In 1831, the great experimentalist Michael Faraday discovered that changes in a magnetic field could induce an electromotive force and current in a nearby circuit – a phenomenon known as electromagnetic induction.
French instrument maker Hippolyte Pixii harnessed these ideas in 1832 in the first practical mechanical generator of electrical current, shown below. Whereas earlier instruments, such as Faraday’s Motor and Barlow’s Wheel, turned electricity into motion, this “magneto-electric machine” was the first to accomplish the opposite trick: turn motion (provided by the person turning the crank) into electricity.
When you turn the hand crank on this device (use the slider to adjust the crank speed), a horseshoe magnet, its north and south poles facing upwards, turns around on a base to which it is attached. Due to the electromagnetic induction discussed above, the moving magnetic field (which moves from the north to the south pole of the magnet) that this turning causes a current to flow in the assemblage of wires positioned above it. These surges of electricity are recorded in the pictured galvanometer.
Due to the spinning of the magnet, the current generated is an alternating current (AC): It flows for a moment in one direction, then reverses course. This is reflected in the galvanometer needle, which jumps in one direction, then the opposite. Arrows on the wires also indicate the invisible current flow. Notice how the machine produces two surges of current for every full revolution of the magnet, but that the electricity from these two surges flows in opposite direction. The changing direction of the arrows denotes the change in current direction. Also, notice that these opposing directions are reflected in the galvanometer, which records the opposing surges, the needle swinging back and forth like a clock's pendulum.
Although alternating current delivers power to toasters and CD players across the world today, back then scientists preferred direct current (DC) for their applications. So a commutator was added to this device to convert the power from AC to DC, making this the first DC generator. Simply stated, the commutator flips a switch momentarily once every revolution, in order to reverse the direction of the current during that brief period. That way, the current generated by the motor always flows in the same direction. Click the DC commutator button, and observe the effect this has on the flow of current (the arrows depicting flow direction in the bottom part of the instrument no longer change direction) and how it is recorded on the galvanometer.
As you experiment with the crank speed, notice the difference in how the galvanometer responds. The faster the magnet turns, the more current is produced, as happens in electromagnetic induction.
Study the detailed images below depicting how electrical flow varies depending on whether or not the commutator is engaged. The red and blue areas depict current flow from the two coils.