Ampere (A)
The ampere explained: how it is defined, what one amp actually represents, the full prefix scale from picoamps up, and typical currents you will meet.
Questions people ask
How much current is dangerous?
Around 1 mA is perceptible, 10 mA makes muscles contract so you cannot let go, and 30 mA across the chest can be fatal. It takes very little.
Why is the amp a base unit rather than the coulomb?
Historically because current was far easier to measure accurately than charge. The 2019 redefinition fixed the elementary charge instead, but the amp kept its base-unit status.
Does this page work offline?
Yes, after the first visit. The service worker caches it, so the reference stays available with no connection.
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Last updated 22 August 2026