Induction
Inductance is a property of certain electrical components to store magnetic energy within a coil, or induce electric current within another coil.
When an electric current flows through a conductor a magnetic field is set up around that conductor. That field can be increased in intensity by winding a coil of the wire so the magnetic field passes through adjacent turns of wire in that coil. Increasing the number of turns, or winding the coil around an iron core increases the strength of that magnetic field. When a current is passed through one such coil, it will cause an electrical current
to flow in a second coil, if it is placed within the magnetic lines of force of the first.
The unit of Inductance is the henry, named after Joseph Henry.
Earth Ground
An Earth Ground is used to provide a return path for the flow of electricity in many applications.
Although at first thought, using the earth as a conductor does not seem to be very efficient, the earth is actually a very good conductor. Although the resistance to the flow of electricity in a shovel full of dirt may be very high, when you use the entire earth itself as a return path the resistance is quite low.
In calculating resistance in parallel, R total= 1/ ((1/r1) + (1/r2) + (1/r3) + ....(1/r etc...)) the answer is always less than the single lowest resistor in the circuit. If you were to calculate the parallel resistance of all of the shovel fulls of dirt between you and the other end of the earth ground circuit, you can see the resulting resistance would be very low.
One Way Conduction
"One Way Conduction" is called rectification. A Diode provides for rectification of an alternating current - changing it from alternating current to direct current.
Braun noted that two crystals making point to point contact had a high resistance to current flow in one direction, and a low resistance in the other direction. A single crystal in contact with a point of wire performed equally well. Materials such as galena, iron pyrite, fused silicon, and carborundum were used in detector circuits in the early years of radio.
The Fleming Valve was a vacuum tube Diode. In a partial vacuum (inside the glass envelope) it consisted of a wire filament heated by a direct current, and a metal plate mounted a short distance away. When heated, the filament emitted electrons which were attracted to the plate.
Coherer
The coherer is based upon the large resistance offered to the passage of electric current by loose metal filings, which decreases under the influence of radio frequency alternating current.
Early coherers consisted of a glass tube, two or three inches long with 2 metal electrodes placed a fraction of an inch apart. Within the tube metal filings surrounded the electrodes. In the presence of radio frequency energy the filings cling together - cohered - and allowed a current to pass. This current was used to actuate a telegraph sounder.
The problem of the filings continuing to cling together after the removal of the RF energy was solved by tapping the coherer with a small mallet attached to the sounder after the arrival of each signal.
Heterodyne
In a heterodyne circuit, two frequencies are mixed to create a sum and difference of the original two frequencies.
If a 100 kHz. frequency and 60 kHz. frequency are mixed - you will get two new frequencies of 160 kHz and 40 kHz. Each of the new frequencies has the characteristics of the original two frequencies.
Triode
The addition of a Grid to the Fleming Diode by Lee DeForest, creating the Triode, was his outstanding achievement. That simple 'grid' of wire between the plate and the filament allowed the tube to be used as an amplifier, and a far more sensitive detector. A very small signal placed on the grid could control a much larger current - the basis for amplification.
The early triodes contained residual gas - a symptom of the vacuum pumps of the time. These 'soft' tubes actually made more sensitive detectors than 'hard', or more perfect vacuum tubes developed later. On the downside, the soft tubes had shorter life, were more unstable, and were very noisy.
Regeneration
Armstrong's regenerative circuit was an outstanding development for radio. A portion of the radio frequency energy in the plate circuit of a triode detector was fed back into the input of the circuit - reinforcing the incoming signal. Armstrong recorded in his journal - "Great amplification obtained at once!" This circuit revolutionized radio of the day, and is one of the basic concepts in electronics.
Oscillator
One of the characteristics of the regenerative circuit was that as gain was increased beyond a certain point the circuit would begin to oscillate, or create a self sustaining feed back. Armstrong noted this, and realized the days of alternators and spark gap transmitters could be ended. The same circuit used to receive signals could also be used to send them!
In receivers of the day this oscillation was coupled into the receive antenna, and would begin to radiate signals in the area - interfering with other receivers. This problem prompted the development of new types of receivers.
