Page:Popular Science Monthly Volume 91.djvu/328

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��Popular Science Monthly

��The moving plate is pivoted on a vertical arm supported by two horizontal wires under tension, whose section is shown at 8, 9, and which are in turn held by the frame io. The lower end of the vertical

���A combined electrostatic telephone and car- bon microphone that is sensitive and rugged

arm carries an electrode which dips into the carbon granules of the differential microphone amplifier II. This variable- resistance cell has two opposing contact surfaces 12 and 13, and is in circuit with the battery 14 and the divided primary winding of the telephone transformer 15, 16. The telephones 17 are connected to the secondary 18.

In operation the supporting wires are stretched to the tightness which tunes them to vibrate at the group frequency of the desired incoming signals. Currents in- duced in the antenna by the arriving waves produce opposing charges upon the plates 7, 6 and cause an attraction. This moves the lever toward the contact 12 and away from 13, so changing the current in both branches of the primary of the telephone transformer. By suitably winding these two coils the effects upon the secondary are made to add, and the change of current resulting in the circuit containing the telephone causes it to respond. By this resultant action it becomes possible to secure responses to comparatively weak signals of the desired group frequency, while interference of other spark frequencies is largely reduced.

The same apparatus may be used on the heterodyne principle, by adding a local source of sustained waves which will inter- act with the incoming signals to produce musical-toned beats. In this case the sen- sitiveness of the device is still further In- creased. The tension of the supporting wires is adjusted to the pitch of the beat-note.

��Strong Wireless Signals in Winter Time

A SERIES of tests lasting over two years were completed some time ago, with the object of finding out how much stronger radio signals between two selected stations would be in winter than in summer. The test signals were sent nearly every day during that time, and the amount of power sent and the intensity of signals received were carefully measured. It was found that the best time of year was from November to February, and that then the messages were about six times as loud as during the months from May to August.

��A Testing Set That Does Not Use a Battery

THE testing set illustrated, which does not use a battery or magneto in the circuit, is novel and interesting. The current used is set up by the action of the saliva on the zinc and copper plates. While it is not recommended for constant use it can be worked in case of emergency.

The mouth piece is made of wood or fiber cut tapering at one end. It is about 2 in. long, Y2 i n - wide and % in. thick. A piece of sheet zinc is cut 2 in. long and 34 in. wide, also a piece of sheet copper of the same dimensions. These metal strips are fastened to the edges of the insulator so that there is no contact between them. Solder a small screw-eye on the outer end of each piece of metal for terminals.

���The current for making the test is set up by the action of the saliva on the metal

Connect a single head receiver in series as shown and place the block in the mouth. A distinct click may be heard when a clear circuit is made. — Albert Fertick.

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