On March 7, 1876, the United States Patent Office issued Alexander Graham Bell patent No. 174,465. Entitled "Improvement in Telegraphy," this patent came to be described as the most valuable patent ever issued. In 1912, the Franklin Institute recognized Bell's success in achieving the electrical transmission of articulate speech with the Elliot Cresson Medal.
Bell's telephone was able to electrically transmit articulate speech thanks to the collaboration of three main parts: the undulatory current, the electro-magnet, and the armature. In his patent, Bell explains that electrical undulations are created by "gradual changes of intensity exactly analogous to the changes in the density of air occasioned by simple pendulous vibrations."
Bell explains the way in which magnets are capable of producing an undulatory current, describing the interaction between a permanent magnet and an electro-magnet. A permanent magnet is a piece of magnetic material that retains its magnetism after it is removed from a magnetic field, while an electro-magnet is defined as a magnet consisting essentially of a coil of insulated wire wrapped around a soft iron core that is magnetized only when current flows through the wire. When a permanent magnet is caused to approach the pole of an electro-magnet, the permanent magnet induces a current of electricity in the coils of the electro-magnet. When the permanent magnet recedes, that action causes a new current of opposite polarity to appear on the wire. If you cause that permanent magnet to vibrate in front of the electro-magnet, it induces an undulatory current of electricity in the coils of the electro-magnet. How rapidly these undulations repeat corresponds to the rapidity of the vibrations of the magnet. Their polarity corresponds to the direction of the permanent magnet's motion, and their intensity corresponds to the amplitude of the magnet's vibration.
Undulations are caused by the vibration or motion of bodies capable of inducing action. In the case of the telephone, the voice is the capable body that induces undulations. Bell depicts a telephonic circuit in a drawing accompanying his patent, showing a circuit where one armature is across from another (see Figure 7 at right). Each armature is loosely attached at one extremity to an electro-magnet, and at the other to the center of a stretched membrane. A cone is used to converge sound vibrations upon the membrane. When a sound is uttered into the cone, that motion sets the membrane in vibration, and the vibration of the membrane in turn causes the armature to take part in the motion. The armature's motion then creates electrical undulations on the circuit. When represented graphically, these vibrations are similar in form to the initial vibrations caused by the sound that was made into the cone. A sound similar to that uttered into the cone is thus heard to proceed from the cone attached to the opposite end of the circuit.
This is the printed patent drawing illustrating Alexander Graham Bell's method of telegraphy, which is considered the first demonstrated usage of the system known as the telephone. Courtesy The National Archives and Records Administration, National Archives at College Park (College Park, MD)
Page 2. Courtesy The National Archives and Records Administration, National Archives at College Park (College Park, MD)