Every once in a while, we come across some historical information that reminds us about just how far we have come in such a short period of time; I was cleaning out my office and found a paper I had written years ago based on an old technical manual covering the history and development of the telephone. Within that book, I had discovered some interesting information related to the history of how Alexander Graham Bell had developed the first audio voice transmission.
The book related that the forerunner of the telephone was the telegraph developed by Samuel F. Morse in 1838. This device used electricity to transmit a signal across a set of conductors to a remote location. In effect, a conductor was strung from one location to another with a battery connected through a keying device to this conductor. By interrupting the circuit or making and breaking the circuit with the keying device, signals of the same make-and-breaks were heard at a sounding device at the remote location. Morse developed a code for the various letters of the alphabet, permitting communications over a long distance.
In 1854, a Frenchman by the name of Charles Bourseul theorized that if signals could be transmitted over wire—as done by Morse code—the same should be able to be done with sound. If a person was to speak near a movable disk and the disk vibrated at the same frequency as the voice, the disk could be designed to make and break an electrical circuit similar to the one for the telegraph. At the remote end would be another disk that would vibrate at the voice frequency, but instead of unintelligible electrical impulses, the person’s voice would be heard. Even though Bourseul was not successful in his efforts, his theory paved the way for others.
Just seven years later, Philip Reis, following the experiments of Bourseul, successfully built a transmitter and receiver that would transmit musical tones across electrical conductors. His transmitter consisted of a thin membrane or diaphragm mounted across the opening of a box. The diaphragm had a thin copper strip connected to a platinum disk located in the center of the diaphragm. Almost touching the platinum disk was a platinum contact point fixed to a stationary piece of copper fastened to the box and connected to one end of a battery. A mouthpiece was cut into the box on the other side and, as the sound from the musical instruments entered the box, the sound waves caused the diaphragm and plate contact to make-and-break at the same frequency as the musical instrument.
The receiver of Reis’s device consisted of a metal knitting needle wrapped with insulated wire coils that were connected to the battery’s other side and to the interconnecting wires from the transmitter. The make-and-break from the transmitter caused a varying current flow through the coiled wires around the knitting needle, causing needle magnetization at the pulse current rate. The needle was mounted on a box with an opening over the needle’s location. The box operated as a sounding board for the sounds produced by the magnetized and demagnetized needle, resulting in the fundamental frequency and pitch of the musical instrument. Very crude but successful.
In the 1870s, Alexander Graham Bell used a slightly more complex diaphragm and receiving device than Reis but had the same general concept. Bell’s transmitter consisted of an electromagnet mounted in a horizontal position in front of the vibrating diaphragm. Bell had a slightly movable armature connected to the shaft of the electromagnet, so the diaphragm moved back and forth as the current increased and decreased.
The receiver was a similar device with the electromagnet mounted vertically in a soft iron cylinder. The cylinder was slightly sprung away from the top and could vibrate at the same frequency as the increased and decreased current caused by the sound wave at the transmitter. However, when he was trying to recreate the Reis experiment, Bell placed the contact points much closer than Reis had done. This slight deviation permitted voices to be reproduced; the adjustment error was just enough to permit recreation of the voice modulation.
The scope of Article 800 in the 2005 National Electrical Code (NEC) covers telephone, telegraph, outside wiring for fire and burglar alarms and similar central station systems, as well as telephone systems not connected to central station systems. There is a change proposed in Article 800 covering communications circuits for the 2008 NEC that will simplify the scope of this article and it states, “This article covers communications circuits and equipment.” This is not an error but will help modernize the article. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at email@example.com.