How to Read Measuring Instruments in IGCSE Physics

Reading instruments correctly is tested directly in Paper 6 and indirectly in Papers 2 and 4. Errors in reading scales lose easy marks.

General method

1. Identify the quantity and unit shown on the scale.
2. Determine the value of each small division.
3. Read the scale at eye level (to avoid parallax error).
4. Record the reading to the precision of the instrument.

How to find the value of one small division

Count the number of small divisions between two labelled marks. Divide the difference between the labelled values by the number of divisions.

Example: A thermometer has marks at 20 and 30 degrees C, with 10 small divisions between them. Each small division = (30 - 20) / 10 = 1 degree C.

Rulers

Read from the nearest mm. State readings as e.g., “23.5 cm” or “235 mm”. Always read at eye level, perpendicular to the ruler.

Measuring cylinders

Read the bottom of the meniscus (the curved liquid surface). Position your eye level with the liquid surface. Record to the nearest half division.

Thermometers

Read to the nearest division. Mercury or alcohol thermometers have a small time lag; wait for the reading to stabilise before recording.

Ammeters and voltmeters

Note the range and sensitivity. An ammeter reading 0 to 5 A with 50 divisions gives 0.1 A per division. A micro-ammeter may read in milliamps. Always check whether the meter reads in A, mA or micro-A.

Digital meters

Read the displayed value directly. Note that digital meters may fluctuate; record the steadiest value or an average.

Zero error

Before using any instrument, check whether it reads zero when nothing is being measured. If it does not, record the zero error and subtract it from every reading.

Parallax error

When a pointer sits above the scale (as in analogue meters), viewing from an angle gives a false reading. Always read directly from the front, perpendicular to the scale.