The Birmingham age-herald., 1922-05-07
VACUUM TUBE
 AS AMPL|F|ER
 AND DETECT0R
When Used as a Detector It Acts Like Valve in Water System-Heart of
 Receiver

The genesis of the vacuum tube, which is such an important factor in radio receiving apparatus. was de- scribed in an article in this section last week In this story. 1t is my in- tention to describe just how the won derful instrument acts as a detector and amplifier. Before doing so, it might be well to point out one of the unusual possibilities of this instru- ment that is now engaging the atten- tion of scientists.
 Recently l was at demonstration of one of these tubes producing all ternating current of any desired fre- quency with total output of 10 kilo- watts of power. This means that in the very near future the vacuum tube will entirely discard the bulky and cumbersome machinery now used for the production of alternating cur rent.
 Feedback System Discovered
 In the last article. l described the development of the vacuum tube from the time that Edison first ob- plication by Professor Fleming; and finally, the addition of the grid by Dr. Lee DeForest. Up to this time we had a vacuum tube, but it's pos abilities were really unknown until Edwin H. Armstrong, young Amer ican amateur. discovered the feed back principles of transforming it from a simple vacuum tube into an electrical oscillator-a discovery which has given to it all of it's re- markable possibilities and made 1t really and truly the modern Alad- dins lamp.
 
 vacuum tube functions as detector Is as follows: As soon as the fila ment Is lighted train of electrons is thrown off from the filament and these lombard the plate. They are in reality, particles of negatively charged matter. which actually bom- bard the plate with terrific impact They afford path for negative electricity to pass between the fila ment and the plate and in order to assist the passage of this electricity, the plate is charged with a positive potential. The grid enables us to control the action of the tube for this purpose.
 When the incoming electro-mag- netic waves pass over the aerial, a very high frequency alternating cur rent Is induced in the oscillating cir cult connected with the aerial, which passes this current on to the vacuum tube. The vacuum tube will only all low the negative half of this alter hating current to pass through it. It completely checks the positive half and therefore acts as an electric valve in exactly the same way as the ordinary valve operates in water pipe system.
 PulsatinG Current Works Phone
 We have, therefore, on the other side of the vacuum tube. current of electricity that Is pulsating cur rent flowing in one direction only and the telephone receiver is now able to respond to 1t. That is the manner in which the tube acts as detector. As an amplifier, 8 slightly different condition exists and different action takes place. In this case, we have what Is known as trigger effect" and the action of the tube in amplifying can be best described by using the trigger effect as an analogy.
 If you study the mechanism of rifle you will realize that when you cock the gun you have to use a great deal of force to pull the hammer back against the spring in the gun. When you pull the hammer back, what you actually do is to store up SCl tafi duiOuHt Ol energy Tn the spring of the gun, which lies dor mant until such time as you bring it into play by, pulling the trigger A little thought will show you how minute the pressure is that you ex- ert on the trigger itself when firing the gun. put by releasing the trig- ger you bring into play the larger amount of energy that is stored up in the spring, and this forces the hammer against the cartridge in the gun with such force that the cart ridge is exploded. This is exactly what the vacuum tube does--it con trols the larger amount of energy that is stored up in the battery con nected to the plate on the vacuum tube: When the weak, incoming cur rent reaches the grid or "control ele- ment" of the tube. it releases this stored-up energy by pulling the grid trigger and permitting the plate cur rent to come into play in the circuit. This action. of course, can be dupli- cated In successive stages of amplifi- cation.
 Regeneration Described
 It Is this action that is also brought into play in the regenera- tive circuit. The tube, having acted as detector. then passes it's slightly increased current through the induc- tance in the plate circuit. and this inductance. being inductively coupled to the oscillating circuit. induces a still stronger current in that, osciilat- ing circuit. which again passes through the vacuum tube releasing still greater energy in the plate cir cult and this action passes around again performing the same function. always simultaneously. until the cer- tain maximum condition is reached. This regenerative action can best l.be explained by using the analogy of the ordinary telephone Some | times, when the telephone rings and you lift on the hook and central still continues ringing. you occasionally get peeved and place the receiver to the mouthpiece of your telephone. The result is that gradually and rap idly you get tremendous buzzing effect that finally reaches loud, and terrific whistle. What has taken ! place is simply this. The sound vibrations received in the telephone receiver have been fed back into the mouthpiece of the transmtter and the transmitter, of course, increa ses 8 the sound and passes 1t around the circuit again to the receiver. which l then again transfers. it to the trans hitter of the mouthpiece until the maximum, final strength Is attained which results in the terrible whis- tling bowling sounds that appear in the telephone.