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

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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

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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 11 wires placed on the ground, under the surface, or suspended a few feet above the ground running parallel to the an- tennae. The earth connection is as im- portant, if not more so, than the aerial. In concluding I will say that Wire- less Telephony can be received on any Audion Receiving set, and on a Crys- tal set, provided that the oscillations are well modulated and the transmit- ting station sufficiently close. The Radio Engineer has a very broad and comparatively new field before him. Sending pictures through the ether is in the process of beginning. And sending electrical energy, in worth while quantities and small dissi- pation losses, without wires, will some day be mastered by a Radio Engineer. Bear this in mind. Radio is not the mysterious something which most peo- ple think ; it is an exact Science.