Cardkey is the name of the company that started an Industry and were based in Silicon Valley, California. They were the first company to develop and widely distribute "Electronic Access Control Systems". Once a division of Greer Hydraulics, Inc., they were bought out by Johnson Controls who are famous for manufacturing Air Conditioning Systems in January 1999. In the UK they had offices in Manchester and Reading and sold their systems to companies such as British Telecom, Shell and BP. The companies original readers used cards which were made from Barium Ferrite and worked by magnetically attracting and repelling locking/unlocking cores within the Reader Module mechanism. From there they were the first to develop Wiegand Cards and readers which were again magnetically based but were more reliable and did not require calibration as used in the Barium Ferrite readers.