Blondel's theorem, named after its discoverer, French electrical engineer André Blondel, states that, in a system of N electrical conductors, N-1 electrical meter or wattmeter elements, when properly connected, will measure the electrical power or energy taken. The connection must be such that all potential coils have a common tie to the conductor in which there is no current coil. For example, a 3 phase 3 wire system needs only two watt meters, but a 3 phase 4 wire system (with a neutral) needs three. [1] The theorem is based on a paper delivered by Blondel in 1893 at the International Electric Congress in Chicago.