Whenever a wire cuts through a magnetic field, a potential difference (voltage) appears at the ends of the wire. If the wire is connected to a device such as a lamp or speaker, a current flows and the lamp lights or the speaker cone moves. As the crank is turned, a current is generated in pulses as the magnets pass the coils. If turned slowly, the pulses are evident in the in and out motion of the speaker. When turned rapidly, the lamps glow with a nice even glow, averaging the effect of the rapid pulses.