University of Santa Clara - Redwood Yearbook (Santa Clara, CA)

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THE REDWOOD of times a second. In order to make a Vacuum Tube function properly at all times, as a rectifier of incoming oscillations, we must introduce a condenser and a high resistance of about a raagohm, shunt- ing the condenser, in the grid circuit instead of the grid battery. When radio frequency currents are introduced on the grid circuit from the aerial, it is first made positive and then negative. It must be remembered that only an alternating, oscillating, or a pulsatinji ' current can pass through a condenser. This is a devise composed of two con- ducting surfaces separated by an insu- lator. When the grid is positive elec- trons are drawn over it, but when it is negative they are repelled; thus for each succeeding half oscillation, elec- trons are drawn to the grid, placing a charge in the grid condenser which is negative on the grid side. Since an increasing negative charge on the grid acts to reduce the plate-to- filament current, then, while a group of oscillations are rectified the tele- phone current is reduced ; but the high resistance, known as the grid leak, comes into play and slowly discharges the condenser and allows the grid and plate to come back to their normal state. These variations which occur at each wave train causes the diaphragm of the telephone receiver to vibrate. We will now consider the Audion Valve as a generator of constant ampli- tude oscillations. Take for instance a spark coil of the wipe type. When we make and break the primary circuit from a battery we set up a similar cur- rent in the secondary. If we place the primary of an air core transformer in the plate circuit and the secondary in the grid circuit so that the plus side of the primary is toward the plate and the negative end of the secondary near- est to the grid; when a current flows from the filament to the plate it places a negative charge on the grid by induc- tion through the coil. This state of the grid repels the electronic discharge and since the filament-to-plate current has no medium to travel, it also stops. This break of the current places a positive charge upon the grid, due to the trans- former, and consequently the electron flow to the plate is resumed again. This process is how the tube acts, and is repeated an enormous number of times a second. We will now consider how the out- put of the Audion Oscillator is modu- lated for telephon.y. The essential in- strument which accomplishes this is the microphone transmitter, which functions on the principle of a varying resistance. The devise is made up as follows: small carbon balls or blocks are in contact with each other and touching the diaphragm. Sound makes this vibrate and thus varies the resist- ance of the carbon pieces proportion- ally, due to their differing pressure of contact with each other. Now it can be easily seen that if the microphone be placed between the aerial and ground that the radiated en- ergy will be modulated. How- ever, as very large currents are some- times in the ground circuit, it is impos- sible to make a transmitter of suffi- cient capacity without entering into more serious electrical and construct- ive difficulties, so it is better to obtain a more satisfactory means of modula- tion. A fairly efficient method is that of placing the microphone in the grid cir- cuit, since a very small change in the

