dion was a radio-frequency current modulated by the voice. The output
was amplified by a two-stage audion amplifier and supplied through a
coupling coil to the large antenna of the U. S. Navy Station at
Arlington. Fig. 121 shows the system.

[Illustration: Fig 121]

The audion amplifiers each consisted of a number of tubes operating in
parallel. When tubes are operated in parallel they are connected as
shown in Fig. 122 so that the same e. m. f. is impressed on all the
grids and the same plate-battery voltage on all the plates. As the grids
vary in voltage there is a corresponding variation of current in the
plate circuit of each tube. The total change of the current in the
plate-battery circuit is, then, the sum of the changes in all the
plate-filament circuits of the tubes. This scheme of connections gives a
result equivalent to that of a single tube with a correspondingly larger
plate and filament.

[Illustration: Fig 122]

Parallel connection is necessary because a single tube would be
overheated in delivering to the antenna the desired amount of power. You
remember that when the audion is operated as an amplifier the resistance
to which it supplies current is made equal to its own internal
resistance of _R_{p}_. That means that there is in the plate
circuit just as much resistance inside the tube as outside. Hence there
is the same amount of work done each second in forcing the current
through the tube as through the antenna circuit, if that is what the
tube supplies. "Work per second" is power; the plate battery is spending
energy in the tube at the same rate as it is supplying it to the antenna
where it is useful for radiation.

[Illustration: Pl. XI.--Broadcasting Equipment, Developed by the American
Telephone and Telegraph Company and the Western Electric Company.]

All the energy expended in the tube appears as heat. It is due to the
blows which the electrons strike against the plate when they are drawn
across from the filament. These impacts set into more rapid motion the
molecules of the plate; and the temperature of the tube rises. There is
a limit to the amount the temperature can rise without destroying the
tube. For that reason the heat produced inside it must not exceed a
certain limit depending upon the design of the tube and the method of
cooling it as it is operated. In the Arlington experiments, which I
mentioned a moment ago, the tubes were cooled by blowing air on them
from fans.

We can