Making a microbalance

Class practical

Students can make an amazing instrument which is able to measure the mass of a hair or a grain of sand.

Apparatus and Materials

For each student group

• Wooden block (4 cm x 4 cm x 2 cm)
• Wooden strip (15 cm x 2 cm, for example, a medical tongue depressor)
• Elastic bands (5 cm), 2
• Needle (fine)
• Metal screw (1.2 cm)
• Aluminium support (see Technical notes)
• Drinking straw (waxed paper or plastic)
• Graph paper - at least 100 sheets
• Scissors
• Sensitive laboratory balance

Health & Safety and Technical Notes

Take care with the needles. Issue them in a little plastic box so they don't become lost.

Read our standard health & safety guidance

The aluminium support is made from a bent sheet of aluminium. An alternative support could be made from two plastic credit-card-sized cards, glued to a wooden block.

The screw should be the correct size to screw into the end of the drinking straw.

Have plenty of spare straws available.

A pile of graph paper can be used. It needs to have no margins on the paper and to have 10 x 10 large squares marked on it and each large square divided up into 10 x 10 smaller squares. The pile of graph paper should have a mass of 1 kilogram and this may be conveniently near to 1000 sheets. If this is not possible, then suitable values should be used in order to make the arithmetic easier, all the time discussing order of magnitude calculations.

Once the mass of 1000 sheets is known then the mass of one sheet can be calculated, the mass of one big square and the mass of one small square. This is 10 ^-7 of the original mass of the pile of paper. Grains of sand and short hairs have a mass in this range. So the vertical scale can now be calibrated by using 1, 2, 3. etc., small squares. Less mathematically capable students can measure the mass in terms of squares of paper or be told the real mass once they have begun to grasp the principle of finding the mass of many sheets in order to calculate the mass of a small square.

NB: Paper changes in mass depending on the moisture content.

Procedure

1. Fix the wooden strip to the wood block as illustrated using the elastic bands twisted twice round the block.
2. Insert the small metal screw in one end of the drinking straw.
3. Cut away the other end of the straw with scissors so that it acts both as a pointer and as a little scoop into which items to be weighed can be placed.
4. Find the approximate centre of gravity by balancing the straw on the needle. Then push the needle through the straw to act as the rolling axle. It should be put through just above the long axis of the straw.
5. Use standard masses (paper squares) to calibrate the microbalance. Mark the zero position of the pointer high up on the vertical scale. Put the 1 mm² weight into the scoop and mark the pointer position. Continue adding weights until a uniform scale is constructed.
6. Other things that might be weighed include a flake of mica, a small piece of thread, a small piece of iron wire before and after rusting, a drop of olive oil, and the loss in mass as a drop of ether evaporates.
7. Order of magnitude calculations can be fun. Once the mass of a grain of sand is known estimate and then measure the mass of a beaker of sand.

Teaching Notes

• This amazing instrument is able to measure the mass of a hair or a grain of sand. It is sensitive, even if not overly precise, but it will give excellent order of magnitude values if it is built carefully.
• It is helpful for you to have made a microbalance before the lesson so that students are able to suggest improvements when making their own. Their smaller fingers may find the task easier than you do.
• Once students have a microbalance they will want to know where the weights are. Offer a small piece of paper and students will realize that they do not know its mass. The ensuing discussion may lead to using a known mass of paper, which can be subdivided. Mark the zero of the scale on a piece of paper attached to the upright wooden strip.
• Students could make a microbalance at home. Use a cutaway matchbox as the support, a straw and suitable screw and any suitable scale rigged up from a piece of card.
• The position of the needle (how high up on the cross-section of the straw) determines the sensitivity of the balance, but students can be left to find this out for themselves. However, some students may need help. When help is given, take away the right straw you have set up and give the student a new straw for a fresh attempt.
• Some students will be able to give the mass of the grain of sand in grams and others only in the number of (mm) squares.

This experiment was safety-tested in July 2007