I860.] 071 Atmospheric Electricity. 281 



between two conducting systems, namely, as one, the aluminium needle 

 (5), the repelling plates (7), and the inner coating of the jar, and, as 

 the other, the insulated cage (8). This latter is commonly connected 

 by means of its projecting electrode (10), with the conductor to be 

 tested. The two conducting systems, if through their projecting elec- 

 trodes connected by a metallic wire, may be electrified to any degree, 

 without causing the slightest sensible motion in the needle. If, on the 

 other hand, the two electrodes of these two systems are connected 

 with two conductors, electrified to different potentials, the needle 

 moves away from the repelling plates ; and if by turning the torsion 

 head it is brought back to one accurately marked position, the number 

 of degrees of torsion required is proportional to the square of the 

 difference of potentials thus tested. 



In the ordinary use of the instrument, the inner coating of the 

 Leyden jar is charged negatively, by an external application of electri- 

 city through its projecting electrode (9). The degree of the charge 

 thus communicated, is determined by putting the cage in connexion 

 with the earth through its electrode (10), and bringing the needle by 

 torsion to its marked position. The square root of the number of 

 degrees of torsion required to effect this, measures the potential of the 

 Leyden charge. This result is called the reduced earth reading. 

 When the atmosphere inside the jar is kept sufficiently dry, — this 

 charge is retained from day to day with little loss ; not more, often, 

 than 1 per cent, in the 24 hours. 



In using the instrument the charging electrode (9) of the jar is 

 left untouched, with the aperture through which it projects closed 

 over it by the metal cap referred to above. The electrode (10) of the 

 cage, when an observation is to be made, is connected with the con- 

 ductor to be tested, and the needle is brought by torsion to its marked 

 position. The square root of the number of degrees of torsion now 

 required measures the difference of potentials between the conductor 

 tested and the interior coating of the Leyden jar. The excess, positive 

 or negative, of this result above the reduced earth reading, measures 

 the excess of the potential, positive or negative, of the conductor tested 

 above that of the earth ; or simply the potential of the conductor 

 tested, if we regard that of the earth as zero. 



III. The portable electrometer is constructed on the same elec- 

 trical principles as the house electrometer just described. The mode 

 of suspension of the needle is however essentially different; and a 

 varied plan of connection between the different electrical parts has 

 been consequently adopted as more convenient. In the portable 

 electrometer, the needle is firmly attached at right angles to the middle 

 of a fine platinum wire, tightly stretched in the axis of a brass tube 

 with apertures in its middle to allow the needle to project on the two 

 sides. One end of the platinum wire is rigidly connected with this 

 tube ; the other is attached to a graduated torsion head. The brass 

 tube carries two metal plates in suitable positions to repel tlie two. 



