Passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming radio frequency signal provides just enough power for the tag to transmit a response. Due to limited power and cost, the response of a passive RFID tag is brief — typically just an ID number (GUID). Lack of an onboard power supply means that the device can be quite small: commercially available products exist that can be embedded under the skin. As of 2005, the smallest such devices commercially available measured 0.4 mm 0.4 mm, which is thinner than a sheet of paper; such devices are practically invisible. Passive tags have practical read distances ranging from about 10 mm up to about 6 metres.
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