118 NEGRETTI AND ZAMBEA, HOLBOEN VIADUCT, E.G., 



with and almost touching the two brass arms, when, if the apparatus lias been 

 properly adjusted, if any cloud or portion of air in its vicinity be in an electrical 

 condition it will act by induction upon the metal ball, and the needle will be 

 deflected according to the amount and tension of the electricity. 



The quality of the electricity, if positive or negative, may be ascertained by 

 the use of a rod of glass or shellac as described in directions for using the gold 

 leaf instrument. (No. 163) 



In atmospheric observations the instrument may either be charged with 

 free electricity and the indications of this needle noted at certain intervals, or it 

 may be brought to the same degree of tension as the earth, and the inductive 

 effects of the atmosphere upon it observed. Owing to its greater convenience the 

 former method is now generally adopted, but the variations of the needle under 

 atmospheric influences are far from being understood or reduced to a system. 



In use the Induction Electrometer is placed upon a stand about six feet from 

 the ground, and to bring it into equilibrium of tension with the earth, touch 

 the base of the stem with a conducting wire. When the instrument is removed 

 from the inductive influence it indicates the presence of free electricity by the 

 deflection of the needle. 



A regular and uninterrupted series of atmospheric observations with the 

 Peltier instrument were made by M. Quetelet at the Royal Observatory at 

 Brussels from August, 1844, till December, 1848. 



A strong inductive influence was generally noticed at the approach or 

 cessation of rain. The maximum of atmospheric electricity was indicated in 

 January, the tension of the atmospheric charge progressively diminishing until 

 June, when it attained its minimum. The difference of the tension in these 

 two months was in the proportion of 13 to 1. The results obtained by Mr. R. 

 Birt at Kew are closely in accordance with those of M. Quetelet. 



Peltier's instrument is now constructed with a smaller ball, and without 

 the metal shade as shown in our engraving, and the whole apparatus carefully 

 insulated with ebonite (vulcanite), in place of shellac and resin. Price, 550 



169. Thomson's Quadrant Electrometer complete with Lamp and 



Scales, including directions for use, in Mahogany Case, with Lock and Key. 



36 



170. Professor Sir W. Thomson's Portable Atmospheric 

 Electrometer (attracted disc), fully described in Negretti and Zambra's 

 Treatise on Meteorological Instruments, paragraph 135, pp. 130 and 131. 



Price with Electrophorous fitted in Mahogany Box 12 



171. We have still to note the want of a portable and simple, but at the 

 same time, accurate instrument to denote the electrical condition of the 

 atmosphere. Many forms of Electrometers lately devised are but of little 

 use to ordinary observers from their complex construction. 



