Balance

Balance, an instrument for the estimation of mass. The most general form is that of a horizontal beam, supported at its centre, with scale-pans hanging symmetrically from each end. The instrument admits of very great refinement of detail. For instance, to ensure perfect freedom of motion the beam is supported by an agate knife-edge on an agate plane fixed to the central standard, and the scale-pans are similarly supported on agate planes at each end of the beam. With ordinary balances as in general use in laboratories, one milligram difference may be detected in a load of one kilogramme, i.e. one part in a million. The general conditions for the accuracy and delicacy of a balance are: (1) the beam should be horizontal when the pans are unloaded, a condition generally attained by a small screw adjustment; (2) the arms of the balance should be of equal length, otherwise a load at the end of the longer arm will counterpoise a heavier load at the other end. The error produced by this inequality may be removed by weighing the body in each pan separately, and then taking the square root of the product of the two weighings; thus, if the object counterpoise 3 grms. in one pan, and 3.1 in the other, its true mass will he square root of 3 x 3.l. Borda's method of double weighing also eliminates this error. If the body in one pan counterbalance a definite quantity of matter in the other pan, and if a weight w does also, then w is the weight of the body; this is the principle of the method; (3) the sensibility, i.e. the amount of turning of the beam for a given small difference in load, should be great. This requisite is very important, and to satisfy it the beam should be light, the arms as long as possible under the circumstances, and the centre of gravity of the beam should be close to the point of support. But this condition satisfied, the beam takes a long time to come to rest, oscillating slowly backwards and forwards to each side of the mean position. Hence a method has been devised of estimating the required mass by observation of the oscillations of the beam. This method of oscillation is invariably adopted in accurate work.