Loudspeaker
Reis's speaker worked by magnetostriction. In his first receiver he wound a coil of wire around an iron knitting needle and rested the needle against the "F" hole of a violin. As current passed through the needle, the iron shrank and a click was formed. The image shown below is a more advanced version where the iron bar is clamped to a cigar-box-shaped resonator. This receiver is very insensitive. It produces weak sound but has good fidelity. It requires very high current and is a current-sensitive device rather than a voltage-sensitive device.
Reis was marginally successful. This instrument could transmit continuous musical tones but produced indistinct speech. In 1865, however, British scientist David E. Hughes used Reis' telephone with "good results".
Read more about this topic: Reis Telephone