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
