Voice Over Wire: Imagine That
Come gather 'round, little ones, as Old Mr. Steve blathers on about the history of wire. We left off in the April 7 issue just as the telephone was being invented.
On June 25, 1876, Emperor Dom Pedro of Brazil listened to the receiver of an early telephone at the Philadelphia Centennial Exposition. "My word! It talks!" he exclaimed.
But you had to alternately talk and listen to the same device, a difficult task. Eventually, by 1878, Bell provided an identical device to both talk and listen. Also into this fray entered Thomas Alva Edison (1847-1931), one of the most prolific inventors ever, with 1,082 patents at the time of his death.
Bell had invented the moving coil "earpiece," while Edison had invented the carbon microphone. Each tried to re-invent the other half until they realized that they needed to join forces. In 1886 they did so. Francis Blake Jr. (1850-1913) added an improvement in the carbon microphone and the modern telephone was born.
A wall-box was the first telephone produced by Western Electric and set the style for telephones for the next 50 years, with an adjustable mouthpiece and the "salt shaker" earpiece.
Meanwhile, in England, the Post Office, which was in charge of all "communications," got wind of this new-fangled invention. William Preece, head of the Post Office, was called before Parliament and questioned about this new invention. He is reported to have said, "The Americans have need of the telephone because of the great distances between cities. We British, however, have plenty of messenger boys."
A Western Union memo of the same period was equally unflattering: "The telephone has too many shortcomings to be seriously considered as a means of communication. The device is inherently of no value to us."
In 1877, Bell married and went on an extended honeymoon, leaving his assistant, Thomas Watson (1854-1912), to continue work on the telephone. He developed a reproduction device in 1882 that used a balanced armature moved by an electromagnet, the same mechanism used for early speaker designs a quarter-century later.
Now here, boys and girls, I run into an interesting sidelight of history. I have been told that Eli Whitney, inventor of the cotton gin and of modern manufacturing (where pieces are made to be interchangeable), also invented a machine to weave cotton around a bare wire.
Noise Rejection Improvement
|Parallel ||0 dB ||-|
|3 per foot ||23 dB ||23 dB |
|4 per foot ||37 dB ||14 dB|
|6 per foot ||41 dB ||4 dB |
|12 per foot ||43 dB ||2 dB |
The fact that this machine was invented is without doubt. And the timing (in the 1870s) would have been about the right time. But not even the Eli Whitney Museum could corroborate this story. So if you, dear reader, know who invented the fabric wire braider, I would love to hear.
These machines still exist, and every manufacturer who makes wire and cable has dozens, even thousands, of them. Their original design was to make ... shoelaces!
A bare wire run through the machine was covered with cotton. Fancier insulated wire was covered with silk, which made a tighter, stronger covering and was the insulated wire of choice well into the next century.
You may recall that silk-covered wire was the choice of Joseph Henry, when he made the first true inductors in 1830.
By 1883, Horace Lamb (1849-1934) and Oliver Heaviside (1850-1925) noted that, as frequencies or "oscillations" increase on a wire, the signal tends to move to the surface of the wire. The concept of skin effect was born.
By 1888, Heinrich Hertz (1857-1894) showed that the propagation of signals down wires moved at the same speed as signals through air. These signal through the air he described as "electromagnetic radiation." He also showed that signals though air can best be received when the receiving circuit is "resonant" with the originating circuit.
What other miracles could he have discovered if he had not died of blood poisoning at the age of 37?
The unit of frequency, the Hertz, was named for him. If you remember "cycles per second," consider yourself a geezer.
We are, of course, set for wireless transmission of all sorts. As this is the story of wire and cable, we will try valiantly to ignore all that "wireless" stuff.
Many of today's common wire configurations came about due to research, and some luck, in the 1880s and '90s.
When telephones were attached to telegraph systems, in which one wire was on a pole and the other conductor was "ground," the noise on the phone was unbearable. Modern engineers (that's you, dear reader) often wondered where this noise came from. There was no radio, no television, almost no man-made sources of electrical noise.
A clue came when someone noticed that the noise dropped at night. Yes, the source of noise was the sun! But how could one get rid of that?
Do the twist
This is where researchers for the Bell System, eventually to be called Bell Labs, started.
They noticed something curious: when the pair of wires used to run a telephone were twisted together, the noise went down. When the wires in a pair were untwisted, run in parallel or otherwise separated, the noise went up.
We can characterize this noise reduction as seen in the accompanying table, although the term "decibel" used here was not used until 1929, another chapter in our story.
As you can see twisting the wires at all makes a huge difference. More and more twisting buys you less and less. From six per foot (lay length of 2 inches) to 12 per foot (lay length of 1 inch) gets you a measly 2 dB for all your work. More twists mean you use up more wire, so lots of twists can be expensive. Besides, how much noise rejection does a telephone need? 30 dB? So most of the phone cable was twisted by-guess and by-golly and worked just fine, giving 20-30 dB of noise rejection.
We'll revisit this subject, lay length or twists-per-inch, when twisted pairs come back into play as data cables in the 1980s and '90s. Even now, more than a century later, our table still applies. And super-tight twisting of pairs does not result in the "best" twisted pairs. But we will save that for later.
Of course, this is only the beginning of twisted pairs. More amazing twists and turns lie ahead. Don't change that dial!