146 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
cycles per second, as measured by a standard wave meter such as is 
now commonly used in wireless telephony and telegraphy. ‘This 
source of high-frequency electromotive force was induced upon the 
high-frequency line wire in a similar manner to that described in the 
station at the Bureau of Standards, with the result that one of the 
wires of the twisted pair was made to carry simultaneously the bat- 
tery telephonic currents from the two transmitters, the high-frequency 
oscillations of about 100,000 cycles per second, applied at the Bureau 
of Standards, and the high-frequency oscillations of about 71,000 
cycles per second, applied at the laboratory. No influence from these 
conditions was perceptible upon the excellence of the battery trans- 
mission and reception of speech either way. 
DUPLEX TELEPHONY, USING METALLIC CIRCUIT. 
(A) BRIDGING ARRANGEMENT. 
The next experiments pertained to the standard metallic circuit 
as universally used on telephone toll lines in congested districts. 
The electric constants of this line have already been given. 
The next step was to remove entirely the earth connections from 
the metallic circuit and superimpose both telephonic circuits upon 
the same pair of wires, as shown in figure 6, in which the high-fre- 
quency apparatus, shown diagrammatically in figure 5, is bridged 
across the line wires A and A’. G is the source of sustained high- 
frequency oscillations; C, is the tuning condenser of the oscillatory 
circuit; L, is the tuning coil of the oscillatory circuit; P is the pri- 
mary of the oscillation transformer; A is the ammeter; M is the 
transmitter microphone; S is the secondary of the oscillation 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’ is the line tuning con- 
denser; L’ is the line tuning inductance; P’ is the primary of the 
oscillation transformer; S’ is the secondary of the oscillation trans- 
former; L’’ is the tuning inductance in the oscillatory circuit; C’’ is 
the tuning condenser in the oscillatory circuit, between which and 
the telephone F the detector D is operatively connected. 
The local battery telephone sets are connected across the line wires 
in the usual manner. In both sets, 1 is the microphone transmitter; 
2 is the local battery; 3 is the induction coil; 4 is the ringing system, 
including the bell and hand generator; 5 is the switch hook; 6 is the 
telephone receiver. 
Since the high-frequency apparatus as commercially developed in 
the wireless telegraph art was used, each of the units was variable 
and had been previously carefully calibrated by reference to the 
standards of the Bureau of Standards. The coupling coils were of 
