As you spin the wooden rod, the rotation of the coils through the field of the magnet causes the magnetic flux through the coils to change in a periodic manner, thus inducing a fluctuating EMF in them, which moves a fluctuating current through them. The ends of the coils are connected to one pair of LEDs via a pair of commutators and brushes, for DC output, and to a second pair of LEDs by a pair of slip rings and brushes, for AC output. If you spin the rod in one direction (counterclockwise, facing the handle end), the green LED on the DC output flashes with each half-turn, while the red one remains dark, and on the AC output, the green LED and red LED flash on alternate half-turns. Spin the rod in the opposite direction, and the red LED on the DC output flashes while the green LED remains dark, and the LEDs on the AC output flash on alternate half-turns.

Demonstrations 72.03 – EMF induced by moving magnet, and 72.06 – EMF induced by moving conductor, illustrate the production of a current in a coil via the induction of an electromotive force by the changing of the magnetic flux through the coil, which we can express by the following equation: E = -NdΦ_B/dt, where E is the induced EMF, N is the number of turns in the coil, and the derivative is the change in magnetic flux with time. This is the physical principle that forms the basis for generators that produce electric power. This demonstration provides an illustration of how such equipment works. Fixed to the wooden rod, between the pole faces of a permanent magnet, is a pair of coils (wound in series). The north pole of the magnet is at the rear, and the south pole is at the front, so the field points from the rear towards the front. The coils are wound in the clockwise direction. That is, when the coil is vertical, if we start with the wire on the bottom of the shaft and follow it through the coil, it enters, goes around in a clockwise direction and exits at the top. As you rotate the coils in the magnetic field, they go through a cycle during which at one position, the magnetic flux is perpendicular to the plane of the coils, so that there is maximum flux through the coils, to where it is parallel to the coils and the flux through the coils is zero, then perpendicular again, with the flux at maximum in the opposite direction to that in the first maximum, then parallel again, with the flux at zero, then perpendicular again, as at the start. As a result, the EMF induced in the coils, and thus the current produced by it, fluctuates in a sinusoidal manner during the cycle. Exactly how this manifests itself in the output of the generator, however, depends on how we make the connections to tap the current produced in the coils.

The wires at the ends of the coils run along the length of the rod, at positions 180 degrees apart, in the plane between the two coils (so parallel to the plane of the coils). Where the pair of brushes at left in the photograph touches the rod, two commutators, one connected to each end of the coils, provide contact with the brushes. The commutators are conductors that sit across from each other on the shaft, and which span just less than 180 degrees each. If we imagine the rod rotating towards us, so counterclockwise as we look from the handle end towards the magnet (top of the coils moving towards us), then when the coils are just past vertical, the top commutator (let’s call it a) is just coming into contact with the front brush, and the bottom commutator (we’ll call it b) is just coming into contact with the rear brush. During the first quarter turn, the flux is going from maximum to zero, which means that the magnetic field associated with the current set up by the induced EMF must reinforce the external field , which is implied by the minus sign in the equation above (see demonstration 72.09 – Lenz’s law), and the current must flow in a counterclockwise direction, so from a to b. (See demonstration 68.13 – Right-hand rule model.) In the next quarter turn, the flux increases from zero to maximum, and the current must now flow in such a direction as to cancel the external magnetic field, now clockwise, but now the coils are facing in the opposite direction, so the current still flows from a to b. The rear brush is thus positive with respect to the front brush, and current flows through a diode whose anode is connected to the rear brush and whose cathode is connected to the front brush, in this case, the green LED. At the end of the first half-turn of the rod, the commutators have now switched places. Starting into the next half turn, the original electrical contacts are broken, and, again, the commutator on top comes into contact with the front brush, the bottom commutator comes into contact with the rear brush, and the cycle starts again. Thus, for every counterclockwise half-turn, we obtain a fluctuating current that flows out of the rear brush and back into the front brush.

If we reverse the direction in which we rotate the shaft, the first quarter-turn begins when the bottom commutator begins contact with the front brush, and the top commutator begins contact with the rear brush. Again, during the first quarter turn, the flux decreases from maximum to zero, and again, to reinforce the external magnetic field, the current must flow in a counterclockwise direction from a to b, but this time, commutator a is touching the rear brush, and commutator b is touching the front brush. Through the next quarter-turn, the flux again increases from zero to maximum, and again, to cancel the external magnetic field, the current must flow in a clockwise direction, but, again, the coil is now flipped, and the current again flows from a to b, but, as noted above, commutator a is now touching the rear brush, and commutator b is touching the front brush. So now the front brush is positive with respect to the rear brush, and current flows in the opposite direction to that in the first example, and the same thing happens during the next half-turn. Now, the front brush is positive with respect to the rear brush, and current flows through a diode whose anode is connected to the front brush, and whose cathode is connected to the rear brush, in this case, the red LED.

The pair of brushes at right in the photograph make contact with the coils via a pair of slip rings. These are rings that afford a conducting surface over the entire circumference of the rod, one connected to each end of the coils. Thus the front and rear brushes are always connected to the same ends of the coils; the electrical contact is never broken. This means that on every half-turn of the shaft, the current reverses, because the orientation of the coils is 180 degrees to that for the previous half-turn. Thus, on alternate half-turns of the shaft, the rear brush is positive with respect to the front, and then the front brush is positive with respect to the rear, and diodes connected in opposite directions across the brushes flash on alternate half-turns of the shaft. To see this clearly, we can continue either of the two analyses above, for a second half-turn. Let’s say that the left-hand brush is connected to wire a through its slip ring, and the right-hand brush is connected to b. For rotation of the shaft in either direction, we saw that in both the first and second quarter-turns, current flowed from a to b, so the right-hand brush should be positive with respect to the left-hand brush. Now, at the start of the second half-turn, the magnetic flux again begins to decrease, so to counter this decrease, the magnetic field due to the induced current must be in the same direction as the external field, and the current must flow in a counterclockwise direction. Now, however, a is on the bottom and b is on the top, so this means that the current flows from b to a. In the last quarter turn, the flux again increases, so the magnetic field due to the induced current must cancel the external field, and the current must flow in the clockwise direction. a, however, is now on top, and b is on the bottom, so the current again flows from b to a. So during the second half-turn, the left-hand brush is positive with respect to the right-hand brush. Thus, the green and red LEDs, which are connected in opposite directions between the two brushes, flash on alternate half-cycles. The analysis is, as before, similar for clockwise rotation of the shaft.

The maximum change in flux occurs when the coils are near their horizontal position, where they are most closely aligned with the magnetic field. The minimum change in flux occurs when the coils are near the vertical position. The transition through the vertical position is where the current falls to zero and rises again, for DC current, or where it falls through zero and changes direction (or rises through zero and changes direction) for AC current.

As we can see from the foregoing analysis, what determines the direction of the current made by this generator are the direction of the external magnetic field and the positions of the connections relative to the orientation of the coils. (If we rotated the commutators 180 degrees from their current position, keeping the connections the same, then the red LED on the DC output would light for counterclockwise rotation of the shaft, and the green LED would light for clockwise rotation.) We can also see that the faster we spin the coils, the greater the change in flux with time, the greater the induced EMF, and the greater the current produced in the coils. You can show this fairly clearly by spinning the shaft at different speeds. The faster you spin the shaft, the more brightly the LEDs glow.