Each division of the scale represents an exact twentieth of an inch in the 

 length of the spark. The actual cord of each division is about a tenth. The 

 divisions are, of course, not perfectly equal to each other : they serve very 

 well to estimate to fortieths, or less. 



We observe a tolerably near approximation to coincidence between the 

 lengths of sparks as measured by this instrument and the degrees of tension 

 exhibited by the Henley. 



A Bennetts gold-leaf electroscope, in form a little differing from fig. 10, 

 has been sometimes used for discovering the length of time which has elapsed 

 between the alternations in kind- of electricity during rain, &c., and verg rarely 

 for ascertaining whether our conductor was charged or not on other occa- 

 sions*. 



A wire A, terminating below in a pair of forceps, carrying the paper by 

 which the leaves are suspended (in Bennet's manner nearly), passes through 

 a glass stopper B, which is ground into a long-necked bottle C, with a me- 

 tallic base D, and a strip of brass (E) is bent and screwed to the inside of D. 

 The neck of C is well-covered with sealing-wax by heating both inside and 

 outf. 



If required, this instrument can be suspended from an arm, as R (fig. 6), 

 and a chain hooked on a ring in its base, but here we depart from the prin- 

 ciple of uniform insulation, and therefore seldom use a Bennet's electroscope 

 in this manner, but merely touch the conductor with it. 



DiSTINGUISHER. 



The distinguisher, which we have found most convenient for ascertaining 

 the kind of electricity possessed by the conductor, &c. at any given time, 

 and in all tensions except the verv lowest, is of the sort represented by 

 fig. 11. 



A is a wire connected with a brass tube which forms the interior coating of 

 a very thin glass tube C. B is an exterior coating of the same kind, and these 

 two coatings are at about three-fourths of an inch distant from each extremity 

 of this little Leyden jar. The intervals D C and B C are coated with melted 

 sealing-wax inside and out. A thus prepared is inserted through a stopper, 



* In measuring low intensities, and particularly small quantities of electricity, the mode of 

 insulation called ' Singer's ' is sometimes very objectionable, for this reason ; the wire (as A) 

 carrying the gold leaves, or other pendulums, becomes partly the interior coating of a charged 

 glass cylinder, and part of the cap of the instrument becomes the exterior coating ; the con- 

 tact of the electrometer with the l)ody whose electric tension is to be ascertained, lowers 

 consequently, and sometimes materially, the tension of that body itself. The charge received 

 by such an instrument is retained well, principally by reason of these associated metallic 

 coatings, &c., and it seems to lose electricity more slowly than it does, because it has more 

 to lose than it seems to have. 



t The principal conductor, its appendages and instruments in the electrical observatory, 

 hitherto described, were chiefly executed by Mr. Newman of Regent Street, and do him very 

 great credit. 



