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An electric fence is a barrier that uses painful or even lethal high-voltage electric shocks to deter animals or people from crossing a boundary. The concept of the electric fence was first described in Mark Twain's A Connecticut Yankee in King Arthur's Court, in 1889, as a defensive weapon. Electric fences were used to control stock in the United States in the early 1930s, and developed further in both the United States and New Zealand. Electric fences have improved significantly since the early days. Improvements include: Polyethylene insulators, improvements in electrical design of the charger, and changes in laws.