186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
at any particular instant being substantially constant. These varia- 
tions in the direct current supply of the oscillator tube result in 
corresponding variations of its high frequency output so that the 
result is a modulated radio frequency current supplied to the antenna. 
The telephone transmitter furnished one of the most difficult 
problems in connection with the design of the transmitting set by 
reason of the fact that this transmitter operates under conditions 
of such extreme noise, due to the wind and engine exhaust. After a 
very extended period of development the present form of transmitter 
was perfected, its improvement 
over the ordinary form being 
PEEP roughly indicated by the state- 
Fe a : : 
rit tryttert}] ment that the ratio of;;noise 
|| ITI IT | signals to speech signals in the 
output of this transmitter is 
probably less than 1 per cent 
of the same ratio for a trans- 
mitter of the usual type. 
Reference to the schematic 
diagram indicates the adjust- 
ments of the transmitting set 
for different wave lengths to in- 
clude only a variable inductance 
ee anan and a variable capacity. Inas- 
rTiTT?TLreriett_tei. much as the apparatus is ad- 
justed for a particular wave 
length before the airplane leaves 
the ground, an artificial an- 
tenna whose constants approximate those of the normal antenna is 
used for making such adjustments on the ground, 
PLATE CURRENT=MILS 
“2.3.45, 6. 7 B. GAONINZNTIF LS 16 
FILAMENT CURRENT 
Fic. 2.—Characteristic curve, regulator tube. 
RECEIVING SET. 
The circuits of the receiving set are likewise shown schematically 
in figure 8. Reference to this figure will indicate that the circuit 
comprises a single resonant circuit, a vacuum tube detector, two 
vacuum tubes used as amplifiers, and a special helmet containing 
the receivers. The detector and amplifier are not particularly novel 
in any respect, but the helmet containing the receivers is of very 
unusual construction. The same noise conditions which were men- 
tioned above for the telephone transmitter likewise affected the recep- 
tion of signals in the telephone receivers. It was accordingly neces- 
sary to develop a special sound-insulating helmet in which the tele- 
phone receivers were mounted in such a way as to exclude almost 
completely from the aviator’s ears all sounds except those emanating 
