282 Professor W. Thomson, [May 18, 



ends of the needle in contrary directions, and metal stops to limit 

 its angular motion within a convenient range. The conducting system 

 composed of these different parts is supported from the metal cover 

 or roof of the jar, by three glass stems. The torsion head is carried 

 round by means of a stout glass bar, projecting down from a pinion 

 centered on the lower side of this cover, and turned by the action of a 

 tangent screw presenting a milled head, to the hand of the operator 

 outside. The conducting system thus borne by insulating supports is 

 connected with the outside conductor to be tested by means of an 

 electrode passing out through the centre of the top of the case by a 

 wide aperture in the centre of the pinion. A wire cage, surrounding 

 the central part of the tube and the needle and repelling plates, is 

 rigidly attached to the interior coating of the Leyden jar. It carries 

 two metal sectors, or " bulkheads," in suitable positions to attract the 

 two ends of the needle, which however is prevented from touching 

 them by the limiting stops referred to above. The effect of these 

 attracting plates, as they will be called, is to increase very much the 

 sensibility of the instrument. The square root of the number of 

 degrees of torsion required to bring the needle to a sighted position 

 near the repelling plates, measures the difference of potentials between 

 the cage and the conducting system, consisting of tube torsion-head 

 repelling plates and needle. The metal roof of the jar is attached to 

 a strong metal case, cemented round the outside of the top of the jar, 

 and enclosing it all round and below, to protect it from breakage 

 when being carried about. There are sufficient apertures in this case, 

 opened by means of a sliding piece, to allow the observer to see the 

 needle and graduated circle (torsion head), when using the instrument. 

 On the outside of the roof of the jar a stout glass stem is attached, 

 which supports a light stiff metallic conductor, by means of which a 

 burning match is supported, at the height of two or three feet above 

 the observer. This conductor is connected by means of a fine wire 

 with the electrometer, in the manner described above, through the 

 centre of the aperture in the roof. An artificially dried atmosphere is 

 maintained around this glass stem, by means of a metal case surround- 

 ing it, and containing receptacles of gutta percha, or lead, holding 

 suitably shaped pieces of pumice-stone moistened ^with sulphuric acid. 

 The conductor which bears the match projects upwards through the 

 centre of a sufficiently wide aperture, and bears a small umbrella, 

 which both stops rain from falling into this aperture, and diminishes 

 the circulation of air, owing to wind blowing round the instrument, 

 from taking place to so great a degree as to do away with the dryness 

 of the interior atmosphere required to allow the glass stem to insulate 

 sufficiently. The instrument may be held by the observer in his hand 

 in the open air without the assistance of any fixed stand. A sling 

 attached to the instrument and passing over his left shoulder, much 

 facilitates operations, and renders it easy to carry the apparatus to the 

 place of observation, even if up a rugged hill side, with little risk of 

 accident. 



