G1 Measurement and Multimeter Skills

1. Choose the right instrument

Start with the quantity you need, not with the apparatus on the bench. Then check that the instrument has a suitable range. A measuring tape covers a 2 m bench in one go. A 30 cm ruler would have to be moved along seven times, and each move adds another chance of error.

2. Take a trustworthy reading

Follow the same short routine whenever you measure.

1. Check.Is the instrument undamaged, reading zero and able to cover the expected value?
2. Position.Place the instrument correctly and put your eye level with the reading.
3. Read.Record all the digits a digital display shows, or read a scale to the nearest marked division.
4. Record.Write the value and its unit straight away. A bare number such as “12” is incomplete.
5. Repeat.Repeat readings that may vary, such as times. A very different result should be checked, not silently deleted.

Read the scale straight on

If you look at a scale from above, below or one side, the mark you are reading seems to move against the scale, and you read the wrong value. This is parallax error. Put your eye level with the point you are reading and look straight at it.

First work out what one small division is worth. The labelled marks are 10 cm³ apart with five spaces between them, so one small division is 10 ÷ 5 = 2 cm³. Water curves up at the walls of the cylinder; this curved surface is the meniscus. Read its bottom: it is three small divisions above 30, so the volume is 30 + 3 × 2 = 36 cm³. Looking down from above would make the level seem higher than it is.

A laboratory thermometer is read the same way. Keep the bulb fully in the liquid, away from the bottom and sides of the container, stir gently and wait until the liquid thread stops moving. Then read the end of the thread with your eye level with it. Do not lift the thermometer out to read it: the bulb starts to cool or warm as soon as it leaves the liquid.

Check the zero

Before you measure, check what the instrument shows when it is measuring nothing: the balance with an empty pan, the spring balance hanging with no load. If it does not show zero, it has a zero error, and every reading it gives is wrong by that amount.

• On an electronic balance, press the zero (tare) button with the pan empty.
• On a spring balance, adjust the pointer to the zero mark before you hang the load.
• The end of a ruler is often worn, so line the object up with a clear mark instead of the end, then subtract.

Here the rod starts at the 2.0 cm mark and ends at the 7.6 cm mark, so its length is 7.6 − 2.0 = 5.6 cm. Reading only the right-hand end would give 7.6 cm, which is 2.0 cm too long.

If you cannot reset an instrument, correct each reading: corrected reading = reading − zero reading. A balance that shows 0.5 g with an empty pan and 42.3 g with a stone on it gives the stone’s mass as 42.3 − 0.5 = 41.8 g.

Time an interval

A digital stopwatch displays hundredths of a second, but you start and stop it by hand. Your reaction time, about 0.2 s, means each press can come a little early or late. Hand-timed readings are therefore recorded to 0.1 s.

• Use clear start and stop events. For example, start when the front of a trolley crosses the start line and stop when the same point crosses the finish line.
• Time a long enough interval. An error of 0.2 s is a quarter of a 0.8 s interval but only a fortieth of an 8.0 s interval. Use a longer track so the time is several seconds.
• Repeat and average. Early and late presses vary from trial to trial, so the mean of several timings is more trustworthy than any one of them.

3. Determine average speed

Average speed describes how much distance is covered per unit time over the whole measured journey.

average speed=total distancetotal time

If distance is measured in metres and time in seconds, average speed is measured in metres per second, written m/s.

A simple practical method

1. Mark a start and finish line and measure the distance between them with a measuring tape.
2. Release the object in the same way for each trial. Do not give it an extra push unless that is part of the method.
3. Start and stop the stopwatch as the same point on the object crosses each line.
4. Repeat the timing. Keep the distance fixed, find the mean time, then calculate the average speed.

4. Use a multimeter safely

A multimeter can measure several electrical quantities, but you must change both its setting and sometimes its connection. Use only the low-voltage circuits provided by your teacher.

Choose the range and record the unit

Each setting has several ranges. The number on the dial is the largest value that range can show, and it tells you the unit of the display. For example, a meter may offer 200 mA and 10 A for current, and 2 V and 20 V for voltage.

• Start on a high range if you do not know the expected value. If the reading fits a lower range, switch down to see more digits.
• A value too big for the range makes the display show “OL” (on some meters, a single “1”). Switch to a higher range.
• Large currents often need the red lead moved to a separate socket, such as the 10 A socket. Check the sockets whenever you change quantity.
• Record the unit of the range with the number. On the 200 mA range, a display of 125.4 means 125.4 mA, which is 0.1254 A, because 1 A = 1000 mA.

5. Record and check results

A results table should make sense without a spoken explanation. Put the quantity and unit in each heading, keep the same number of decimal places down each column and include every repeat.

The three results are close, so they support an average speed of about 0.50 m/s.

A result that does not fit the pattern of the others is an anomaly. Look for a reason, such as a late stopwatch press, and repeat that trial. If the repeat agrees with the other results, leave the anomaly out of the mean and write why beside it. Keep it in the table.

7. Practise this