In 1890, Edward Branly, of France, showed that metal filings in atube cohered when electric waves acted on them, and this device hetermed a radio conductor; this was improved upon by Sir OliverLodge, who called it a coherer. In 1895, Alexander Popoff, of Russia,constructed a receiving set for the study of atmospheric electricity,and this arrangement was the earliest on record of the use of adetector connected with an aerial and the earth.
This is the wireless, or radio, as the average amateur knows ittoday. But it is by no means the limit of its possibilities. On thecontrary, we are just beginning to realize what it may mean to thehuman race. The Government is now utilizing it to send out weather,crop and market reports. Foreign trade conditions are being reported.The Naval Observatory at Arlington is wirelessing time signals.
Department stores are beginning to issue programs and advertise byradio! Cities are also taking up such programs, and they willdoubtless be included soon among the regular privileges of thetax-payers. Politicians address their constituents. Preachers reachthe stay-at-homes. Great singers thrill thousands instead of hundreds.Soon it will be possible to hear the finest musical programs,entertainers, and orators, without budging from one's easy chair.
Meanwhile, the radio amateur to whom this book is addressed mayhave his share in the joys of wireless. To get all of these goodthings out of the ether one does not need a rod or a gun--only acopper wire made fast at either end and a receiving set of some kind.If you are a sheer beginner, then you must be very careful in buyingyour apparatus, for since the great wave of popularity has washedwireless into the hearts of the people, numerous companies have sprungup and some of these are selling the veriest kinds of junk.
Now there is only one method by which currents of high frequency,or radio-frequency, as they are termed, can be set up by sparktransmitters, and this is by discharging a charged condenser through acircuit having a small resistance. To charge a condenser a spark coilor a transformer is used and the ends of the secondary coil, whichdelivers the high potential alternating current, are connected withthe condenser. To discharge the condenser automatically aspark, or an arc, or the flow of electrons in avacuum tube, is employed.
The reason a vacuum tube detector is more sensitive than a crystaldetector is because while the latter merely rectifies theoscillating current that surges in the receiving circuits, the formeracts as an amplifier at the same time. The vacuum tube can beused as a separate amplifier in connection with either: (1) acrystal detector or (2) a vacuum tube detector, and(a) it will amplify either the radio frequency currents,that is the high frequency oscillating currents which are set up inthe oscillation circuits or (b) it will amplify the audiofrequency currents, that is, the low frequency alternatingcurrents that flow through the head phone circuit.
To use the amplified radio frequency oscillating currents oramplified audio frequency alternating currents that are set up by anamplifier tube either a high resistance, called a grid leak, oran amplifying transformer, with or without an iron core, mustbe connected with the plate circuit of the first amplifier tube andthe grid circuit of the next amplifier tube or detector tube, or withthe wire point of a crystal detector. Where two or more amplifiertubes are coupled together in this way the scheme is known ascascade amplification.
Where either a radio frequency transformer, that is onewithout the iron core, or an audio frequency transformer, thatis one with the iron core, is used to couple the amplifier tubecircuits together better results are obtained than where a highresistance grid leak is used, but the amplifying tubes have to be morecarefully shielded from each other or they will react and set up ahowling noise in the head phones. On the other hand grid leakscost less but they are more troublesome to use as you have to find outfor yourself the exact resistance value they must have and this youcan do only by testing them out.
The Vacuum Tube Amplifier.--This consists of a threeelectrode vacuum tube exactly like the vacuum tube detector describedin Chapter VIII and pictured in Fig. 38,except that instead of being filled with a non-combustible gas it isevacuated, that is, the air has been completely pumped out of it. Thegas filled tube, however, can be used as an amplifier and either kindof tube can be used for either radio frequency or audio frequencyamplification, though with the exhausted tube it is easier to obtainthe right plate and filament voltages for good working.
A Radio Frequency Transformer Amplifying ReceivingSet.--Instead of using a grid leak resistance to couple up theamplifier and detector tube circuits you can use a radio frequencytransformer, that is, a transformer made like a loose coupledtuning coil, and without an iron core, as shown in the wiring diagramat A in Fig. 45. In this set, which gives better resultsthan where a grid leak is used, the amplifier tube is placed in thefirst oscillation circuit and the detector tube in the secondcircuit.
