VIII. MEASUREMENT. 331 



The instrument exhibited was made 15 or 20 years ago. The present condi- 

 tion of the spiral spring shows that it has become elongated through time, 

 without stress, because the hook at its lower end, bearing the disc, rests firmly 

 against the lower stop, with the stop in the lowest position that the micrometer 

 screws allow. 



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 instruments were con- 

 structed on the same electric plan 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 replenisher and 

 gauge for the charge of the jar. 



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

 scribed in NichoPs Cyclopaedia, article Electricity (Atmospheric). 



Sir William Thomson. 



1426. Improved Helmholtz's Quadrant Electrometer. 



T. Rob. Voss, Berlin. 



This is well suited for school use, as it is not very expensive, and its action 

 is, in proportion, as good as that of larger machines. 



1427. Electrometer. E. Stohrer, Leipzig. 



1428. Kohlrausch's Torsion-electrometer. 



Prof. Wiillner, Aix-la-Chapelle. 



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

 of different magnetic, moment. Prof. Wiillner, Aix-la-Chapelle, 



1430. Kohlrausch's Condenser. 



Prof. Wullner, Aix-la-Chapelle* 



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

 and Marburg. 



