180 APPENDIX TO MEMOIR OF PELTIER. 



to assure liimself tliat iluring tlic whole time of the experiment the total loss 

 should be very small, and consequently the difference still smaller. When the 

 experiment, however, lasts a long time, and in damp weather, w'e cannot neglect 

 this difference in the electric state of the balls, since it involves a considerable 

 one in the results. On the other hand, there are many experiments in which it 

 is proposed to measure the successive addition or subtraction of the electric 

 forces, which cannot be done with an instrument of which the active parts are 

 insulated one from the other. 



Peltier corrected these defects l)y applying to the torsion balance two import- 

 ant modifications. In the first place he established a permanent communication 

 between the movable disk and the fixed liall ; secondly, he soldered this last to 

 a metallic rod which projects laterally, and whicli, after having left the ball, is 

 bent vertically in order to receive the condensing plates or any other apparatus. 

 The following is briefly a description of this instrument : 



The torsion balance as modified by Peltier presents at its upper part a micro- 

 meter, like that of Coulomb's balance. A cocoon thread, devoid of torsion, is 

 attached to the windlass of this micrometer by its upper extremity, and bears at 

 its lower end a metallic needle, terminated on one hand by a proof plane, and 

 on the other by a balance weight of gum-lac. The needle has on its lower face, 

 and in the line of prolongation of the cocoon thread, a point of platina descending 

 vertically. This point is immersed in a ca])sule of glass, into which has been 

 previously introduced diluted sulphuric acid. The capsule is carefully surrounded 

 with resin, and rests on a plate of copper. This plate may be raised or lower by 

 means of a Ijent lever, whose leg is situated without and passes underneath the 

 footstand of the instrument. The fixed ball, as has been already said, is sol- 

 dered to a copper rod which projects laterally through the glass case whicli 

 covers the instrument, and then rises vertically, so as to receive the condensing 

 plates or other apparatus. A fine wire proceeds from the rod which supports 

 the fixed ball, and directs itself towards the capsule filled with sulphuric acid ; 

 having reached this capsule the wire is bent at a right angle, and descends into 

 the acid. When the instrument is to be used the capsule is elevated by means 

 of the bent lever; when the experiment is finished it is lowered, and the point of 

 platina and the wire are no longer immersed in the acid. 



Peltier placed, moreover, two graduated circles, one on the footstand of the 

 instrument, and the other on the upper plane of the casing. These two circles 

 correspond ; consequently, if the visual ray is made to pass by the same degrees 

 in the two circles, the deviation of the needle may be read without the possi- 

 bility of error. 



Those who are a little conversant with electricity will readily comprehend the 

 object and advantages of the arrangements hero indicated. By means of these 

 modifications, in effect, the tension remains perfectly equal between the fixed 

 ball and the proof plane, even when the experiments last some time, or when the 

 electricity which is to be measured is either augmented or diminished. 



Electrometer. — We pass now to a description of the electrometer of Peltier.* 

 On a socle or footstand, three decimetres in diameter, is pasted a dial-plate of 

 pasteboard, graduat(^d to 360 degrees ; at five centimetres above the centre of 

 this dial is the extremity of a rod of copper having a section of seven millime- 

 tres. This rod is slightly curved, and bends back almost at a right angle when it 

 arrives above the zero. It then penetrates vertically into the footstand from which 

 it is insulated by resin ; it is there again bent round so as to proceed laterally ; 

 then, at a distance of a few centimetres, it rises vertically to receive the plates 

 of Volta or any other apparatus. On the inner extremity of this rod, just above 

 the centre of the dial, is soldered a small plate of tempered steel, polished and 

 slightly concave. 



* Annates de Cliunie et dc Physique, t. 62, p. 422. 



