A Magnavox permanent magnet loudspeaker from a 1933 advertisement in an electronics magazine. This was just before the development of alnico magnets (which started in 1931) and previous speakers required electromagnets to create strong enough fields for adequate volume. Stronger magnetic alloys allowed this speaker to use a permanent magnet, but requires a long horseshoe shape to produce the required field. The development of alnico in World War 2 allowed a magnet only an inch long to produce the same field, and led to a new generation of compact loudspeakers in the postwar era.