The accompanying drawing, from Roscoe, represents a common form of chemical balance.
The apparatus consists of a perforated brass beam (AA), vibrating about its centre, at which is fixed a triangular knife-edge of agate (C); this rests upon a horizontal agate plane attached to the upright brass pillar. To each end of the beam light brass pans (BB) are hung, each pan being suspended by an agate plane, upon an agate knife-edge fixed on the end of the beam at DD. This arrangement is rendered necessary in order to reduce as much as possible the friction of the edges on their supports, which friction, if unchecked, would seriously impair the sensibility of the balance.
In order to prevent the agate edges being worn away by constantly rubbing on the agate planes, the beam and the ends (DD) are supported by the brass arm (EE), when the balance is not in use, so that the agate surfaces are not in contact. The beam and pans are released when required by turning the handle (F). The movement of the brass arm (EE) is accompanied by means of a rod descending through the upright brass pillar, and resting on a simple eccentric, by the turning of which by the handle (F) it may be gradually raised or lowered. The substance to be weighed (held by a tube, watch glass, &c.) is placed in one of the pans, and weights added one by one in the other, until the beam is in equilibrium: this is ascertained by the long pointer(G) oscillating to an equal distance on each side of the central mark or index, this latter being subdivided into equal spaces, so that the oscillations can be measured. A spirit level is also a necessary appendage to the instrument, since it enables the operator to place the beam on an exactly horizontal level.
The beam of the balance is generally graduated into decimals. This saves the trouble of placing a weight on the scale, since it enables the operator to weigh the milligramme and its fractions by suspending a centigramme rider or hook on or between the indicated points of a graduated line.
The balance is enclosed in a glass case, which serves not only to protect it from dust, but to allow of the weighing being carried on away from the outer air, in which the prevalence of draughts proves a source of considerable annoyance to the weigher. The front of the scale case generally consists of three parts, viz. a fixed centre piece and two lateral frames or doors, all of course of glass. It is of importance that the air inside the balance case should be perfectly dry, since a humid atmosphere would not only affect the weight of many hygroscopic substances when placed in the pans, but likewise be liable to attack the instrument itself. To guard against these casualties a small beaker containing oil of vitriol, or chloride of calcium, or freshly-burnt lime, should be kept in the
case. A balance capable of weighing 70 or 80 grammes in each scale will be found to meet the needs of most chemists.
Fresenius says, “The ACCURACY of a balance depends upon the following conditions:
“a. The fulcrum must be placed above the centre of gravity of the beam.
“b. The suspension points of the scales must be on an exact level with the fulcrum.