University of San Francisco - USF Don Yearbook (San Francisco, CA)

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30 THE J ax ATI AS incoming wave group due to the flow of the rectified current. (2) This is the 44Grid condenser method. ’ With a properly constructed tube it is most sensitive because the plate potential varies at an audio frequency due to the condenser interposed somewhere in the grid circuit. Unidirectional current passage between the filament and the grid rectifies the impressed groups of radio oscillations and the condenser consequently receives an unidirectional charge which, in building up, causes the plate current to decrease. When the incoming oscillations have ceased the condensed charge dissipates through a high-resistance grid-leak and the plate potential returns to normal. The effect of inter-electrode capacities is next to be considered. Remembering the fact that a slight variation of the grid potential greatly changes the strength of the plate current, we can appreciate the fact that this phase of operation has two broad applications with respect to Wireless. Primarily it causes the Tube to be a self-contained amplifier which in no way interferes with its detective qualities, which may even be simultaneous, and secondarily, it is in this respect that the tube is used as an oscillation generator. These characteristics have been fully set forth by their discoverer, (’apt. E. H. Armstrong. In some types of circuits there is a marked difference in the action of the tube than what might have been expected. This is due to inter-electrode capacities. The effect of these is to cause a tube to produce oscillations even if there is no mutual inductance between the input and the output. It is also on account of these that there is an impedence between the cathode and the grid which depends both on these capacities and on the constants of the input circuit. When oscillations are impressed on such a circuit they die out at a rate which varies as the total resistance. If the impedence is negative the total resistance is reduced and there will be a smaller consumption of current in the input circuit and the tube will give an increase of amplification. This increase of amplifi-

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THREE E LEM EXT YAC1TM TV BE 20 crophonic detector might be used for these types of waves but tlie same does not hold for continuous waves which are damped and which make no impression on any detector except at the beginning and end of such a wave and even then it is only evidenced by a click. When the crystal detector was in vogue it was customary to use some sort of mechanical interrupter in the circuit which was called a tikkcr. This tikker broke up the incoming continuous wave some three hundred to one thousand times a second, thus rendering the wave, when rectified, audible. This method was popular because there was no other. At the best it was crude and could be no better than the crystal detector that was used with it. The advent of the Vacuum Tube caused a hundred per cent improvement in both the detection of damped and undamped waves and in the method of rendering the 0. Y. audible after detection. The Vacuum Tube method entirely supplanted the old “Tikker” method. The system by which the ( AY. is rendered audible is sometimes referred to as “Beat reception” but is more properly called “Heterodyne.” The desired effect is obtained in the Vacuum Tube by supplementing the incoming radio frequency wave with a locally generated current which differs in amplitude by an amount which lies within the range of audibility. The resulting fortification and interference which takes place due to periodic synchronism gives rise to a third or “beat current” which, when properly rectified is audible. In this case as in detection the input is a trifle greater than the output due to the addition of the incoming wave to that locally generated by the tube. A brief account of the two methods, by which damped oscillations may be detected, follows: (1) The grid current is maintained with respect to the cathode, so that a negative grid charge will be followed by a slight decrease in the anode current or a positive charge on the grid will produce a relatively high increase in the plate current and the receivers are affected once for each

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THREE E LEM EXT YACVI'M TIRE 31 action is called “Regeneration.” A great step in the development of the Vacuum Tube was made when Captain Armstrong discovered this means of increasing the strength of the input without adding any extra energy from the outside. The “Armstrong regenerative circuit” is subject to endless modifications but the principle is always the same and may be formulated thus: The incoming oscillations, alternating at radio-frequency, enter some form of coupling device. In this, the secondary or detector circuit is tuned to the length of the incoming wave which it receives by inductance from the primary. This induced wave (in the secondary) is reinforced by high frequency waves fed back from the anode or plate of the Vacuum Tube. One wire from the secondary bears these fed-back oscillations. The other connects to the grid and carries the induced incoming current, reinforced by these fed-back oscillations which have the same form as the incoming waves. At this point by the function of the tube, alternating waves of high frequency are generated and are later taken around to strengthen the incoming wave. Simultaneously the Vacuum Tube converts the message-bearing current from radio frequency to one of direct pulsating low frequency, already strengthened by the fed-back waves. The two currents of varying frequency arc now sent through a variometer which serves to tune the high frequency waves escaping from the tube to the same frequency as the incoming wave. By this method the low frequency currents are strengthened from ten to one hundred times and they enter the phones at about four to ten thousand cycles a second. Of course they are accompanied by the other radiofrequency currents, but since these have no effect on the phones they are passed through a condenser provided for that purpose. These waves, after passing through the condenser, go again to the secondary of the coupling device and the circuit is repeated. This system constitutes one branch of radio amplification. The other is accomplished when the current from the plate of the detector tube is coupled to the