3B Scientific Teltron Triode S Helium-filled User Manual

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3B SCIENTIFIC® PHYSICS

Gas Triode S with He-Filling 1000618

Instruction sheet

12/12 ALF

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54

21

1 4-mm plug for connect-

ing anode
2 Anode
3 Grid
4 Heater filament
5 Cathode plate
6 Connection pins
7 Guide pin

1. Safety instructions

Hot cathode tubes are thin-walled, highly
evacuated glass tubes. Treat them carefully as
there is a risk of implosion.

• Do not subject the tube to mechanical

stresses.

• Do not subject the connection leads to any

tension.

• The tube may only be used with tube holder

S (1014525).

When the tube is in operation, the terminals of
the tube may be at high voltages with which it is
dangerous to come into contact.

• Only use safety experiment leads for con-

necting circuits.

• Only change circuits with power supply

equipment switched off.

• Only exchange tubes with power supply

equipment switched off.

If voltage or current is too high or the cathode is
at the wrong temperature, it can lead to the tube
becoming destroyed.

• Do not exceed the stated operating parame-

ters.

When the tube is in operation, the stock of the
tube may get hot.

• Allow the tube to cool before dismantling.

Use of the equipment over long periods and
involving hefty gas discharges can cause loss of
material from the electrodes, which is deposited
on the glass of the tube itself resulting in darken­ing of the surface.

The compliance with the EC directive on elec­tromagnetic compatibility is only guaranteed
when using the recommended power supplies.

2. Description

The gas triode allows recording of the IA – U

A

characteristic of a thyratron, observation of ex­cited and non-excited discharge and of the dis­continuous transfer of energy by helium atoms
in inelastic collision with free electrons.

The gas triode is a helium-filled tube with a pure
tungsten heater filament (cathode) and a round
metal plate (anode) with a wire grid between
them, all inside a clear glass bulb. The cathode,
anode and grid are all aligned parallel to one
another. This planar configuration corresponds
to the conventional symbol for a triode. A circu­lar metal plate attached as a backing to the fila­ment ensures that the electric field between the
anode and cathode is uniform.

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3. Technical data

Gas filling: Helium
Filament voltage: ≤ 7.5 V AC/DC
Anode voltage: max. 400 V DC max.

Anode current: 10 mA typ. at U

= 300 V

a

Grid voltage: max. 30 V
Glass bulb: 130 mm diam. approx
Length of tube: 260 mm approx.

4. Operation

To perform experiments using the gas triode the
following equipment is also required:

1 Tube holder S 1014525
1 DC Power supply 500 V (@115 V) 1003307

or
1 DC Power supply 500 V (@230 V) 1003308

2 Analogue multimeter AM50 1003073

4.1 Setting up the tube in the tube holder

• The tube should not be mounted or removed

unless all power supplies are disconnected.

• Press tube gently into the stock of the holder

and push until the pins are fully inserted.
Take note of the unique position of the guide
pin.

4.2 Removing the tube from the tube holder

• To remove the tube, apply pressure with the

middle finger on the guide pin and the thumb
on the tail-stock until the pins loosen, then
pull out the tube.

5. Example experiments

5.1 Discharge, evidence of positive charge
carriers

• Set up the circuit as in fig. 1.

• To demonstrate the existence of positive

charge carriers (He

at a maximum heater filament voltage U
measure the current I

+

ions) for gas discharge

taking note of the

G

,

F

sign.

5.2 Non-self-sustaining discharge

• Set up the circuit as in fig. 2.

• Record a characteristic curve of I

U

(= UG) for various filament voltages U

A

against

A

(5

F

V …7.5 V).

At about 25 V the anode current I

increases

A

considerably in the gas triode. This increase is

accompanied by the appearance of a blue lumi­nescence. It is apparent that there are many
more charge carriers transporting charge than in
the vacuum triode (since there are He

+

ions as

well as thermal electrons).

5.3 Self-sustaining discharge

• Set up the circuit as in fig. 3.

• Gradually increase the anode voltage U

and determine the striking voltage U

for the

S

A

gas discharge.

• Reduce the anode voltage U

again until the

A

self-sustaining discharge ceases. Record

this extinguishing voltage U

.

E

5.4 Simplified Franck-Hertz-set-up

Experiment for demonstrating discontinuous
energy emission resulting from inelastic colli­sions between between electrons and helium
atoms. The electrons have to travel through a
decelerating reverse-potential field between the
grid and anode, so that they only arrive at the
anode if they possess sufficient kinetic energy.

Only then do they contribute to the current I

A

between anode and ground.

• Set up the circuit as in fig. 4.

• For a reverse polarity U

raise the accelerating potential U
to 70 V and measure the anode current I

• Plot a graph of the anode current as a func-

of 6 V, gradually

R

from 0 V

A

.

A

tion of the accelerating voltage.

Up to an accelerating potential of about 24 V,
the anode current increases but then it drops
suddenly. As the accelerating potential is further
increased the current increases once again but
after another 20 V or so it drops again.

A plot of the anode current should exhibit two
clear maxima. If this is not perceptible, the fila­ment voltage should be lowered somewhat.

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