APPENDIX TO MEMOIE OF PELTIER, 181 



This concave surface is destined to receive the pivot of a needle formed of a 

 very fine copper wire. The needle is a decimetre in length, and is thus equal to 

 the radius of the dial. Its pivot is soldered to its posterior part ; consequently 

 it must be maintained in equilibrium by a small counterpoise of gum-lac. The 

 pivot is of tempered steel, and terminated by as fine a point as possible. The 

 copper wire which forms the needle is slightly curved, in order that its greatest 

 portion in length may be placed in contact' with the rod, and receive from it a 

 greater influence. To give a direction to it we place at the centre, fonning one 

 body with this rod and with the pivot, a very small wire of tempered steel, very 

 feebly magnetized, to which is imparted only the quantity of magnetism rigor- 

 ously necessary to draw the large needle near the horizontal rod. 



To obtain the maximum of sensibility, it is necessary that the movable needle 

 should not be of steel ; for however little magnetism might be given it or be 

 received by it, whether from its position in the magnetic meridian, or through 

 oxidation, it would act on the particles of iron contained in all the coppers of 

 commerce, and thus alter the great sensibility of this instrument. 



The electrometer is covered with a glass cylinder, the upper flat surface of 

 which presents another graduated circle, con-esponding to the lower circle. In 

 this way the visual ray passes by the same degrees in the two cu'cles, and thus 

 no error of parallax is to be apprehended. 



Peltier had further added a movable armature. This was a plate of copper, 

 of the length of the needle, and placed at the same height. It was worked by 

 means of a horizontal lever, situated below the foot-stand. This armature con- 

 siderably augmented the sensibility of the instrument ; unfortunately it some- 

 what embarrassed the phenomena, and the instrument lost in point of exactness; 

 hence Peltier seldom employed it. 



The manner of using this electrometer is very simple. The apparatus is 

 placed in the magnetic meridian, so that the needle may touch lightly the fixed 

 rod. The instrument being thus adjusted, we touch the exterior ball or the 

 i plate with the body charged with the electricity which we wish to measure, and 

 immediately the needle deviates by a certain number of degrees, which may be 

 read on the dial. Thus we have results perfectly comparable; the fixed rod and 

 the movable needle, always in metallic contact, maintain infallibly an equilibrium 

 of electricity. 



However light the indicating needles, their weight occasions a slight friction 

 on the concave plane which supports the pivot. This friction gives to the instru- 

 ment a small resistance which prevents it from obeying at the instant, when 

 very w^eak quantities of electricity are either added or witlldra^vn. To overcome 

 this resistance, it is sufiicient in general to strike lightly on the table which 

 supports the electrometer. 



For those who have an electrometer, but no torsion balance, it is necessary 

 that the electrometer should be capable of being transformed at will to a torsion 

 balance. Peltier added, therefore, to his electrometer different pieces, with a 

 view of rendering this transformation possible and easy. The pieces are the 

 following: 1°. A gallows formed of a foot of copper or ivory, screwed in the 

 pedestal ; of a vertical staff" of glass, 25 centimetres in length, and of a hori- 

 zontal bridge of copper, the free extremity of which corresponds exactly to the 

 centre of the dial-plate. 2°. An apparatus destined to carry the wire and pos- 

 sessing two very distinct movements — one horizontal and circular, the other ver- 

 tical and rectilinear. 3°. A silver wire of the utmost fineness, terminated at its 

 lower extremity by a small cylinder of gum-lac, having beneath it two small 

 copper hooks designed to carry the needle. 



In order to transform the electrometer into a torsion balance, it is enough to 

 lower the wire by means of the vertical and rectilinear movement above indi- 

 cated, to seize the needle with the two hooks, and again sufficiently raise the 

 whole ; by this means, in effect, the indicating needle, instead of being sup- 



