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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.)
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.
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.
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