Here is an excerpt from "The Principles And Practice Of Modern House-Construction | by G. Lister Sutcliffe" published in 1900. The link given is to Part IV,Wiring and Lamps.
http://chestofbooks.com/architecture...ps-Part-4.html
So there you see a plug and socket arrangement in the UK as far back as 1900. It looks like a standard 5A plug. In 1904 Hubbell invented and manufactured the flat-blade plug that became the US standard.