According to Faraday's law of electromagnetic induction, when there is a change in the magnetic fluxΦ through a circuit (coil), an induced emf appears depending on the rate of change of magnetic flux: εind = ΔΦ/Δt. The magnetic flux Φ through the area S equals, by definition, Φ = B?S?cosα, Where B is the magnitude of the magnetic field, and α is the angle between B and the normal to the plane of the circuit. If a rectangular frame with area S rotates at uniform angular velocity making f turns per second in a uniform magnetic field B, the magnetic flux through the frame is a periodic function of time: Φ(t) = B?S?cos()2πft. Thus, a periodic voltage that equals εind = 2πfBSsin(2πft) Appears between the ends of the frame. The amplitude of this voltage is proportional to the angular velocity of the frame. This frame is a model of an alternating current generator. If the terminals of the frame are connected to a special collector device, a direct current generator is created.