A credit card that only works when it hears its owner's voice has been developed by US scientists. Researchers hope that the device, which comes with a built-in voice recognition chip and microphone, will be a weapon in the battle against credit card fraud. Even if thieves know a card's password and personal identification number, they will still have to copy the owner's voice accurately. The trial card was created by scientists at Beecard in California US. The first version is three times as thick as a normal credit card, but researchers believe smaller chips will allow the card to slim down to a more conventional size. The card is apparently the first to put a voice recognition chip, a microphone, speaker and battery into a credit card. To use the card, the owner first presses a button and hears the prompt "Say your password". If the password is correct and spoken by the right person, the card emits an identification signal which is processed by a computer connected to the Internet. Researchers hope to get the card to handle ten transactions per day for two years before its non-replaceable battery runs out.