'Duode' 8-inch loudspeaker unit, 1920s.
© Science Museum / Science & Society Picture Library
Loudspeakers were developed so that several people at once could listen to radio sets. The loudspeaker has a strong permanent magnet shaped like a cylinder. A coil of wire fits between the poles of the magnet within the strong magnetic field, and is attached to the cardboard cone. The varying electric currents pas through the coil making it move to and fro. As the coil moves, the cone moves, making sound waves that correspond to the varying electric currents.