In yesterday’s post covering the invention of the vacuum tube and its principals of operation we learned that small crystals were used as the basis for radio receivers and that inventor Lee De Forest came up with an alternative to the crystals, the triode tube which others later turned into the triode vacuum tube. This was the birth of audio as we know it.
I promised yesterday to tell you more about the crystals that people were using for radios and how years later those mysterious crystals would again play a major role in everything we do today. Here we go.
Around the turn of the century experimenters discovered that certain types of natural crystals would act like a radio receiver for the new fangled Marconi wireless radio signals. No one understood why they worked, they just did. What’s was cool about these crystals was they required no power to operate – no battery, no AC, nothing. All you needed to make a crystal radio was some wire, a crystal and a set of tiny headphones and with a little luck and fooling around you could get in on the new radio era.
Soon commercial versions of the crystal radios became available and within 20 years of their introduction there were millions of crystal radio sets around the world enjoying the new commercial radio broadcasts of the day. What killed the crystal radio was the new Audion tube receiver we learned about yesterday.
So we have two interesting paths: the crystal radio path that started in about 1902 and ran its course through about 1920 and the Audion and later Triode tube radios that started about the same time but didn’t begin their decline until the 1950′s when the transistor was first introduced. Coincidently the transistor’s popularity and subsequent public awareness came about because a fledgeling upstart company, Texas Instruments, needed a mass market appeal product that used the new transistor with enough volume so they could lower manufacturing costs. That turned out to be the transistor radio which also gave another fledgeling upstart company its birth when Akio Morita and Masaru Ibuka decided to take a chance on the new device and start a company called Sony.
These disparate yet related paths started with the mysterious crystal that no one understood. Experimenters had known for years that if you placed a tiny whisker of wire on the surface of a crystal, connected the wire and ground through a coil of wire, you could receive radio signals. The crudest implementation of this weird phenomena required users to carefully place the whisker on just the right spot on the crystal to get the loudest sound through the headphones. These were known as “cat whisker” receivers and sometimes took hours and hours of experimenting to find just the right spot on the crystal.
Enter three physicists: John Bardeen, Walter Brattain and William Shockley. Bardeen and Brattain were traditional physicists who worked for the new Bell laboratories and had been charged with trying to figure out a way to get rid of the vacuum tube in the telephone system. Vacuum tubes, by the late 1940′s were used through the phone system as amplifiers and mechanical relays were used as switches. The nationwide phone system worked – barely – but making long distance calls was something of an ordeal employing thousands of inefficient vacuum tubes and failing relays.
Bardeen and Brattain were intrigued with crystals and were focusing their research on trying to figure out how they worked and if it might be possible to use them as a replacement for vacuum tubes. They had figured out by this time that the reason crystals worked as radio receivers was their curious ability to pass current only in one direction. Radio signals, which are AC, need to be turned into DC so you can operate a headphone. By passing the radio signal through a crystal you cut off half of the AC and what remains is a type of moving DC that goes back and fourth to the music on the radio wave. The two researchers were trying to see if they could get more controlled results using the cat whisker and control more current than what naturally occurred in these crystals.
William Shockley was what we might have called a hippie had this history been written in the 1960′s. By all accounts a wild and crazy guy, completely full of himself in every respect and went to work at Bell Labs with an idea that also involved crystals. Instead of using the cat whisker to physically touch the crystal, Shockley wanted to make a grid of metal (remember the vacuum tube grid from yesterday?) and induce a current “in the air” that could control the crystal without any physical contact. He made numerous attempts at this, all of which failed during this phase of the research. Unknown to Shockley at the time physicist Julius Edgar Lilienfeld and later Oskar Heil (yes, of the Heil Air Motion transformer loudspeaker fame from ESS) had already patented such a device called a Field Effect Transistor (FET) although they never actually made anything that worked.
Bell Labs put the three physicists together in a team headed by Shockley (much to the horror of the other two) and they plowed through several years of work which culminated in the discovery of the solid state diode and soon after the first transistor. Shockley tried to nab credit for the invention and was first to submit it as a field effect device – only to then discover the older patents. Panicked and discouraged Shockley went home and in a stroke of pure genius, figured out what is known today as the junction transistor consisting of three elements (just like the vacuum tube), the base, the emitter and the collector. You’ll recall that in the vacuum tube we had the grid, the anode and the cathode – which loosely translates to the same thing in the transistor.
In 1956 Bardeen, Brattain and Shockley were awarded the Nobel prize for their discovery of the point contact transistor and I can safely suggest the world has and will never be the same. If you want to enjoy some terrific reading on the subject by a writer far more gifted than I read Crystal Fire the birth of the information age by Michael Riordan – a great storyteller.
The transistor can be thought of like the tube with its two current flowing elements (anode and cathode in a tube – emitter and collector in a transistor) and the middle “grid” or “base” that is controlling the flow (like a valve) of the current – only, unlike the vacuum tube, the transistor is always off (remember the tube is always on).
To picture a transistor working we take a battery with one end the + and the other end the -. Place the + end of the battery on the transistor’s collector and the – on the transistors emitter. When you do this in a transistor, nothing happens – no current flows through the battery because the transistor is always off. If the same model were used to explain the tube, current would flow immediately because the tube is always on.
If you take a little bit of the + voltage and apply it to the base of the transistor, a current starts to flow between the emitter and the collector. If we again attach a phono cartridge between the – of the battery and the base of the transistor, then when you play a record on the turntable the small voltage generated by the phono cartridge will make a big voltage flow between the emitter and the collector – should you have your loudspeaker attached at that point you will get music.
Tomorrow: The fundamental differences between tubes and transistors