178 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
The particular characteristics of these various units are described 
in detail hereinafter. 
Wireless telephony is subject to certain particular limitations in 
the same way as wireless telegraphy, notably that of interference due 
to atmospheric electricity or radio signals foreign to the desired 
signal. 
HISTORICAL SUMMARY. 
The first requirement for any wireless telephone station is a source 
of radio frequency current whose amplitude from cycle to cycle re- 
mains constant, except when varied by the modulation imposed upon 
it by the voice current. If variations in its amplitude occur, due to 
other causes, these variations will introduce disturbances which will 
cause the system to be deficient in the effective transmission of 
speech. It is at once evident that the original source of radio fre- 
quency current used in wireless telegraphy, namely, the oscillatory 
discharge of a condenser supplied with energy from a low frequency 
source, is entirely unsuited to the purposes of wireless telephony. 
With the development of the Poulsen arc the first successful at- 
tempts at radio telephony were begun. These attempts involved the 
second factor in a wireless telephone station, namely, that of modu- 
lating the radio frequency current in accordance with the currents 
supplied by a telephone transmitter. The early attempts to accom- 
plish this modulation, by means of microphones inserted directly in 
the antenna circuit or coupled to the circuit in various manners, were 
largely unsuccessful, due to the limitations of the microphone de- 
vices, such as the low current capacity and the small range of varia- 
tion of resistance. 
A second source of radio frequency current is the high frequency 
alternator, which has been developed in various forms and which has 
been likewise used with limited success for wireless telephone trans- 
mission. The same lack of a suitable modulating device handicapped 
the use of the high frequency alternator until the advent of the 
audion or vacuum tube. The characteristics of the vacuum tube have 
been fully described in many recent publications, and will be dis- 
cussed in this paper only in so far as these characteristics are directly 
applicable to the problems of wireless telephony. It will be seen 
from this subsequent discussion that the vacuum tube possesses in a 
remarkable manner the precise characteristics required for the gen- 
eration and modulation of radio frequency current for low power 
wireless telephone stations, and for the detection and amplification 
of radio signals of any character whatsoever. Its influence on the art 
of wireless telephony may well be compared with the influence of the 
gas engine on aviation. 
