286 SEC. 10. ELECTRICITY. 



1423. First Mirror divided Ring (semi-circular) Elec- 

 trometer, used at Kew for recording atmospheric electricity. 



Sir William Thomson. 



A specimen of the curve by which it recorded the atmospheric potential is 

 published in Thomson's reprint, xvi. 292. Specimen sheets of its actual work 

 accompany the instrument. 



1424. First Trial Apparatus, towards mirror quadrant 

 electrometer. Sir William Thomson. 



This instrument was first designed for marine use. The mirror and needle 

 are supported on a stretched bundle of silk fibre, as are the needle and mag- 

 nets of the marine galvanometer. The electric connexion between the needle 

 and the inside coating of the Ley den jar is made by a spiral of fine platinum 

 wire. These peculiarities were tested and found to work moderately well in 

 the trial instrument now exhibited, but have never been repeated ; nor does it 

 seem very desirable they should be repeated, as the balancing of the needle 

 on this plan, with sufficient accuracy for good work at sea, would probably be 

 more troublesome than the object would justify. The electric action of this 

 instrument was found so promising that immediately on the same electric 

 plan were constructed instruments for use on land. The shape and dimensions 

 of the suspended needle and of the electrified surroundings of the mirror are 

 precisely the same as those of the quadrant electrometers now made. The im- 

 provements upon this original working model consist of geometrical slides for 

 the quadrants, mechanical details regarding the suspension, the substitution 

 of a fine platinum wire hanging down into the liquid in the bottom of a tall 

 Leyden jar for the platinum spiral, and the addition of a replenish er. 



1425. Divided Ring (semi-circular) Electrometer, de- 

 scribed in Nichol's Cyclopaedia, article Electricity (Atmospheric). 



Sir William Thomson. 



1426. Improved Helmholtz's Quadrant Electrometer. 



F. Rob. Voss, Berlin. 



The advantages of this instrument are: (1.) That the quadrants with 

 the needle and mirror can be easily removed, so that any change in the needle 

 or misplacement of the mirror may be examined with certainty. 



1427. Electrometer. E. Stohrer, Leipzig. 



1428. Kohlrausch's Torsion-electrometer. 



Prof. Wullner, Aachen. 



1429. Kohlrausch's Sine-electrometer, with two needles 

 of different magnetic momentum. Prof. Wullner, Aachen. 



1430. Kohlrausch's Condenser. Prof. Wullner, Aachen. 



All three pieces of apparatus were manufactured by Th. Schubart, of Ghent 

 and Marburg. 



The detailed description and theory of Kohlrausch's various apparatus may 

 be found as under : 



Poggendorff's Annaleu, Vols. 72 and 7 4 for the torsion-electrometer. 

 Vol. 88 for the sine-electrometer. 



Vols. 75 and 88 for the condenser. 



The apparatus which are exhibited show the forms which Mr. Schubart 

 (late of Marburg, and now of Ghent) now makes. A good description of the 



