Page:The American Cyclopædia (1879) Volume II.djvu/253

 BALANCE 233 from this that the centre of gravity must be under the point of support, and the sensitive- ness of the instrument depends to a great ex- tent on the distance between these two points. This derived degree of sensitiveness varies with the purposes for which balances are to be used. The most delicate balances are those used for physical and chemical investigation ; and in order to secure the greatest possible degree of sensitiveness the conditions are as follows: 1. The centre of gravity of the beam must lie as near as possible under the point of suspen- sion ; it is evident that when this centre of gravity is raised, the point m will be raised also, and the angle sent will become larger, which results in a greater deflection of the beam in case there is no proper equilibrium. Fine balances are provided with an upright rod above their point of suspension, on which a small weight may be screwed up or down, in order to raise or lower the centre of gravity, and so to increase or diminish the delicacy of the instrument. In fig. 1 this rod is represented below, which is only admissible when no great degree of sensitiveness is required, as in this case the centre of gravity is lowered too much. 2. The beam should be as long as compatible with strength. As the distance cd becomes greater in proportion to the length of the arms, any difference in the two weights with which the balance is charged will be the more perceptible the longer the arms are. 3. The beam should also be as light as compatible with strength ; the smaller the weight of the balance itself, the greater the influence of minute differences in the load will be to shift the position of the point d from the centre. Therefore the beams of chemical balances are made like an elongat- ed frame, with large openings between, on the same principle as the walking beams of steam engines are constructed. 4. The points of sus- pension of the two scales must be such that the line uniting them passes exactly through the point of support ; if this line passes under that point, the sensitiveness of the balance will diminish too much when the load is increased. This takes place in any case to a small degree, as no beam is so perfectly inelastic that a slight flexion will not take place under the maxi- mum load. 5. The distances of the points of suspension of the scales a and J from the centre c should be perfectly equal ; this is best verified by changing the weights in the two scales, when if the equilibrium remains un- changed their distances are equal. Some bal- ances have screw arrangements to correct small differences in this respect. In fig. 2 a chem- ical balance is represented as used, in a glass case, which serves to protect it not only from dust, but also against air currents which might prevent a truly sensitive balance from ever coming to rest, and thus make correct weigh- ings impossible. The turning point of the beam, in order to reduce the friction to the least amount, is a knife-edge or triangular prism of hardened steel passing at right angles through the beam, and resting when in use upon polished plates of agate (one each side of the beam), which are set exactly upon the same FIO. 2. Chemical Balance. . horizontal plane. This knife-edge is polished and brought to an angle of 30. The points of suspension are also knife-edges, one set across each extremity of the beam. Great care is required that the line connecting them shall be precisely at right angles with the line passing through the centres of motion and of gravity. The index or pointer is sometimes a long nee- dle, its line passing through the centre, and ex- tending either above or below the beam, or it is a needle extended from each extremity of the beam. In either case it vibrates with the motion of the beam over a graduated arc, and rests upon the zero point when the beam is horizontal. The degrees upon each side of the zero of the scale indicate, as the needle oscil- lates past them, the intermediate point at which this will stop, thus rendering it unnecessary to wait its coming to rest. In order to save the knife-edges from wear, the beam is made, in delicate balances, to rest when not in use upon a forked arm, and the pans upon the floor of the case in which the instrument stands. The agate surfaces, being lifted by means of a cam or lever, raise the beam off its supports and put it in action ; or the supports, by a similar contrivance, are let down from the beam, leaving it to rest upon the agate ; the pans in the latter case must always remain sus- pended. However perfectly a balance may be made, there is always great care to be exer- cised in its use. Errors are easily made in the estimation of the nice quantities it is used to determine. The sources of some are avoided by a simple and ingenious method of weighing suggested by Borda. The body to be weighed is exactly counterpoised, and then taken out of the pan and replaced by known weights, added till they produce the same effect. A false balance must by this method produce cor-