144 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1911. 



SILENT EARTH CIRCUITS. 



The electromagnetic constants of tlie apparatus employed in tele- 

 graphy and telephony over wire circuits are of the order of magnitude 

 of microfarads and henrys, and since no attempt is made at tuning, 

 these are constructed at present with no provision for continuously 

 varying the units. 



In wireless telegraphy and telephony these electromagnetic con- 

 stants are of the order of magnitude one thousand times smaller, or 

 are expressed in thousandths of microfarads and of henrys; further- 

 more, these forms of apparatus are provided with convenient means 

 of continuously varying their values for tuning. 



In the operation of providing tuning elements for earth connections 

 there is at the same time afforded a certain means of eliminating any 

 harmful disturbances from the earth, for the condensers employed 

 for tuning to frequencies above audition possess an impedance to the 

 frequencies involved in speech and also any disturbances from the 

 earth, which effectively prevents the passage of any disturbance of 

 audible frequency. These condensers offer a comparatively free pas- 

 sage to the electrical oscillations of the frequencies here being con- 

 sidered. When such earth connections are selectively tuned with the 

 line to frequencies entirely above audition it is evident that no audible 

 frequencies, either in the earth itself or from the line, can pass. Sim- 

 ple experiments proved the efficiency of this arrangement, and when 

 the metallic telephone circuit, equipped with a standard local battery 

 set, was connected to earth in the manner described, the operation 

 of the battery set was perfectly quiet and equally good with and with- 

 out such earth connections. 



The point was now reached where the road was clear for duplex 

 telephony, and for this purpose the apparatus and methods employed 

 in wireless telephony were applied to one of the wires of the metallic 

 circuit as though it were an antenna. The actual arrangement of this 

 circuit is shown in figure 4, in which G is the source of sustained high 

 frequency oscillations; C is the tuning condenser of the oscillatory 

 circuit; 1/ is the tuning inductance of the oscillatory circuit; P is the 

 primary of the oscillation transformer; A is the ammeter; M is the 

 transmitter microphone; S is the secondary of the ocsillation trans- 

 former in the line circuit ; C is the tuning condenser in the line circuit ; 

 L is the tuning inductance in the line circuit ; A' is the ammeter in the 

 line. At the receiving end of the line C x is the line timing condenser; 

 L t is the line tuning inductance ; P x is the primary of the oscillation 

 transformer; Si is the secondary of the oscillation transformer; L/ 

 is the tuning inductance in the oscillatory circuit; e/ is the tuning 

 condenser in the oscillatory circuit, between which and the tele- 



