45, CORNHILL, EC, AND 122, BEGEtfT STREET, W., LONDON. 117 



Zamboni's dry galvanic pile of 800 pairs, about a quarter of inch, in diameter ; 

 and when the plates are pressed together, 2 \ inches in length. The bent wires 

 above the pile terminate in two plates, P and If, which are the poles of the 

 pile. These plates are parallel and opposite to each other. Their opposite 

 sides are slightly convex, and gilded ; between is suspended a fine gold leaf, ~D 

 G, which is attached to the metal conductor, C D. If the leaf hang 

 exactly between the two plates, it is equally attracted by each, and will be in a 

 state of repose. The apparatus is protected by a glass shade, having an opening 

 at the top through which the metal wire, C D, passes, insulated by being con- 

 tained in a glass tube, which is cemented to the glass shade by means of shellac. 

 A metal plate is attached to the wire rod in connection with the gold leaf to 

 convey to it the electricity to be tested. The electricity to be tested will be 

 conveyed by the metal wire to the gold leaf, and the latter will immediately 

 move towards the plate which has the opposite polarity. This electroscope is, 

 beyond doubt, one of the most delicate ever constructed, and is well adapted to 

 show small quantities of positive and negative electricity. Price, 880 



165. Volta's Straw Electrometer, with graduated Arc, for estimating 

 the amount of electric force by degrees of divergence. Price, 220 



166. Cavallo's Pith. Ball Electroscope, (fig. 122) with graduated Arc 

 for estimating the amount of electric force .... Price, t 10 



167. Ditto ditto with Stopcock, fig. 123 . .220 



168. Peltier's Tension Electrometer,* (fig. 124) according to Mr. 

 Latimer Clark, was in all its essential parts first described and illustrated by 

 Dr. Thomas Milner in the year 1 733. The instrument described as the inven- 

 tion of Peltier in the Report of the British Association, 1849, and termed the 

 Induction Electrometer, is constructed as follows : 



It consists of a light metal ball of about 4J inches diameter mounted on 

 a brass rod, terminating in a flattened oval or heart-shaped aperture. In the 

 centre of this aperture is placed a fine steel point on which is suspended a light 

 copper or aluminium wire needle, with a small magnetic needle mounted on it 

 at right angles. Two light metal rods or arms are extended from opposite 

 sides of the support of the ball of the same length as the copper needle. 

 Below these rods is a graduated circle, for estimating the value of the deflection 

 of the needle in degrees. The support of the ball and centre of the needle is 

 very carefully mounted and insulated on ebonite, and the whole mounted on a 

 mahogany base with three adjusting screws. A cylindrical glass cover is 

 placed over the graduated circle and indicating needle to protect them from 

 currents of air, dust, &c. 



In use this electrometer is very carefully placed in such a position that the 

 magnetic needle shall cause the light copper wire index needle to lie parallel 



* Peltier's Electrometer as used by Professor Palmier! at the Observatory on Mount Vesuvius. 