Since the radio frequency transformer has no iron core the highfrequency, or radio frequency oscillating currents, as they arecalled, surge through it and are not changed into low frequency, oraudio frequency pulsating currents, until they flow through thedetector. Since the diagram shows only one amplifier and one radiofrequency transformer, it is consequently a one step amplifier;however, two, three or more, amplifying tubes can be connected up bymeans of an equal number of radio frequency transformers when you willget wonderful results. Where a six step amplifier, that is, where sixamplifying tubes are connected together, or in cascade, thefirst three are usually coupled up with radio frequency transformersand the last three with audio frequency transformers. A radiofrequency transformer is shown at B and costs $6 to $7.
An Audio Frequency Transformer Amplifying ReceivingSet.--Where audio frequency transformers are used for stepping upthe voltage of the current of the detector and amplifier tubes, theradio frequency current does not get into the plate circuit of thedetector at all for the reason that the iron core of the transformerchokes them off, hence, the succeeding amplifiers operate at audiofrequencies. An audio frequency transformer is shown at A inFig. 46 and a wiring diagram showing how the tubes areconnected in cascade with the transformers is shown atB; it is therefore a two-step audio frequency receivingset.
A Six Step Amplifier Receiving Set With a Loop Aerial.--Byusing a receiving set having a three step radio frequency and a threestep audio frequency, that is, a set in which there are coupled threeamplifying tubes with radio frequency transformers and threeamplifying tubes with audio frequency transformers as described underthe caption A Radio Frequency Transformer Receiving Set, youcan use a loop aerial in your room thus getting around thedifficulties--if such there be--in erecting an out-door aerial. Youcan easily make a loop aerial by winding 10 turns of No. 14 or16 copper wire about 1/16 inch apart on a wooden frame two feeton the side as shown in Fig. 47. With this six step amplifierset and loop aerial you can receive wave lengths of 150 to 600 metersfrom various high power stations which are at considerable distancesaway.
How to Prevent Howling.--Where radio frequency or audiofrequency amplifiers are used to couple your amplifier tubes incascade you must take particular pains to shield them from one anotherin order to prevent the feed back of the currents through them,which makes the head phones or loud speaker howl. To shieldthem from each other the tubes should be enclosed in metal boxes andplaced at least 6 inches apart while the transformers should be set sothat their cores are at right angles to each other and these alsoshould be not less than six inches apart.
While a vacuum tube detector has an amplifying action of its own,and this accounts for its great sensitiveness, its amplifying actioncan be further increased to an enormous extent by making the radiofrequency currents that are set up in the oscillation circuits reacton the detector.
Since the regenerative action of the radio frequency currents isproduced by the detector tube itself and which sets up an amplifyingeffect without the addition of an amplifying tube, this type ofreceiving set has found great favor with amateurs, while incombination with amplifying tubes it multiplies their powerproportionately and it is in consequence used in one form or anotherin all the better sets.
The units of inductance commonly used in radio work are themillihenry, which is the thousandth part of a henry; and thecentimeter of inductance, which is one one-thousandth part of amicrohenry.
AMMETER, HOT-WIRE.--High frequency currents are usually measured bymeans of an instrument which depends on heating a wire or metal stripby the oscillations. Such an instrument is often called a thermalammeter, radio ammeter and aerial ammeter.
AMPLIFIER, MAGNETIC.--A device used for controlling radio frequencycurrents either by means of a telegraph key or a microphonetransmitter. The controlling current flows through a separate circuitfrom that of the radio current and a fraction of an ampere willcontrol several amperes in the aerial wire.
WAVE LENGTH BAND.--In wireless reception when continuous waves arebeing sent out and these are modulated by a microphone transmitter thedifferent audio frequencies set up corresponding radio frequencies andthe energy of these are emitted by the aerial; this results in wavesof different lengths, or a band of waves as it is called. 59ce067264