ill remember
from our discussion of the regenerative circuit for receiving. For
example, also, in any receiving set the vacuum tube which detects is
usually amplifying. In the regenerative circuit for receiving continuous
waves by the heterodyne method the vacuum tube functions as a generator
of high-frequency current and as a detector of the variations in current
which occur because the locally-generated current does not keep in step
with that generated at the transmitting station.

Another example of a vacuum tube performing simultaneously two different
functions is illustrated in Fig. 120 which shows a simple
radio-telephone transmitter. The single tube performs in itself both the
generation of the radio-frequency current and its modulation in
accordance with the output of the carbon-button transmitter. This audion
is in a feed-back circuit, the oscillation frequency of which depends
upon the condenser _C_ and the inductance _L_. The voice
drives the diaphragm of the transmitter and thus varies the resistance
of the carbon button. This varies the current from the battery,
_B_{a}_, through the primary, _T_{1}_, of the transformer
_T_. The result is a varying voltage applied to the grid by the
secondary _T_{2}_. The oscillating current in the plate circuit of
the audion varies accordingly because it is dependent upon the grid
voltage. The condenser _C_{r}_ offers a low impedance to the
radio-frequency current to which the winding _T_{2}_ of
audio-frequency transformer offers too much.

[Illustration: Fig 120]

In this case the tube is both generator and "modulator." In some cases
these operations are separately performed by different tubes. This was
true of the transmitting set used in 1915 when the engineers of the Bell
Telephone System talked by radio from Arlington, near Washington, D. C.,
to Paris and Honolulu. I shall not draw out completely the circuit of
their apparatus but I shall describe it by using little squares to
represent the parts responsible for each of the several operations.

First there was a vacuum tube oscillator which generated a small current
of the desired frequency. Then there was a telephone transmitter which
made variations in a direct-current flowing through the primary of a
transformer. The e. m. f. from the secondary of this transformer and the
e. m. f. from the radio-frequency oscillator were both impressed upon
the grid of an audion which acted as a modulator. The output of this
au