2(M D. W. Shea — Calibration of au Electrometer. 



Art. XVII. — Calibration of an Electrometer / by D. W. Shea. 



The mathematical theory * of the quadrant electrometer 

 leads to the general formula : 



9=a (A-B) (C—J (A + B), 



in which 6 represents the moment of the couple which turns 

 the needle, A, B, C, the potentials of the two pairs of quad- 

 rants and of the needle respectively, and a is a constant which 

 defines the sensibility of the instrument. The deflection of 

 the needle is proportional to the moment 0. Hence, if a is a 

 constant f as the theory supposes, the deflection of the needle 

 should be proportional to the product 



(A-B) (C-i(A+B), 



and the curve of calibration obtained in any given method of 

 setting up the electrometer should have a perfectly definite and 

 constant form. But in the various forms of the quadrant elec- 

 trometer, and in the different methods of setting up the same 

 instrument, the curves of calibration obtained correspond in a 

 very irregular manner with the curves given by theory.;}; 



Some observations with an electrometer of the Mascart form, 

 which show variations apparently due to change in the sensi- 

 bility with variation in the temperature,§ are given in the fol- 

 lowing pages. It is possible that they may be of interest to 

 those who use this form of electrometer. 



The results here given were obtained with the electrometer 

 set up in the following manner : 



The needle was suspended by a bifilar suspension consisting 

 of a single fiber of cocoon silk, the ends of which were fastened 

 to the drum and the loop to the hook on the upper end of the 

 rod carrying the needle. The two parts of the fiber were sep- 

 arated as widely as the construction of the instrument admits 

 of. The needle was charged by a water-battery. The positive 

 pole of the battery was attached to the electrode connecting 

 with the inner coating of the Leyden jar, and the negative pole 

 and the electrometer case were to ground. The water-battery 

 was made up of four hundred cells of zinc-copper elements, ar- 

 ranged in boxes of eighty cells each. The difference of poten- 



* Maxwell, Electricity and Maguetism, vol. i, p. 311. 



f Hopkinson, Phil. Mag., V, xix, p. 297. 



X Mouton, Journal de Physique, vi, p. 13. 



Benoit, Journal de Physique, vi, p. 118. 



Boltzmann, Pogg. Ann., bd. cli, p. 487. 



Hallwachs, Wiedemann's Ann. der Phys. und Chem.. xxix, p. 35. 



§ Hallwachs, ibid., p. 41. 



