Sound recording and reproduction
In the late 1930s the tape recorder appeared. Tape recorders use a magnetic tape as a medium, with a recording head to store the sound on the tape. Sound waves are converted into an electrical signal inside the recorder. This signal goes into the head, where it changes the polarity of very small magnets. Tape moves past the head at a constant speed, and its magnetic particles are rearranged by these magnets to a pattern that represents the sound wave. These magnetic patterns are much like the small grooves of a cylinder or disc record in the way that they represent the vibration energy of sound waves.