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Show This Is the fourth of a series of articles ar-ticles by Prank tieorge. wireless expert, explaining ih principles of radio. By FRANK GEORGE. Checked and approved by radia section, U. S. bureau of standard. Any receiving Inat rumen t that will receive radiograph messages will also receive voice. It Is estimated that ther are 700.000 receiving Instruments throughout the country, and that Installations are 1n- reuairiK st the rate of mor than 600 sets a month. To receive radiograph and radiophone message no license is required. Commercial radio ntm cost from $25 ftir an Instrument i hat will receive int-snaac over a r.idiuw of iwenty-flv miles to $.60 for a net that will catch messages frv-m almost any distance. Any amount of money can be spent on t he set tn the purchase of highly sensitive sensi-tive parts. SET IS CHEAP. A radiophone receiving set that can 1 const rue tetl fur i to $1 ! hns recently re-cently been designed by the Cnltcd Htates bureau of standards In connection connec-tion with t he boys' and girls radio club work of the department of agriculture. agri-culture. Thla set will enable one to hear mrssaKes sent from medlnm power sending stations within an area or utMiut ten ririus. I'nder favoruble conditions hlrh power stations sending on 200 to 600 1 meter wnve lengths can be heard within fifty miles. The apparatus can i be made by any one; only a few of the parts need to b purchased. i The essential purts of the set are th aniennti, a lightning switch, ground connect Ions, the receiving Instrument and phones. Ordinary telephone head receivers are used, Th nntenan Is simply a seveniy-flve-foot lenvth of copper wire suspended outdoors at a 1 height of thirty feet from the ground. SIMPLE CONSTRUCTION, Th lightning swltrh prevents dnm-sge dnm-sge to the set .y lightning and also serves ss a IlKhtning nd for the house. The tuner, panel and crystal detector comprising the receiving Instrument are all hum made Th tuner la fjLshinned ut of an oatmeal oat-meal box and some copper wire: th crystal detector Is simply a tested crystal, a cork and some screws and wire, and ihe panel on which switch arms and contact ncri are mounted Is a plec of wood from a packing box. Details of construction arc contained In a circular which may h obtained on request from the I'nlted State bureau bu-reau of standards, Washington. A more improved aet designed by th bureau nf standard for demonstration purposes In th nearby reception of metgfs from hiKh power stations la shown iti the illustration. Th complete com-plete sot weighs only twenty pounds and. including the aerial, can be packed In a box one foot square. Mn his next artil Frank fieorge will tell how a small tube permits tha j hcuring of radio wavea ) |