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Washington Times, February 26, 1919, page 3:

Awed  Visitors  Listen  to  "Pretty  Baby"  Played  by  Wireless  Phonograph

    Uncle Sam lifted the lid from his box of electrical wonders out at the Bureau of Standards last night and permitted a hundred or more visitors to peep into the hitherto jealously guarded mysteries of America's latest marvels of invention.
    The Government permitted a limited number of visitors to visit the bureau to see a specially arranged exhibit showing the electrical devices that hastened the downfall of the Kaiser's armies.
    Talking to Paris and Berlin by wireless, filling an auditorium with music transmitted by radio-telephone from a building several hundred yards away, talking through thin air over wireless telephones, firing a stream of machine gun bullets through a propeller making 2,000 revolutions a minute--these were some of the marvels that made the visitors gasp.

U.S.  Accuracy  Puzzles.

    During the closing months of the war the German artillerymen were confused and astonished by the accuracy of the American gunners, made possible by an instrument exhibited last night. This instrument, known as a sound ranger, consisted of three stations set up in the front line trenches. When a German gun opened fire the sound waves sent out were received and recorded automatically by each of the three stations.
    Then a mathematically inclined artillery officer at each station would plot the position of the gun with relation to the other two stations and within five minutes American shells would begin dropping in to visit the German gun crew with immediate and disastrous results. It was in perfecting this device that Capt. Ernst Weiber, of the Bureau of Standards, gave his life on the western front.

Music  Through  Air.

    One of the most interesting features to many of the visitors was the transmission of music through the air. A group of signal corps men and navy radio men grouped about a radio-telephone set in a little room in the wireless building played "Pretty Baby," and other musical hits on a phonograph placed before the mouthpiece of the wireless phone.
    The music was transformed into electric waves and passed through the air to the other phone set placed in the bureau auditorium. Here the electrical waves were transformed once more to sound waves, increased in volume, and the visitors seated in various parts of the auditorium were treated to an aerated rendition of the music which was being created many yards away. By this arrangement Washington merrymakers will soon be able to dance to the music made by an orchestra on one of New York's roof gardens.
    Among other wireless, radio, and telephone exhibits was an automatic telephone switchboard of the kind proposed not long ago for Washington. By this device connections are automatically made by the caller and the phone bell at the number called is rung at intervals of about fifteen seconds until the call is answered or until the party making the call breaks the circuit by hanging up his receiver.

2,000  Turns  a  Minute.

    The apparatus set up for demonstrating the synchronization of a machine gun with the airplane propeller was put into actual operation, and a belt of cartridges was fired through a metal plate affixed to a propeller revolving at a speed of 2,000 revolutions a minute. At this speed the tips of the propeller blade travel at a speed of seven miles a minute, or 420 miles an hour. Yet none of the bullet holes in the metal plate were within six inches of the blades.
    By the use of the radio telephone an aviator can locate the exact position of a landing field with which he is unfamiliar, even on the darkest night, by means of a large electric coil throwing off sound waves, which are intensified by the approach of the airplane. The closer the pilot approaches the field the louder the noise he hears.