School of Physics Lecture Demonstration Manual

Ef-3 Conduction of Electricity Through Gases

Published: Tuesday 14 March 2006 - Updated: Tuesday 29 March 2011

Aim

To demonstrate the phenomena associated with electrical discharge under reduced pressure.

Apparatus

Diagrams

Click on pictures to enlarge.

Conduction Thru Glass Diagram

Description

The apparatus is connected as shown in the diagram, care being taken that the H.T. leads do not contact earth at any point. Complete darkness is required for the details of the discharge to be seen in the lecture room. The rotary pump is started after the H.T. of the induction coil is applied.

The pressure at which the various phenomena are observed is detailed below.

 

PRESSURE (mm of Hg)

OBSERVATION

50

start of discharge

 

violet streamers

10

positive column

1

striations and Faraday dark space

0.5

Crookes dark space

0.1

striations vanish

 

negative glow, Crookes dark space

0.02

cathode glow, luminescence

 

At the completion of the demonstration switch off the induction coil, stop the pump and leak air slowly into the discharge tube.

Safety notes

Electrical safety 

Sharps safety 

Maintenance 

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