278 Professor W. Thomson,' [May 18, 



obtained by the application of Peltier's mode of obsei*vation with his 

 metrical electroscope. The much more accurate electrometer, and 

 the greatly improved mode of observation invented by Delmann, have 

 given for the electric intensity, at any instant, still more precise results ; 

 but have left something to desire in point of simplicity and convenience 

 for general use, and have not afforded any means for continuous obser- 

 vation, or for the introduction of self-recording apparatus. The 

 speaker had attempted to supply some of these wants, and he explained 

 the construction and use of instruments which he had planned for this 

 purpose, which were now exhibited to the meeting. 



Apparatus for the observation of atmospheric electricity has 

 essentially two functions to perform ; to electrify a body with some 

 of the natural electricity or with electricity produced by its influence ; 

 and to measure the electrification thus obtained. 



The measuring apparatus exhibited consisted of three electrometers, 

 which were referred to under the designations of (I.) The divided 

 ring reflecting electrometer. (II.) The common house electrometer ^ 

 and (III.) The portable electrometer. 



(I.) The divided ring reflecting electrometer consists of: 



(1 ) A ring of metal divided into two equal parts of which one is 

 insulated, and the other connected with the metal case (5) of the 

 instrument. 



(2) A very light needle of sheet aluminium hung by a fine glass 

 fibre, and counterpoised so as to make it project only to one side of 

 this axis of suspension. 



(3) A Leyden phial, consisting of an open glass jar, coated outside 

 and inside in the usual manner, with the exception that the tinfoil of 

 the inner coating does not extend to the bottom of the jar, which is 

 occupied instead by a small quantity of sulphuric acid. 



(4) A stiff" straight wire rigidly attached to the aluminium needle, 

 as nearly as may be in the line of the suspending fibre, bearing a light 

 platinum wire linked to its lower end, and hanging down so as to dip 

 into the sulphuric acid. 



(5) A case protecting the needle from currents of air, and from 

 irregular electric actions, and maintaining an artificially dried atmo- 

 sphere round the glass pillar or pillars supporting the insulated half 

 ring and the uncoated portion of the glass of the phial. 



(6) A light stiff" metallic electrode projecting from the insulated 

 half ring through the middle of a small aperture in the metal case, to 

 the outside. 



(7) A wide metal tube of somewhat less diameter than the Leyden 

 jar, attached to a metal ring borne by its inside coating, and standing 

 up vertically to a few inches above the level of the mouth of the jar. 



(8) A stiff" wire projecting horizontally from this metal tube above 

 the edge of the Leyden jar, and out through a wide hole in the case 

 of the instrument to a convenient position for applying electricity to 

 charge the jar with. 



