The FILAMENT-BEARING BALANCE JACK DE MENT 1907 South East 48th Avenue, Portland, Oregon
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HE balance is regarded by many as the most valuable instrument in the scientific laboratory. While balances and other forms of apparatus for weighing substances are present in most laboratories, the use of the delicate fine-edged bearing type of balance is often restricted to those laboratories where finances are not of prime importance. There is an indication that in many cases the price of a weighing machine is proportionate to the sensitivity of that machine. The substitution or construction of a special type of very sensitive and yet economical form of bearing will in many cases enhance the value of the ordinary chemical balance in the laboratory where the use of the delicate analytical balance is prohibitive due to price. Also, the use of this different typeof bearing will often merit its use in so-termed "sensitive" balances; that is, an ordinary knife-edge bearing balance may be converted into a semimicro balance by use of thefilament-bearing principle. It is obvious that in most cases the less contact between moving parts of a balance, with a subsequent minimum of friction, the more sensitive the machine will be. Using this idea, it is possible for the instrument maker or mechanically minded laboratory worker to modify balance bearings in the following manner: It was found that extremely delicate balances could be constructed by the use of long balsa wood beams suspended on bearings made of a single human bair strung through the eye of a needle! These experimental balances were surprisingly sensitive. Fractions of a milligram could be weighed with ease. Th$ human bair, needle-eye type of bearing was very strong, for in most cases the use of maximum loads (20-40 grams) did not impair its efficiency or affect its sensitivity. One such balance, using a human hair for a bearing, has been in use for over three years. The same hair has been suitable for moderate use during this period. While in many cases the use of the wood beam, human hair, needle-eye type of bearing might be warranted, it is suggested that either wire net or thin sheets or tubes of very light metal be used for beams. In many cases the use of aluminum and its alloys, as well as alloys of beryllium and others, bas provided very suitable beams. However, the ordinary laboratory balance as con-
strutted commercially will usually be appropriate for conversion to the filament type of bearing. The bearing and its construction will be more or less a t the discretion of the worker but the general principle of using the beam suspended upon a tense filament is utilized. Choice of the material for the fiber involves consideration of type and intensity of usage. Human hairs will often suffice in machines where work is of the semimicro type. Care must be used with hairs or other fibers of an organic origin, due to the fact that gases and volatile solids and liquids tend to act upon the fiber with a subsequent weakening or rupture. Metal filaments (wires) are perhaps most suitable in that hardness, high tensile strength, and small size are the desirable features which may be incorporated in such a filament. For ordinary purposes the use of steel music wires of 00000 to 00000000 (0.0100" to 0.0083") gage will generally be suitable. It is obvious that the type of balance as well as the use to which the filament bearing is to be put is the determining factor in the construction of filament-bearing balances. Factors of sensitivity will obviously vary but sensitivity may be regulated by tension upon the supporting filament. The bearing rest may be of seyeial types. The use of a needle eye fix& to the beam with the fiber strung through i t a t right angles constitutes a very practical arrangement. However, sensitivity is often increased by the use of another wire strung on the beam and resting a t right angles to the supporting wire and parallel to the beam. In using a filament bearing for increasing sensitivity of an ordinary balance, or perhaps in the construction of a new type of micro balance, several factors might be considered as fairly basic: 1. Mechanical effects of use upon the bearing. Soft materials or highly ductile, brittle, elastic, or weak substances are obviously unsuitable. 2. Size of the filaments and the area of contact, as well as load and tension of the filament, will often determine or sometimes restrict the sensitivity. 3. Chemical action upon the filament bearing should be reduced to a minimum by use of desiccants and of inactive materials for construction.
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