Being able to generate a voltage by moving a conduc- tor through a magnetic field is extremely useful as it provides us with an easy way of generating electric- ity. Unfortunately, moving a wire at a constant linear velocity through a uniform magnetic field presents us with a practical problem simply because the mechani- cal power that can be derived from an aircraft engine is available in rotary (rather than linear) form!
The solution to this problem is that of using the rotary power available from the engine (via a suitable gearbox and transmission) to rotate a conductor shaped into the form of loop as shown in Fig. 4.2. The loop is made to rotate inside a permanent magnetic field with opposite poles (N and S) on either side of the loop.
There now remains the problem of making contact with the loop as it rotates inside the magnetic field but this can be overcome by means of a pair of car- bon brushes and copper slip-rings. The brushes are spring loaded and held against the rotating slip-rings so that, at any time, there is a path for current to flow from the loop to the load to which it is connected.
The opposite sides of the loop consist of conduc- tors that move through the field. At 0° (with the loop vertical as shown at A in Fig. 4.4) the opposite sides of the loop will be moving in the same direction as the lines of flux. At that instant, the angle, θ, at which the field is cut is 0° and since the sine of 0° is 0 the generated voltage (from EBlvsinθ) will conse- quently also be zero.