While working at the Copenhagen Telephone Company, Valdemar Poulsen invented the telegraphone. The device recorded human speech by alternating magnetization of a wire. Poulsen patented this first functional magnetic audio recorder in 1898. He became interested in magnetic recording because he was frustrated that telephone callers could not leave a message when the party at the other end was not available. At the Paris Exposition of 1900, the telegraphone won the gold medal. This exposure encouraged Poulsen and Soren Lemvig Fog to set up a Danish Corporation and laboratory to manufacture telegraphone machines.

His invention was never a commercial success, and his business partnerships and company went bankrupt. The development of the telegraphone was plagued with conflicts over money, research direction and problems of technology transfer. Many felt his invention came too early, and it was doomed, since without an amplifier, magnetically recorded voices were hard to hear. But the magnetic recording technology he developed laid the groundwork for the audio, video and computer storage industries. Magnetic storage is necessary for personal computers, the Internet, digital recordings, walkmans and other consumer electronic products.

Poulsen also is credited with helping make low-wave radio broadcasting possible by 1920. In 1903 he invented an arc generator for use with wireless telegraphy.