A SENSITIVE STUDENT BALANCE C . C. KIPLINGER ..
West Liberty, West Virginia C
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N RECENT years the balance has become an important tool in the laboratory phase of first-year chemistry. The newer manuals emphasize the quantitative experimental evidence underlying the fundamental concepts of the subject, thus following the historical method in the laboratory development of the course. The problem of providing adequate facilities for this type of work is no easy one when viewed in the light of small budgets and the high cost of satisfactory balances. This problem in our institution forced me to resort to the "law of necessity" and invent a style of instrument which has demonstrated its merits during the past semester. I constructed five of these "junk balances," all of equal sensitivity, although no two are exactly alike in minor details, this indicating that a satisfactory balance of this type is not an accidental creation, but can be readily duplicated. The cost of material is negligible, the construction simple, and a minimum amount of time is required
for its completion. Next year we plan to have each student build one for his individual use and bring him in 6rst-hand contact with the fundamentals of balance construction and maintenance. An approximate list of materials includes a widemouth salt bottle of about five hundred grams capacity with a cork to fit; two short lengths of 6-mm. glass rod; two wire paper clips; four one and one-quarter inch slender shank screws; a stove bolt one and threesixteenths of an inch by two inches; two two-inch iron or brass strips; some sheet tin (from a coffee can); some linen thread or fishline; one one-fourth of an inch triangular file; one wire clothes hanger and a rectangular parallelepiped of light wood, 20 cm. X 4 cm. X 2 cm. for the beam. If carefully cut, the symmetry of the latter permits an easy determination of its geometric center. A bit of adhesive tape and a fine needle complete the requirements. The center of a side face of the wood bar is located.
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and a hole drilled just large enough to allow the file knife-edge to pass through. A three and one-half cm. length of the latter is broken off, and two faces ground smooth on carbomndum until a sharp edge is formed a t the junction of these faces. The hole in the bar is
replacements. In fact, the ease of replacement of wearing parts is one of the best features of this design. Our soft glass rods have been replaced but once during the past semester. Pyrex rods would be more durable. The pointer scale in the type favored by my students is a graduated card attached by paper clips to an inverted J-shaped piece of heavy wire with a closed loop on the shorter limb to limit the motion of the pointer. The longer limb of the J is forced through a cork in a bottle of suitable height. When arranged as shown in the photograph the loop-stop permits the removal or addition of objects with a minimum disturbance of the system. Pieces of wire or wire solder wrapped about
bushed with a piece of tin, the knife-edge inserted in position a t a right angle to the bar and one or two long wood screws screwed down from the top of the beam directly above the knife-edge so as to hold it firmly in position. By the use of a straight edge a hne is drawn the screw above the knife-edge serve to raise the center lengthwise of the beam tangent to the knife-edge and of gravity. In lien of these, two rectangular strips of parallel to the sides. This is done on each face, and metal are screwed by one end to the back face of the the lines are joined across the ends. Two slender wood beam a t equal distances from the knife-edge. The screws are screwed into the ends so that the rims of the latter arrangement is preferred as they both serve to heads extend about 0.5 mm. above the plane of the counterbalance as well as raise the center of gravity. The equality of beam-arm length is established by lines. A sharp notch is cut on the top of the screw head to a depth of 0.5 mm. while holding the file a t a screwing one pan support screw in or out and filing new notches until a satisfactory adjustment is obtained. A seventy-degree angle to the above plpe. A 20-cm. length of heavy wire cut from the coat pin or needle is attached to the pointer end to permit hanger is inserted in one end of the beam above the h e r readings. In the second type shown, the constructional differnotched screw and a long, heavy, screw in the other end ence lies chieflv in the use of a small bolt to hold the for a counterbalance. Pan stirrups are made of elongated C-shaped pieces knife-edge in position, as well as support the pointer of wire cut from wire paper clips. The pans are tri- and the wire-and-small-bolt device for altering the angular pieces of tin with turned-down edges and spun center of gravity. The beam is a short section of broom concavities in their centers, suspended by short lengths handle. Wire riders of suitable weight are used for the final adjustment. of linen thread or fishline. If care be taken to insure sharp knife-edges, glass Two short lengths of glass rod inserted vertically through the cork of the salt-bottle support comprise notches that do not bind the knives, knife-edge and pan the knife-edge supports of the balance. Their upper support notches in one plane and the center of gravity ends have been notched by soften in^ in a hot flame and of the system only slightly below this plane, the average immediately forcing the-plastic t&minals against a balance will show a sensitivity of from 1 mg. to 5 mg. With extra care in ninety-degree metallic edge, holding the rod a t a slight for a load of one to fifty . prams. angle to the edge so as to give a sloping edge. It is construction and adjustment and protection from air well to make several of these a t one time, as not all will currents they approach an analytical balance in sensibe equally good, and some can be held in reserve for tivity.
