3B SCIENTIFIC PHYSICS U19157 Manuel D'utilisation page 11

•
Reduce the anode voltage again until the dis-
charge ceases. Record this extinguishing volt-
age U , too.
E
Another experiment can show how an excited dis-
charge can continue without the exciting trigger.
•
To this end set the anode voltage to about 10 V
more than the extinguishing voltage.
•
Irradiate the gas-filled space between the elec-
trodes with UV light.
•
Once the anode voltage is applied, turn off the
mercury lamp.
The discharge that was excited (by UV light) still
continues although the exciting trigger has been
removed.
5.3 Inelastic collisions between electrons and
helium atoms
The electrons emitted from the cathode are accel-
erated into the space between the cathode and the
grid (held at earth potential) and, having passed
through the mesh of the grid, only arrive at the
anode (at -6 V) if they possess sufficient kinetic
energy to overcome the field acting against them.
Electrons collide with helium atoms, either elasti-
cally or, at certain kinetic energies, inelastically
whereby they give up some of their energy. Elec-
trons that overcome the field cause a current to be
measured between the anode and ground.
•
Set up the experiment as in fig. 4. Dim the
lights in the room.
•
For a reverse polarity U
the accelerating potential U
and observe the anode current I
Up to an accelerating potential of about 24 V, the
current increases (elastic collision) but then it drops
suddenly. Electrons are now colliding inelastically
with the He atoms and thus losing kinetic energy.
This causes the helium atoms to become excited
and they start to emit quanta of light.
As the accelerating potential U
the current I increases once again but after an-
A
other 20 V or so it drops again.
A plot of the anode current should exhibit two
clear maxima. If this is not perceptible, the fila-
ment voltage should be lowered somewhat.
5.4 Characteristics of a thyratron
5.4.1 Recording the I – U
A
tron
To investigate the function of the grid, determine
the anode current I as a function of the grid volt-
A
age U at constant anode voltage U
G
•
Set up the experiment as in fig. 5a. Dim the
lights in the room and apply the filament volt-
age.
•
Set the grid voltage U
of 6 V gradually raise
R
from 0 V to 70 V
A
.
A
is further increased
A
characteristic of a thyra-
A
.
A
to -20 V.
G
•
Set the anode voltage U
•
Increase the grid voltage U
2 V then go back to -20 V. Plot the change in
the anode current I
At about 10 V the gas discharge 'strikes'. Further
changes to the grid have no effect on the anode
current (see fig 5b) because all the positive charge
carriers produced by the discharge accumulate
around the grid (where the potential is lowest) and
cannot pass on their charge.
5.4.2 Striking and extinguishing voltages as a func-
tion of the grid voltage
•
Set up the experiment as in fig. 5a. Dim the
lights in the room.
•
Set various grid voltages U
etc.) and determine the relevant striking and
extinguishing voltages by raising and lowering
the anode potential U
5.4.3 Thyratron rectifier
To use the gas triode as a rectifier it needs to be set
up with an alternating anode potential from an
isolating transformer. An oscilloscope is used to
measure the voltage drop across a resistor that is
directly proportional to the changes in the anode
current.
•
Set up the experiment as in fig. 6. Dim the
lights in the room.
•
Apply voltage to the filament.
•
Apply the anode potential with a grid voltage
U of 0 V.
G
•
Reduce the grid voltage down to -20 V and
observe the change in the anode current on
the oscilloscope
5.4.4 Thyratron oscillator
To generate oscillations of various frequencies, a
capacitor connected in parallel with the anode is
charged via a resistor. When the 'striking' voltage U
is attained, the capacitor quickly discharges. When
the capacitor voltage drops below the extinguishing
voltage conduction ceases and the capacitor
charges back up again. The frequency of the oscil-
lation f is not only dependent on the time constant
RC but on the negative potential U
which affects the 'striking' voltage U
that changing the grid voltage changes the fre-
quency of oscillation f.
•
Set up the experiment as in fig. 7. Dim the
lights in the room.
•
Apply a voltage of -20 V to the grid.
•
Perform the experiment with anode potentials
between 100 V and 150 V.
•
In each experiment, gradually increase the grid
voltage until the gas discharge begins.
3
to 100 V.
A
to 0 V in steps of
G
.
A
(-20 V, -15 V, -10 V
G
.
A
at the grid,
G
. This means
S
S