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- The Radio Amateur's Hand Book - 30/44 -


Receivers, He is able to catch not only Code Messages but the regular Broadcasting Programs from Stations Twenty and Thirty Miles Distant.]

The Crystal Detector.--This is known as the _Rasco baby_ detector and it is made and sold by the _Radio Specialty Company_, 96 Park Place, New York City. It is shown in Fig. 96. The base is made of black composition and on it is mounted a standard in which a rod slides and on one end of this there is fixed a hard rubber adjusting knob while the other end carries a thin piece of _phosphor-bronze wire_, called a _cat-whisker_. To secure the galena crystal in the cup you simply unscrew the knurled cap, place it in the cavity of the post and screw the cap back on again. The free end of the cat-whisker wire is then adjusted so that it will rest lightly on the exposed part of the galena.

[Illustration: Fig. 96.--Rasco Baby Crystal Detector.]

The Tuning Coil.--You will have to make this tuning coil, which you can do at a cost of less than $1.00, as the cheapest tuning coil you can buy costs at least $3.00, and we need the rest of our $5.00 to invest in the earphone. Get a cardboard tube, such as is used for mailing purposes, 2 inches in diameter and 3 inches long, see A in Fig. 97. Now wind on 250 turns of _No. 40 Brown and Sharpe gauge plain enameled magnet wire_. You can use _No. 40 double cotton covered magnet wire_, in which case you will have to shellac the tube and the wire after you get it on.

[Illustration: Fig. 97.--How the Tuning Coil is Made.]

As you wind on the wire take off a tap at every 15th turn, that is, scrape the wire and solder on a piece about 7 inches long, as shown in Fig. 99; and do this until you have 6 taps taken off. Instead of leaving the wires outside of the tube bring them to the inside of it and then out through one of the open ends. Now buy a _round wood-base switch_ with 7 contact points on it as shown at B in Fig. 97. This will cost you 25 or 50 cents.

The Headphone.--An ordinary Bell telephone receiver is of small use for wireless work as it is wound to too low a resistance and the diaphragm is much too thick. If you happen to have a Bell phone you can rewind it with _No. 40_ single covered silk magnet wire, or enameled wire of the same size, when its sensitivity will be very greatly improved. Then you must get a thin diaphragm and this should _not_ be enameled, as this tends to dampen the vibrations of it. You can get a diaphragm of the right kind for 5 cents.

The better way, though, is to buy an earphone made especially for wireless work. You can get one wound to 1000 ohms resistance for $1.75 and this price includes a cord. [Footnote: This is Mesco, No. 470 wireless phone. Sold by the Manhattan Electrical Supply Co., Park Place, N.Y.C.] For $1.00 extra you can get a head-band for it, and then your phone will look like the one pictured in Fig. 98.

[Illustration: Fig. 98.--Mesco 1000 Ohm Head Set.]

How to Mount the Parts.--Now mount the coil on a wood base, 1/2 or 1 inch thick, 3-1/2 inches wide and 5-1/2 inches long, and then connect one end of the coil to one of the end points on the switch, and connect each succeeding tap to one of the switch points, as shown schematically in Fig. 99 and diagrammatically in Fig. 100. This done, screw the switch down to the base. Finally screw the detector to the base and screw two binding posts in front of the coil. These are for the earphone.

[Illustration: Fig. 99.--Schematic Layout of $5.00 Receiving Set.]

[Illustration: Fig. 100.--Wiring Diagram for $5.00 Receiving Set.]

The Condenser.--You do not have to connect a condenser across the earphone but if you do you will improve the receiving qualities of the receptor.

How to Connect Up the Receptor.--Now connect up all the parts as shown in Figs. 99 and 100, then connect the leading-in wire of the aerial with the lever of the switch; and connect the free end of the tuning coil with the _ground_. If you have no aerial wire try hooking it up to a rain pipe that is _not grounded_ or the steel frame of an umbrella. For a _ground_ you can use a water pipe, an iron pipe driven into the ground, or a hydrant. Put on your headphone, adjust the detector and move the lever over the switch contacts until it is in adjustment and then, if all your connections are properly made, you should be able to pick up messages.

[Illustration: Wireless Set made into a Ring, designed by Alfred G. Rinehart, of Elizabeth, New Jersey. This little Receptor is a Practical Set; it will receive Messages, Concerts, etc., Measures 1" by 5/8" by 7/8". An ordinary Umbrella is used as an Aerial.]

APPENDIX

USEFUL INFORMATION

ABBREVIATIONS OF UNITS

Unit Abbreviation

ampere amp. ampere-hours amp.-hr. centimeter cm. centimeter-gram-second c.g.s. cubic centimeters cm.^3 cubic inches cu. in. cycles per second ~ degrees Centigrade C. degrees Fahrenheit F. feet ft. foot-pounds ft.-lb. grams g. henries h. inches in. kilograms kg. kilometers km. kilowatts kw. kilowatt-hours kw.-hr. kilovolt-amperes kv.-a. meters m. microfarads [Greek: mu]f. micromicrofarads [Greek: mu mu]f. millihenries mh. millimeters mm. pounds lb. seconds sec. square centimeters cm.^2 square inches sq. in. volts v. watts w.

PREFIXES USED WITH METRIC SYSTEM UNITS

Prefix Abbreviation Meaning

micro [Greek: mu]. 1 millionth milli m. 1 thousandth centi c. 1 hundredth deci d. 1 tenth deka dk. 10 hekto h. 1 hundred kilo k. 1 thousand mega m. 1 million

SYMBOLS USED FOR VARIOUS QUANTITIES

Quantity Symbol

capacitance C

conductance g

coupling co-efficient k

current, instantaneous i

current, effective value I

decrement [Greek: delta]

dielectric constant [Greek: alpha]

electric field intensity [Greek: epsilon]

electromotive force, instantaneous value E

electromotive force, effective value F

energy W

force F

frequency f

frequency x 2[Greek: pi] [Greek: omega]

impedance Z

inductance, self L

inductance, mutual M

magnetic field intensity A

magnetic flux [Greek: Phi]

magnetic induction B

period of a complete oscillation T

potential difference V

quantity of electricity Q


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