WIRELESS TELEPHONY—SLAUGHTER. 179 
VACUUM TUBB. 
The requirements of a radio frequency generator may be grouped 
as follows: 
The desired frequency may range roughly between the values 
of 15,000 and 6,000,000 cycles per second. 
The frequency for any particular generator may be required 
to vary over a considerable range, in some cases as much as 
several octaves. In other cases, a single value of frequency is 
sufficient. " 
The frequency when set at a particular value should remain 
substantially unaffected by ordinary changes in the physical or 
electrical conditions associated with the station. 
The current delivered by the generator should approximate 
a sine wave as closely as possible. 
The required output for different classes of stations may vary 
from less than one watt to several hundred kilowatts. 
The efficiency of the generator should be reasonably high 
though not necessarily comparable with the efficiencies obtained 
from ordinary types of generators. 
When used as an oscillator or radio frequency generator associated 
with properly designed circuits, the vacuum tube will meet all of 
these requirements, with the exception that the power output of 
vacuum tubes as at present constructed is limited to not more than a 
few hundred watts per tube. 
The requirements of a modulator for radio telephony may be 
grouped as follows: 
The modulator should be actuated by the current from 
an ordinary microphone telephone transmitter or its equiva- 
lent. 
The modulator should faithfully reproduce, in its effect 
upon the radio frequency current, the wave-form of the 
telephone or speech current. 
The modulator should be capable of almost completely 
modulating the output of radio frequency generators whose 
power outputs may cover the range indicated above. 
These requirements are fulfilled to a remarkable extent by the 
vacuum tube used in properly designed circuits. 
Although the vacuum tube was invented in 1906, its development 
into a sufficiently practical form to be useful for wireless telephony 
was comparatively slow. This development was greatly accelerated, 
beginning in 1912, when the American Telephone & Telegraph Co. 
became interested in the vacuum tube for use in telephone repeaters. 
Rapid improvements were made in the design and construction of 
vacuum tubes, and at the same time experiments were conducted look- 
