ON THB KEW OBSERVATORY. 123 



Electrometers, &c. 

 The voltaic electrometers, which we used at first for the observations, were 

 Volta's No. 1, or standard, O, fig. 2, and his second P, so modified that the 

 straws suspended within square glass bottles with metallic bases, &c. were 

 not suspended from the bottles themselves ; but finding it difficult to avoid 

 parallax and distortion by uneven glass, &c., I have endeavoured to improve 

 these electrometers, and since the 16th of June we have used the following 

 form, having first taken special pains to render the new instruments as nearly- 

 accordant with the old as possible (vide Experiments, post\ 



P (fig. 4) is a front view and O (fig. 5) a side view of a brass case (instead 

 of a bottle) exactly 2 English inches wide internallj^ and furnished with 

 plates of thin plate glass fixed by brass plates, &c. to its front and back : the 

 back plate is ground to semi-transparency. The radius of the ivory scale p 

 is equal to the length of the pair of straws Q ii. e.) 2 Paris inches, and the 

 scale is graduated "in half Paris lines. The scale of No. 1 counts single de- 

 grees, and each degree of No. 2 corresponds with five of No. 1. 



The straws are suspended by hooks of fine copper wire inserted into their 

 hollows and passing freely through holes in the flattened ends of the wire R, 

 at the distance of half a line from each other. R passes through a glass tube 

 S, covered with sealing-wax by heating the tube (not by spirit varnish). T is 

 a cover cemented upon S, and, when the instrument is not in use, closing P. 

 U is a ring to which R and S are attached, and V (fig. 5) is a knife-edged 

 piece of steel riveted into a slit in U*. 



The base ( W X Y) of the instrument consists of three parts. W is a cylinder 

 with a kind of flange, w^, and is screwed firmly down upon a circular plate X. 

 Y is a stout ring turning with friction about the smaller part of W, and X is 

 secured firmly on the table of G by a bolt, screw-washer and nut Z, the bolt 

 passing through a hole in G much larger than itself. The lower part, or 

 plinth of P, has a shallow cavity beneath into which id^ fits easily. 



A is a tubular arm attached to Y, and carrying a steel wire B, which sup- 

 ports an eye-piece C ; this can be adjusted and fixed at the required height 

 from Y, in the same manner as Q (fig. 3). The distance of C from P, .when 

 in use, is one English foot. 



R R (fig. 6) is a horizontal tubular arm fixed upon one of the vertical arms 

 Q (fig. 3), and S S (fig. 6) are two little tubes witii stoppers which slide into 

 R and turn on their common axis ; s' s^ are notches cut down to the diameter 

 of S S, and the horizontal parts of V (vide fig. 5) fit these notches. 



Hence it is obvious, that when the adjustments have been made, an elec- 

 trometer-case can be properly placed upon its base W, &c., and the straws 

 Q, &c. suspended from S at exactly their proper height, without destroying 

 the insulation of the warmed glass pillar (for it is necessary to handle P only), 

 that U, &c. will then hang with sufficient freedom without liability to turn on 

 their axis, and that C can be brought to exactly its proper position for noting 

 the degrees onjo>, indicated by the divergence of Q. In like manner O can 

 be removed and closed (as shown in the side view, fig. 5) without destroying 

 the insulation, and finally, the whole of PX, &c. can be adjusted to make 

 the straws accord with the zero point of />' (when unelectrified) and firmly 

 fixed there f. I will not enter upon further particulars concerning the man- 

 ner of using the sight-piece C in estimating fractions of degrees. 



* Cleverly suggested to me by Mr. Robert Murray. 



t The Astronomer Royal has improved the manner of placing and displacing these elec- 

 trometers at Greemvich. 



