312 



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. 



