Analogue Apparatus & Interpolation (OCR AS Physics): Revision Note

Analogue Apparatus & Interpolation

Analogue Apparatus

• The type of analogue apparatus needed depends on the measurements to be made:

Analogue Apparatus Table

• A large disadvantage of analogue instruments is parallax error

•  This occurs when a scale reading is not read directly from the needle, but at an angle, therefore, seeming like a slight difference in reading

  • This can be fixed by a mirror behind the needle. When the needle and its mirror image are aligned, then the reading is correct

• Parallax error is minimised by reading the value on a scale only when the line of sight is perpendicular to the scale readings (ie. at eye level)

• Examples of where parallax error is common are:

  • Determining the volume of liquid

  • Making sure two objects are aligned

  • Reading the temperature from a thermometer

Reading the value of the needle head-on (left image) looks different to reading it from the right (right image). This is parallax error 

Interpolation

• Sometimes a pointer on an analogue meter can fall between two scale markings

  • In which case, interpolation between the scale markings is needed

• Interpolation is the processes of estimating unknown values that fall between known values

  • For example, if a straight line passes through two points of known value, the point midway between them can be estimated

• This is why calibration is very important

• For example, a vernier caliper includes the main scale with certain scale markings

  • To then read between these markings, a vernier scale is used for accurate interpolation between the smallest divisions on the main scale

  • This means a reading can be obtained to a greater number of decimal places

The length of the wire falls between two scale markings