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Page:Popular Science Monthly Volume 90.djvu/651

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Popular Science Monthly

��635

��There is no need of any rectifying crystal or other complex conducting system if the incoming oscillations are passed through a high-speed commuta- tor. The tone- wheel, formerly used at the Tuckerton Transatlantic Station, is an apparatus whose operation has been described as mechanical rectifica- tion. A nearly identical device is shown in U. S. Patent 1,172,017, which w^as applied for by R. A. Fessenden in 1906 and was issued during the past year. The drawing indicates an antenna I connected to earth 2 through a primary inductance 17, which serves both to tune the aerial system and to couple that circuit with the secondary coil 18. The upper terminal of 18 leads to a rotating contact arm 5 which passes at high speed over segments 7, 8 of a large commutator. The lower secondary con- nection is taken through a telephone 20 and the wire 37 to the group of alternate segments marked 8 and connected to-

[ gether through 8a.

I If the brush 5 is rotated at a high rate

it is possible to secure synchronism betv^een the time of contact and the

k alternations of the received-wave cur- rents. By arranging matters so that the rotating contactor rests upon a segment of the No. 7 group during each negative half- wave of received current and upon a No. 8 segment throughout each positive half-wave, a positive direct current will flow through the telephone whenever waves are received. Thus the system acts mechanically to convert the radio-frequency alternating current into direct current suitable for producing telephonic effects if interrupted. As shown in the diagram, only one-half of the wave-current is used ; by connecting another ear-piece, such as 20, to the remaining group of segments (No. 7) and carrying the circuit to the lower end of 18, both halves will be made eflfectiv'e.

When receiving sustained waves it is merely necessary to run the commutating contact at a speed slightly different from synchronism, if a musical note is desired. The interruptions then gradu- ally change in relation to the half-waves, so that for a time the rectification is positive and then for a time negative. The rapidity of change of rectification polarity depends upon the difference between the interruption frequency and that of the waves.

��Making an Inclosed Rotary Spark Gap

SEVERAL enclosed gaps such as are herein described hav'e been made by the writer and have not only proved very efficient but also have reduced the noise of the spark a great deal.

The dimensions given in this article

���The rotor is made from an aluminum zinc combination casting to prevent wearing

are of course applicable only to one size motor and may have to be changed. If the amateur has no motor, one of about i/io horsepower, which is either induc- tion or series wound, should be procured, as this power will allow for a larger and more accurate rotor. If an induction motor is used, the number of points or lugs on the rotor should be such that it will give a frequency of any odd multiple of 120, such as 360, 600, 840 or 1080 sparks per second. This will give a clear note or 60 cycles without any undertone. The rotor (Fig. i), is made from an aluminum zinc combination casting, as

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��Dimensions of the parts required for at- taching the handles to the spark-gap frame

this will not wear away very fast and is easy to work. Pure copper can also be used if one has a motor of }/% horse-

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