gets an electric shock of 250-milliwatts . The boundaries are drawn by GPS and exist only as a line on a computer. There are no wires or fixed transmitters at all. The cattle took less than an hour to learn to back off when they heard the warning noise. The scientists indicated that commercial units were up to 10 years away. Another type of invisible fence uses a buried wire that sends radio signals to activate shock collars worn by animals that are "fenced" in. The system works with three signals. The first is visual (white plastic flags spaced at intervals around the perimeter in the fenced-in area), the second is audible (the collar emits a sound when the animal wearing it approaches buried cable), and finally there's an electric shock to indicate they have reached the fence. Other invisible fences are wireless. Rather than using a buried wire, they emit a radio signal from a central unit, and activate when the animal travels beyond a certain radius from the unit. See also Animal rights Brain implant Cruelty to animals Microbotics Necrobotics Optogenetics Surveillance tools References External links Animal testing Animal welfare Biocybernetics Bioethics Biology experiments Pest control Radio control Remote control Working animals Articles containing video clips