CIE A Level Physics复习笔记1.2.3 Measurement Techniques

• Common instruments used in Physics are:
  • Metre rules - to measure distance and length
  • Balances - to measure mass
  • Protractors - to measure angles
  • Stopwatches - to measure time
  • Ammeters - to measure current
  • Voltmeters - to measure potential difference
• More complicated instruments such as the micrometer screw gauge and Vernier calipers can be used to more accurately measure length

• When using measuring instruments like these you need to ensure that you are fully aware of what each division on a scale represents
  • This is known as the resolution
• The resolution is the smallest change in the physical quantity being measured that results in a change in the reading given by the measuring instrument
• The smaller the change that can be measured by the instrument, the greater the degree of resolution
• For example, a standard mercury thermometer has a resolution of 1°C whereas a typical digital thermometer will have a resolution of 0.1°C
  • The digital thermometer has a higher resolution than the mercury thermometer

Measuring Instruments Table

Micrometer Screw Gauge

• A micrometer, or a micrometer screw gauge, is a tool used for measuring small widths, thicknesses or diameters
  • For example, the diameter of a copper wire
• It has a resolution of 0.01 mm
• The micrometer is made up of two scales:
  • The main scale - this is on the sleeve (sometimes called the barrel)
  • The thimble scale - this is a rotating scale on the thimble

• The spindle and anvil are closed around the object being measured by rotating the ratchet
  • This should be tight enough so the object does not fall out but not so tight that is deformed
  • Never tighten the spindle using the barrel, only using the ratchet. This will reduce the chances of overtightening and zero errors
• The value measured from the micrometer is read where the thimble scale aligns with the main scale
  • This should always be recorded to 2 decimal places (eg. 1.40 mm not just 1.4 mm)

How to operate a micrometer

Vernier Calipers

• Vernier calipers are another distance measuring tool that uses a sliding vernier scale
  • They can also be used to measure diameters and thicknesses, just like the micrometer
  • However, they can also measure the length of small objects such as a screw or the depth of a hole
• Vernier calipers generally have a resolution of 0.1 mm, however, some are as small as 0.02 mm - 0.05 mm
• The calipers are made up of two scales:
  • The main scale
  • The vernier scale
• The two upper or lower jaws are clamped around the object
  • The sliding vernier scale will follow this and can be held in place using the locking screw
• The value measured from the caliper is read when the vernier scale aligns with the main scale
  • This should always be recorded to at least 1 decimal place (eg. 12.1 mm not just 12 mm)

The vernier caliper reading is read when the vernier scale aligns with the main scale