BALANCE. The instrument most com monly known by the term balance is a su perior sort of scales, executed with all the precision necessary for the nicest operations of physics, and particularly of chemistry.
A balance should be so sensible that, when equipoised, a very small additional weight in either scale may overcome the friction and adherence of the pivot by which it rests ; and the diminution of friction to the utmost pos sible extent is accomplished by givinn. the supports a high polish and attaching a giving edge pivot to each side of the beam. The knife-edges must not be so sharp as to cut the supports ; and, to prevent them from becoming too blunt, they are in some balances removed from the supports, when the instruments are pot in use, by an apparatus for the purpose.
The sensibility of a balance is estimated by the angular deviation of the beam from a horizontal position when a very small weight is placed in one scale : [thus, if one grain placed in a scale of each of two balances should make the beam of the first incline two degrees, and that of the second four degrees, the latter balance would be twice as sensible as the former. The quantities weighed in delicate
balances are usually small. A balance made by Ramsden for the Royal Society, weighing ten pounds altogether, turned with the ten millionth of that quantity, or with about the thousandth part of a grain.
A balance should be made as much as pos sible of brass. Steel and iron are apt to acquire magnetic properties. It should also be enclosed in a glass case, with doors for communication.
Mr. Cotton's beautiful apparatus, employed at the Bank for weighing sovereigns, is de scribed under GOLD-WEIGIIING MACHINE. The more ordinary balances for commercial purposes are noticed under STEELYARD; WEIGHING MACHINES.