An electric fence causes a psychological barrier that keeps livestock in and wild animals out. A pulsating electric current is sent along the fence wire, from an energizer which is grounded. When the animal touches the fence it completes an electrical circuit between the fence wire and the ground. When this circuit is completed the animal will receive a short, sharp but safe shock. The shock is sufficiently memorable that the animal never forgets and knows not to go close to the fence. Because the electric fence is a psychological barrier, it does not require great strength to be effective. However, it must be well designed and constructed to absorb some pressure from animals, snow and wind. The energizer must have enough power for the length of the fence and for the animals being controlled.