Either a ruler or a caliper.
A caliper is more accurate.
a small cm ruler An effective method to measure a fingernail would be to use a sliding caliper
ruler filler caliper etc..
Ruler. Micrometers. Caliper.
A caliper, a ruler and a tape.
A diameter is measured by using a ruler. However, you also could use a digital caliper to measure the diameter of a circle.
You measure it. A ruler and a caliper are useful.
It depends how accurately you can read the scale. A ruler would only be accurate to 1 mm. If the caliper has a vernier scale it should be ten times better
It depends what they are measuring. A caliper does better with cylinders and things like that. A ruler does better with a flat distance between two marks.
No, but you could use a ruler instead.
The vernier caliper is more accurate that a plastic ruler. That's because the caliper is designed to be used and effectively read to measure small distances. And it does this better than a "regular" ruler because it has a vernier scale built into it. It's like an "amplified ruler" attached to the device that has increased accuracy. A link is provided to the Wikipedia article on the vernier caliper and to the one on the vernier scale. Check out the pictures and skim the details. You should be able to pick it up with a bit of thought.