What does a pet microchip actually do?
Considerably less than most owners assume — it is a permanent identity number, not a tracker, and it only works if the database entry behind it is current.
What it is. A glass-encased transponder about the size of a grain of rice, implanted under the skin between the shoulder blades by injection. It contains no battery.
How it works. A scanner emits a radio field; the chip draws power from that field and transmits back a unique number. That is the entirety of the interaction. The chip stores no name, address or medical history.
The number is then looked up in a database which holds the keeper's contact details. This is where the system succeeds or fails.
What it does not do:
It is not GPS. It cannot be located, tracked or pinged. It only works when an animal is physically in front of a scanner — typically at a vet, a rescue centre, or a local authority.
It does not update itself. A chip registered to an address you left four years ago returns an unreachable owner. This is the single most common reason microchipped animals are not reunited, and it is entirely preventable.
It does not prove legal ownership on its own, though it is strong evidence and is frequently treated as such in practice.
The legal position in the UK. Microchipping is compulsory for dogs, and has been extended to cats in England, with keepers required to keep the registered details up to date. Failure to comply can attract a fine.
Practical points:
Check your details annually. Most databases let you log in and confirm.
Update before you move, not after.
Chips can migrate slightly or, rarely, fail — ask your vet to scan at the annual check.
A collar tag is still worth having, because a member of the public who finds your animal cannot scan it. The two systems complement each other.