Crystal Detectors and Telephonic Speech

The Radio Amateur's Hand Book, by A. Frederick Collins is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here: [LINK TO TABLE OF LINK]. Chapter III: Simple Telegraph and Telephone Receiving Sets

III. SIMPLE TELEGRAPH AND TELEPHONE RECEIVING SETS

With a crystal detector receiving set you can receive either telegraphic dots and dashes or telephonic speech and music. You can buy a receiving set already assembled or you can buy the different parts and assemble them yourself. An assembled set is less bother in the beginning but if you like to experiment you can hook up, that is, connect the separate parts together yourself and it is perhaps a little cheaper to do it this way. Then again, by so doing you get a lot of valuable experience in wireless work and an understanding of the workings of wireless that you cannot get in any other way.

Assembled Wireless Receiving Sets.--The cheapest assembled receiving set [Footnote: The Marvel, made by the Radio Mfg. Co., New York City.] advertised is one in which the detector and tuning coil is mounted in a box. It costs $15.00, and can be bought of dealers in electric supplies generally.

This price also includes a crystal detector, an adjustable tuning coil, a single telephone receiver with head-band and the wire, porcelain insulators, lightning switch and ground clamp for the aerial wire system. It will receive wireless telegraph and telephone messages over a range of from 10 to 25 miles.

Another cheap unit receptor, that is, a complete wireless receiving set already mounted which can be used with a single aerial is sold for $25.00. [Footnote: The Aeriola Jr., made by the Westinghouse Company, Pittsburgh, Pa.] This set includes a crystal detector, a variable tuning coil, a fixed condenser and a pair of head telephone receivers. It can also be used to receive either telegraph or telephone messages from distances up to 25 miles. The aerial equipment is not included in this price, but it can be bought for about $2.50 extra.

Assembling Your Own Receiving Set.--In this chapter we shall go only into the apparatus used for two simple receiving sets, both of which have a crystal detector. The first set includes a double-slide tuning coil and the second set employs a loose-coupled tuning coil, or loose coupler, as it is called for short. For either set you can use a pair of 2,000- or 3,000-ohm head phones.

Photograph unavailable--original © Underwood and Underwood.
General Pershing Listening In.

The Crystal Detector.--A crystal detector consists of: (1) the frame, (2) the crystal, and (3) the wire point. There are any number of different designs for frames, the idea being to provide a device that will (a) hold the sensitive crystal firmly in place, and yet permit of its removal, (b) to permit the wire point, or electrode, to be moved in any direction so that the free point of it can make contact with the most sensitive spot on the crystal and (c) to vary the pressure of the wire on the crystal.

A simple detector frame is shown in the cross-section at A in Fig. 10; the crystal, which may be galena, silicon or iron pyrites, is held securely in a holder while the phosphor-bronze wire point which makes contact with it, is fixed to one end of a threaded rod on the other end of which is a knob. This rod screws into and through a sleeve fixed to a ball that sets between two brass standards and this permits an up and down or a side to side adjustment of the metal point while the pressure of it on the crystal is regulated by the screw.

A crystal of this kind is often enclosed in a glass cylinder and this makes it retain its sensitiveness for a much longer time than if it were exposed to dust and moisture. An upright type of this detector can be bought for $2.25, while a horizontal type, as shown at B, can be bought for $2.75. Galena is the crystal that is generally used, for, while it is not quite as sensitive as silicon and iron pyrites, it is easier to obtain a sensitive piece.

The Tuning Coil.--It is with the tuning coil that you tune in and tune out different stations and this you do by sliding the contacts to and fro over the turns of wire; in this way you vary the inductance and capacitance, that is, the constants of the receiving circuits and so make them receive electric waves, that is, wireless waves, of different lengths.

The Double Slide Tuning Coil.--With this tuning coil you can receive waves from any station up to 1,000 meters in length. One of the ends of the coil of wire connects with the binding post marked a in Fig. 11, and the other end connects with the other binding post marked b, while one of the sliding contacts is connected to the binding post c, and the other sliding contact is connected with the binding post d.

Fixed and Variable Condensers.--You do not require a condenser for a simple receiving set, but if you will connect a fixed condenser across your headphones you will get better results, while a variable condenser connected in the closed circuit of a direct coupled receiving set, that is, one where a double slide tuning coil is used, makes it easy to tune very much more sharply; a variable condenser is absolutely necessary where the circuits are inductively coupled, that is, where a loose coupled tuner is used.

A fixed condenser consists of a number of sheets of paper with leaves of tin-foil in between them and so built up that one end of every other leaf of tin-foil projects from the opposite end of the paper as shown at A in Fig. 13. The paper and tin-foil are then pressed together and impregnated with an insulating compound. A fixed condenser of the exact capacitance required for connecting across the head phones is mounted in a base fitted with binding posts, as shown at B, and costs 75 cents. (Paper ones 25 cents.)

A variable condenser, see C, of the rotating type is formed of a set of fixed semi-circular metal plates which are slightly separated from each other and between these a similar set of movable semi-circular metal plates is made to interleave; the latter are secured to a shaft on the top end of which is a knob and by turning it the capacitance of the condenser, and, hence, of the circuit in which it is connected, is varied. This condenser, which is shown at D, is made in two sizes, the smaller one being large enough for all ordinary wave lengths while the larger one is for proportionately longer wave lengths. These condensers cost $4.00 and $5.00 respectively.

