with enough current to flip their discs; those powered on only the vertical or horizontal line see only 1/2 of the required force (as flux is proportional to current, which in turn is proportional to the voltage). Those on unpowered lines also do not flip. Typically, the driving scheme works its way from top to bottom, powering each horizontal line "on" and then powering the needed vertical lines to set up that row. The whole process takes a few seconds, during which time the sound of the discs being flipped over is quite distinctive. Other driving schemes use diodes to isolate non-driven solenoids, which allows only the discs whose state needs changing to be flipped. This uses less power and may be more robust. History The flip-disc display was developed by Kenyon Taylor at Ferranti-Packard at the request of Trans-Canada Air Lines (today's Air Canada). By the time the system had been patented in 1961, TCA had already lost interest and Ferranti's management didn't consider the project very interesting. The first big opportunity for this system came in 1961 when the Montreal Stock Exchange decided to modernize its method of displaying trading information. Ferranti-Packard and Westinghouse both bid on the project, Westinghouse using an electro-luminescent technology. Ferranti won the contract after demonstrating the system with a mock-up they built in a disused warehouse across the street from the exchange's new offices, using hand-painted dots moved by hand to show how the system would work. The dots were slowly replaced with operating modules as they became available. The $700,000 system () was beset by delays and technical problems, but once it became fully operational it was considered very reliable. The systems were relatively expensive because of their manual construction, typically completed by women who "sewed" the displays in a fashion very similar to the construction of magnetic-core memory. Worse, Ferranti signed maintenance contracts that were, by 1971, losing $12,000 a month. A re-organization of the engineering and maintenance department addressed the problems, and prices started to fall. By 1977 the system had won sales with half the world's major stock exchanges. As prices fell, they were soon found in wider roles, notably that of highway signs and information systems for public transport. In Europe and in the United States, vane displays based on the same technology became popular for displaying prices at gasoline stations. In 1974 Ferranti started a project to