144 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
SILENT EARTH CIRCUITS. 
The electromagnetic constants of the 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; L’ 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; 
Lis the tuning inductance in the line circuit; A’ is the ammeter in the 
line. At the receiving end of the line C, is the line tuning condenser; 
L, is the line tuning inductance; P, is the primary of the oscillation 
transformer; S, 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- 
