Homemade Scales for Weighing Things

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Homemade Scales for Weighing Things


 
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    Oftentimes one finds that something needs to be weighed (massed), and there is no balance or scales available. Not to worry! A scales that is usually good enough for most measurements is readily made from a pencil (F), ruler, two light cardboard pans (P), and coins (C). The mechanics all follows the law of levers. The following is for a scales useful for small weights or masses.

    How to set up your scales. (By the way, the word is plural because there are two "pans" on it. Besides using the plural, diminishes the confusion when you speak of a scale or a dial.)

  1. Find a FLAT, STIFF ruler that has inches and/or metric inscribed on it
  2. Find some light cardboard such as is attached to the backs of writing tablets. Cut out TWO squares that are about 3 inches on a side.
  3. Using drops of glue placed very near the ends of the ruler on the side with the numbers, affix the two squares you've cut out. Allow the glue to dry.
  4. Place the ruler upon a pencil such that the pencil is at the very MIDDLE of the ruler (using the numbers).
  5. As the cardboard squares might not be of equal weight, trim the heavier one such that the ruler will balance at it mid-point. You now have a scales.

    Now for your known weights against you will weigh or mass things. Coins are quite regular when newly manufactured. So obtain a few that are not more than 5 years old and are not damaged or very worn. Here are the weights of the most common coins in the USA:

dime = 2.2 g; penny = 2.5 g; nickel = 5.0 g; quarter = 5.7 g
(For the weights of other nations' coins, you can ask a dealer in collector's coins.)


Using Your Scales

    Suppose you need to weigh out 4 grams of a powder (notice: only 1 significant figure!) And suppose that your scales is constructed from a 12-inch ruler.

  1. First you must do some math: you need to know where to place a coin or coins on the ruler.
    1. Say that you select as your weight the nickel, which is 5.0 grams.
    2. Using the formula under the picture you determine that you should place your nickel exactly ___ from the fulcrum,
      X = (4 g x 6 inches)/5 g = 24/5 = 4.8 inches
    3. Place the nickel 4.8 inches away from the fulcrum, which is at the 6-inch mark
    4. At this point in time, the nickel will cause the ruler to tip its direction.
    5. Onto the MIDDLE of the cardboard square, slowly spoon on (THOU SHALT NOT DUMP!) small amounts of your powder until the ruler just tips lifting the nickel's side of the ruler.
    6. You will have 4 grams of powder on the square.

    There are other TRICKS you could discover for yourself, but why waste time. Here are three. (A) It is often easier to measure volumes than weights, so adjust your volume so that, in the above example, you could use 4.4 grams. Then all you would have had to do is add two dimes to the center of one cardboard square and balance with 4.4 grams of powder. (Again: your directions said that you needed make 100 ml that contained 4 grams of your powder and then you were going to use 50 ml of that solution. It's just as good to make 110 ml containing 4.4 g and use 50 of it.)

    (B) When making up reagents for quantitative analysis, remember to think! Every reagent is in excess except for ONE. Know your chemistry of the analysis. Carefully measure out THE critical reagent, and then make sure you have added enough of the others, which can often measured out rather crudely.

    (C) When measuring out components for growing microbes in petri plates or flasks, you can be extremely free. Do you weigh out your meat and vegetables to the nearest 0.01 gram? No! Bacteria and fungi are much less fastidious than you are! This author's students once determined how much freedom there was in making growth media for E.coli. They found they could add three times more than the recipe called for and the bacteria would grow just as well. And if they added only a tenth of what was called for the bacteria grew faster! The most critical component of petri plate making is usually the amount of agar - you don't want either mush or concrete. The bacteria don't care, but you do!


For even smaller weighings

  In principle this wire model is the same as that of the larger "ruler model," above. Here are a few of its salient points.

  1. The end of the wire is bent downward and then horizontal again so that the overall center of gravity of the scales will be lower than the fulcrum (where the thread [red] is tied) to the wire. This will prevent the scales from tipping over and dumping what you have placed on the "pan."
  2. You may wish the pan to be made of metal foil or plastic so that it will not absorb drops of liquids (if you are to be weighing liquids such as drops of water from the teflon experiment).
  3. With the pan empty, you shall first want to determine the balance or equivalency point of the scales by sliding the thread's attachment point to and fro until the wire hangs horizontally. Once you have found this point, you will want to use a knife blade to scratch a circle around the wire to prevent the thread's slipping to and fro. (If your notching was imprecise, trim small bits from [or add very small drops of glue to] the "pan" until the scales will become horizontal.)
  4. You shall want to have a "V" of wire to be your sliding weight that you will add to the long arm and be able to move it along the wire until it balances the mass you have added to the pan. You will want to know the mass of this slider. A jeweler should be able to give you its weight.
    THOU SHALT NOT LOSE THIS SLIDER!
  5. As with the ruler-scales, you will move the slider until it will balance the unknown's mass. In similar fashion the inverse proportions of distance will be made.
  6. A more precise wire-scales can be made IF you have a hanging "pan" rather than a fixed one. In a hanging pan, the placement of the sample doesn't matter as the pan's hanging point on the wire will be fixed. With a fixed pan it is important that you place your samples exactly in the center of the pan.


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