VIII. MEASUREMENT. 289 



producing daily records of the fluctuations, &c. of atmospheric electricity, 

 which were discussed by Professor Everett, and the results published, together 

 with a description of the instrument, in the Philosophical Transactions for 

 1868, Pt. I. 

 It has since been replaced by an improved quadrant electrometer. 



1435. Singer's Gold Leaf Electroscope, for lecture pur- 

 poses. Large size. Elliott Brothers. 



1436. Quadrant Electrometer, being a modification of Sir 

 VV. Thomson's delicate quadrant electrometer, used for measuring 

 the difference of potential between two conductors. 



Elliott Brothers. 



1437. Peltier Electrometer, for measuring the electrical 

 tension of a charge by the repulsion of a light aluminium needle, 

 which receives a directive force through a very small magnet 

 attached to it. Elliott Brothers. 



1438. Capillary Electrometer, after Lippinann. 



R. Jung, Heidelberg. 



A glass tube a, filled to a height of about 85 cent, with mercury, and 

 ending below in a fine point, dips in a cylinder b, so that its point presses 

 spring-wise against the wall, where there is a microscope c placed horizontally. 

 The cylinder contains mercury in its lower part, and dilute sulphuric acid 

 about the point of the tube ; and in the mercury terminates one pole, pro- 

 tected by a glass tube from the sulphuric acid. Above, the long tube is! con- 

 nected with a small glass bow, in which a platinum wire is fused, which, 

 reaching down into the mercury in the tube, forms the upper electrode. The 

 other end of the bow is connected by caoutchouc tubing with au air press, 

 which, on its other side, is connected with a manometer. A little mercury is 

 first forced through the fine point by menus of the press. The microscope is 

 then so placed that the zero point of the eye-piece micrometer coincides with 

 the image of the meniscus of mercury in the capillary tube ; then the electric 

 source to be measured is brought into the circuit of the apparatus, its negative 

 pole being connected with the upper electrode. The mercury forthwith 

 retires, and can only be brought back by a determinate pressure with the 

 press. 



1439. Coulomb's Torsion Balance, for measuring the 

 potential of electricity. Elliott Brothers. 



1439a. School Form of Coulomb's Torsion Balance. 



Harvey, Reynolds, and Co. 



b. GALVANOMETERS. 



1440. Galvanometer with variable resistance. 



The Director of the Physical Laboratory of the Uni- 

 versity of Groningen. 



This galvanometer can be used for hydro-electric as well as thermo-electric 

 currents. By its aid the dependence of the sensibility of a galvanometer on 

 its resistance can be demonstrated. 



39508. T 