About Telephone Receivers.--There are a number of makes of head telephone receivers on the market that are designed especially for wireless work. These phones are wound to resistances of from 75 ohms to 8,000 ohms, and cost from $1.25 for a receiver without a cord or headband to $15.00 for a pair of phones with a cord and head band. You can get a receiver wound to any resistance in between the above values but for either of the simple receiving sets such as described in this chapter you ought to have a pair wound to at least 2,000 ohms and these will cost you about $5.00. A pair of head phones of this type is shown in Fig. 14.

Connecting Up the Parts--Receiving Set No. 1.--For this set get (1) a crystal detector, (2) a two-slide tuning coil, (3) a fixed condenser, and (4) a pair of 2,000 ohm head phones. Mount the detector on the right-hand side of a board and the tuning coil on the left-hand side. Screw in two binding posts for the cord ends of the telephone receivers at a and b as shown at A in Fig. 15. This done connect one of the end binding posts of the tuning coil with the ground wire and a post of one of the contact slides with the lightning arrester or switch which leads to the aerial wire.

Now connect the post of the other contact slide to one of the posts of the detector and the other post of the latter with the binding post a, then connect the binding post b to the ground wire and solder the joint. Next connect the ends of the telephone receiver cord to the posts a and b and connect a fixed condenser also with these posts, all of which are shown in the wiring diagram at B, and you are ready to adjust the set for receiving.

Receiving Set No. 2.--Use the same kind of a detector and pair of head phones as for Set No. 1, but get (1) a loose coupled tuning coil, and (2) a variable condenser. Mount the loose coupler at the back of a board on the left-hand side and the variable condenser on the right-hand side. Then mount the detector in front of the variable condenser and screw two binding posts, a and b, in front of the tuning coil as shown at A in Fig. 16.

Now connect the post of the sliding contact of the loose coupler with the wire that runs to the lightning switch and thence to the aerial; connect the post of the primary coil, which is the outside coil, with the ground wire; then connect the binding post leading to the switch of the secondary coil, which is the inside coil, with one of the posts of the variable condenser, and finally, connect the post that is joined to one end of the secondary coil with the other post of the variable condenser.

This done, connect one of the posts of the condenser with one of the posts of the detector, the other post of the detector with the binding post a, and the post b to the other post of the variable condenser. Next connect a fixed condenser to the binding posts a and b and then connect the telephone receivers to these same posts, all of which is shown in the wiring diagram at B. You are now ready to adjust the instruments. In making the connections use No. 16 or 18 insulated copper wire and scrape the ends clean where they go into the binding posts. See, also, that all of the connections are tight and where you have to cross the wires keep them apart by an inch or so and always cross them at right angles.

Adjusting the No. 1 Set--The Detector.--The first thing to do is to test the detector in order to find out if the point of the contact wire is on a sensitive spot of the crystal. To do this you need a buzzer, a switch and a dry cell. An electric bell from which the gong has been removed will do for the buzzer, but you can get one that is made specially for the purpose, for 75 cents, which gives out a clear, high-pitched note that sounds like a high-power station.

Connect one of the binding posts of the buzzer with one post of the switch, the other post of the latter with the zinc post of the dry cell and the carbon post of this to the other post of the buzzer. Then connect the post of the buzzer that is joined to the vibrator, to the ground wire as shown in the wiring diagram, Fig. 17. Now close the switch of the buzzer circuit, put on your head phones, and move the wire point of the detector to various spots on the crystal until you hear the sparks made by the buzzer in your phones.

Then vary the pressure of the point on the crystal until you hear the sparks as loud as possible. After you have made the adjustment open the switch and disconnect the buzzer wire from the ground wire of your set. This done, be very careful not to jar the detector or you will throw it out of adjustment and then you will have to do it all over again. You are now ready to tune the set with the tuning coil and listen in.

The Tuning Coil.--To tune this set move the slide A of the double-slide tuner, see B in Fig. 15, over to the end of the coil that is connected with the ground wire and the slide B near the opposite end of the coil, that is, the one that has the free end. Now move the slide A toward the B slide and when you hear the dots and dashes, or speech or music, that is coming in as loud as you can move the B slide toward the A slide until you hear still more loudly. A very few trials on your part and you will be able to tune in or tune out any station you can hear, if not too close or powerful.

Photograph unavailableoriginal-- © Underwood and Underwood.
The World's Largest Radio Receiving Station. Owned by the Radio Corporation of America at Rocky Point near Point Jefferson, L.I.

Adjusting the No. 2 Set.--First adjust the crystal detector with the buzzer set as described above with Set No. 1, then turn the knob of your variable condenser so that the movable plates are just half-way in, pull the secondary coil of your loose-coupled tuner half way out; turn the switch lever on it until it makes a contact with the middle contact point and set the slider of the primary coil half way between the ends.

Now listen in for telegraphic signals or telephonic speech or music; when you hear one or the other slide the secondary coil in and out of the primary coil until the sounds are loudest; now move the contact switch over the points forth and back until the sounds are still louder, then move the slider to and fro until the sounds are yet louder and, finally, turn the knob of the condenser until the sounds are clear and crisp. When you have done all of these things you have, in the parlance of the wireless operator, tuned in and you are ready to receive whatever is being sent.

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Collins, A. Frederick. 2002. The Radio Amateur's Hand Book. Urbana, Illinois: Project Gutenberg. Retrieved April 2022, from https://www.gutenberg.org/files/6934/6934-h/6934-h.htm#chap03

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