How do we measure the weight of a substance?
Weight of a substance cannot be measured with a common balance. For this purpose we use spring balance. It consists of a spring with a metal hook attached to its lower end from which a weight can be suspended. A pointer is provided with the spring, which slides over a graduated scale given on the casing. When an article is suspended from the hook, the spring elongates in proportion to its weight. We can determine the weight of the body by watching the position of the pointer on the scale.
In modern days, a special type of balance is used in big shops, laboratories, pediatric clinics etc. This machine has a pan, fitted on a base block. A clock-like dial is fitted at one side of the base block. The dial is graduated in gram and milligram scale. When any object is placed on the pan it gives a downward pressure to compress a spring and the indicator shows the proper weight of that object. Nowadays digital weighing machines are also in use in many shops.
Collect a lid of a jar. Make three holes at equal distance along the edge of the lid. Tie three threads of equal length at three holes. Make a knot taking the three free ends of threads.
Now tie a rubber band at the knot. Take a thin wire and insert its one end into the knot. Keep the other end of the thin wire extended freely like a pointer. Now place a wooden scale on a table with its one end extended from the edge of it. Place one or two heavy books on the other side of the scale. Now stick a white card board from the edge of the table just below the scale. Hang the pan from the scale with the rubber band. Find the position of the pointer over the card board. Mark it with a pen and write 101 beside it. Now if a one rupee coin is placed on the pan the rubber band will elongate and the thin wire pointer will go down. Note the position and mark it as 1’ on the card board. Put a few more similar coins one after another and mark the positions of the pointer each time in similar way. Now the balance is ready to measure weight.
If you like to measure the weight of a few marbles, put those items on the pan. Note the position of the pointer, If it points towards the mark 15 then the weight of those marbles will be equal to that of five one rupee coins.
If the rubber band is very hard it will hardly stretch by the small weight of a coin. So use soft and highly elastic rubber band.
This balance works like a spring balance. But instead of a spring a rubber band is used here. So you can call it a rubber band balance.