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THE REDWOOD He called his devise the Electrical In- dicator and it later become known as the Edison Effect . Almost twenty years afterward, an English Physicist, Dr. J. A. Fleming, by substituting a cylindrical plate in- stead of the extra filament discovered that this devise could be used for Radio Telegraph reception. The principle upon which Fleming ' s invention func- tions is as follows: When the filament is heated to a state where light is given off there is an emission of electrons radially from it. Now, an electron is the smallest particle of matter known to science, and is supposed to carry the tiniest possible charge of negative elec- tricity on its travel from one body to another. In the case of an ordinary electric lamp these particles hit the glass walls. But if we place a small metal band around the filament and connect a set of batteries between the filament and the plate, so that the negative pole of the battery is attached to the negative side of the filament, and the positive pole to the plate, these negative elec- trons will be greatly attracted to the metal plate because of its positive na- ture. Should the plate be made electro- negative by reversing the polarity of the plate battery, the negative elec- trons will be repelled. It is simply in accordance with the fundamental law of magnetism and electricity, — that like attracts unlike and repels like. Now, if we place a milammeter in the plate circuit we will find that a current flows in one direction only when the plate is electro-positive in nature, and no current passes through the meter when it is electro-negative. This demonstrates that the electrons form a conductor over which the cur- rent flows. It can be seen from above, that if an alternating current is substi- tuted for the plate potential only the positive half of the cycle will be able to pass, thus we have in the two ele- ment valve a very powerful rectifier. These are known commercially as the General Electric Tungar and the West- inghouse Rectigon Rectifiers. In order to convert radio oscillations into sound they must be converted into the form of a pulsating direct current so as to be able to actuate the telephone receiver. This is because the pole of the receiver has a definite magnetic flux, and to impose an alternating cur- rent upon it would ruin its sensitive- ness. So it is seen that the purpose of the Fleming Oscillon, or two element valve in radio telegraph or telephone recep- tion is that of rectification. Now if we place a third element, a helical coil of wire, called the grid, be- tween the plate and the filament and connect it electro-positively in series with a battery and a milammeter to the negative side of the plate battery, a current in the same direction is regis- tered on both the grid and plate milam- meters. Should we make the grid electro- negative the flow of electrons from the filament is repulsed, as both are nega- tively charged and they necessarily op- pose each other. Therefore the current to the plate, having no electronic con- ductor to travel on, is quite suddenly stopped. One can understand from this that the grid acts as an automatic inter- rupter. Since it has no mechanical parts to move, it seemingly has no iner- tia, and can therefore open and close the plate circuit an enormous number

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10 THE REDWOOD grid voltage causes a great variation in the electron flow from the filament to the plate. A slight improveme nt is to place the microphone inductively in the grid circuit by means of a transformer having a high ratio between the prim- ary and the secondary, in order to step up the voltage of the microphonic vari- ations before placing them on the grid. This means is known commonly as the absorption method. A very superior form of modula- tion is known as the constant current system. By this is meant that the plate current of the oscillator is not varied by modulating the grid potential of the tube, but the plate supply is regulated by the results of an output of another Audion Valve, whose grid current has been changed by a microphone in its circuit. In other words, if a constant if the supply of the tube nearest the source of current is fed to two tubes with their plates wired in parallel, and source is varied, a slight potential drop will be effected on the plate of the sec- ond Audion, which will vary the ampli- tude of its oscillations. The method of modulation, where a separate tube is used as a modulator and another as the oscillator, is called the Heissing System. There are several other appliances used in a telephone set, which will be merely enumerated as their functioning will appear obvious: A motor genera- tor, which will supply from 500 to 1500 volts direct current for plate potential, depending upon the size and range of the set. A Voltmeter is required to measure this. A filter circuit is needed to smooth out the ripples of this supply so that a nearly constant amplitude di- rect current may be had. This devise is composed of two II 2 Henry choke coils and two high voltage test con- densers, which are shunted across the coils. A twelve volt Storage Battery is ne- cessary to light the filaments. A Ther- mo-couple or a Hot wire ammeter of suitable range is needed to measure the radiation current in the aerial. This meter must not be of the ordinary mag- netic type as the radio-frequency cur- rents would not go through the arma- ture coil, and a high frequency current stays on the outside of the conductor; consequently, the meter would give a reading which would be correct only for such an instrument of this type with one turn on the armature coil. Before closing, the purpose of the aerial will be stated. Its function is two fold: (1) To radiate energy in the form of electro-magnetic waves; (2) To absorb part of the energy radiated by a distant transmitter; briefly it is that of sending out the impulses and of receiving them. There are four gen- eral types of antennae suitable for transmission, viz. : the vertical or fan type, the umbrella aerial, the inverted L flat top antenna, and the T aerial. The vertical and umbrella antennae radiate well in all directions, while the flat top styles are slightly directional, that is, they radiate the current better in one direction than in another. How- ever, since all aerials are governed by the following two conditions all types find a use: (1) By the length of the wave to be radiated; (2) By the space available for erection. The connection from the transmit- ting apparatus to the earth should be as direct as possible and the conductor ought to be one of high conductivity. The ground connection is sometimes a water pipe, buried plate or a system of