Superheterodyne
Many engineers had struggled with the problems encountered in amplifying radio frequency signals above 200 kHz. Armstrong recognized that using a heterodyne method of reducing the frequency of the carrier would solve that problem.A Heterodyne circuit mixes two frequencies to obtain a sum and difference frequency with the modulation intact. Armstrong's circuit mixed the radio frequency with a local oscillator frequency to obtain a lower frequency - near 45 kHz. which was easy to amplify.
A-Battery, B-Battery, C-Battery
Battery operated sets needed several different voltages to operate.
The A battery was the filament battery - providing low voltage (3 to 6 volts common) to light the tube filaments. These batteries would last from 1 to 3 weeks, and could usually be recharged.
The B battery provided plate voltage for the tubes (60 to 130 volts common) and would last from two to six months. These could not be recharged.
The C battery provided Bias or negative voltage to the tubes, and added to the longevity of the B battery. The C battery had to be replaced every six to twelve months.
Super-regenerative
The super-regenerative receiver had a special circuit to prevent oscillation so the set could operate at maximum amplification. The circuit however provided very poor selectivity, and could not be used where many stations were on the air.AC Heaters
Consumers were anxious to get away from battery operation of radios - but until the invention of AC tubes the only option was the use of rather expensive battery eliminators. The addition of a cathode surrounding the filament isolated the AC current from the source of electrons in the tube, and eliminated many of the problems of using AC in radio circuits.
Screen Grid
The screen grid tube, or Tetrode is made by placing another electrode between the grid and the plate, and acts as a shield between them. Plate current in a screen grid tube is largely independent of plate voltage, and made possible much higher amplification than with a triode.Directional Antenna
A single radiator of an antenna radiates nearly equal amounts of RF energy in all directions. By placing longer and shorter elements - Reflectors and Directors near the Driven element, the signal can be made stronger in a specific direction.AVC
When radio receivers were designed to be sensitive to weak signals, they were too sensitive for strong local stations. A person tuning the radio would have one hand on the tuning dial, and another on the volume control to prevent a loud blast of sound when a local station was found.
The Automatic Volume Control, or AVC reduced the gain of the RF amplifiers when a strong station was tuned, and to some degree compensated for fading of signals.
Electrolytic Detector
As Fessenden worked to develop voice transmission, it became clear that the radio detectors available would not be suitable for his purpose. Although they worked fine for recieving the damped oscillations of spark transmitters, the 'coherer' and related types of detectors were not continuously operating -- essential for recieving audio signals. Fessenden developed what he called a 'Hot-Wire Barretter' that would work to recieve AM signals, even though it was not very sensitive.The invention of the Electrolytic Detector was one of those fortunate accidents that happen from time to time. As part of the process of creating the barretter, he left a filiment immersed in acid too long, until only a stub remained in contact with the acid. Fessenden noted that it was responding well to radio signals being generated nearby, and he had his new detector.
Leyden Jar
First described in 1745, a Leyden Jar was the earliest form of capacitor, or condensor.
A capacitor has the ability to store an electric charge.
The Leyden Jar consisted of a glass jar, an insulating stopper in the top of the jar, and a metal rod passing through the stopper and into an electrolyte of mercury, water or wine.
Short Wave
Describing 20 Megacycles as a 'Short-wave' isn't really very accurate. One wave length at 20 Megacycles would actually be close to 50 feet in length. But in comparison to 833 Khz -- one of the first frequencies assigned to broadcasting -- it was short. 833 khz has a wavelength of over 1,120 feet.
Your local FM station on 99.5 mhz has a wavelength of about 10 feet, and they may use a microwave link to their transmitter of 950 mhz with a wavelength of just under 1 foot.
These frequencies are still much longer in wavelength than many which are being used for radio communications all around us.
A.K. Time Capsule
At the 1929 dedication of his new factory, Arthur Atwater Kent placed a number of items inside a copper box, which was then soldered tightly shut and placed inside a hollowed out marble cornerstone. Among the items, five newspapers published that day in Philadelphia, a copy of the speech for the dedication, and an Atwater-Kent Model 55 radio - fresh off of the assembly line. The new factory was reportedly one of the largest plants in the world, and at its peak of production produced some 5000 sets per day.
The artifacts from the time capsule will be on display at the Department of Veterans Affairs in Philadelphia.
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