Discovery of the Electron (English)

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1-7: The Discovery of the Electron

[Vacuum Discharge, the Cathode Rays]

Put a pair of plates

(electrodes)

in a glass tube

and apply a several kV

high voltage potential

between them.

When the gas pressure

in the tube

becomes very low

(lower than 0.1

atmospheric pressure),

an electric discharge

will take place.

This is called

vacuum discharge.

We can see a striped

pattern in the tube.

An example is shown in

the picture below.

When the gas pressure

in the tube

becomes about

0.000001 atmospheric pressure,

the striped pattern

will disappear

and the inside

of the tube will

become dark.

However this does

not mean

the discharge stopped;

a current is still

passing between the electrodes.

Namely, something is flowing

between the electrodes.

These are called

the cathode rays.

The equipment

to generate the cathode rays

was named

Crookes tube

after its inventor

W. Crookes

(UK, 1832 - 1919).

In order to investigate

the properties of

cathode rays

we place a cross

in the Crookes tube

as seen in the

following figures

and set a screen

of a fluorescent substance

beyond the cross

also inside the tube.

Then we can watch

a shadow of the cross

on the screen.

This implies

that the cathode rays

emitted from the cathode

toward the anode

travel in straight lines

to the screen.

[What are the Cathode Rays?]

J. J. Thomson

(UK, 1856 - 1940)

investigated the true nature

of cathode rays

(1897).

There have been

three famous British physicists

named "Thomson"

in the history

of science.

Do not confuse them.

(1)

W. Thomson

(William Thomson,

1824 - 1907).

"Lord Kelvin".

The unit of absolute

temperature K

is given after his name.

(2)

J. J. Thomson

(Joseph John Thomson,

1856 - 1940).

Under consideration

on the present page.

J. J. Thomson had

many achievements

including not only

the discovery of the electron

but the proposal

of the atomic model

discussed later

and

many other discoveries.

(3)

G. P. Thomson

(George Paget Thomson:

1892 - 1975)

A son of the above

J. J. Thomson.

G. P. Thomson

confirmed the diffraction

of electrons

with the use

of a metsllic crystal

to prove the wave

nature of electrons

independently of

the works of US physicists,

C. J. Davisson

and L. H. Germer.

The apparatus used

by J. J. Thomson

to investigate

the properties of

cathode rays

is schematically shown

in the following

figure.

Its basic idea

is essentailly the same as that

of the Crookes tube.

J. J. Thomson presumed

that the cathode rays

that came out of

the negative electrode

(cathode)

were a collection (beam) of the same particles with negative charges.

If they are so,

these particles

emitted from the cathode

would be pulled

by the positive

electrode (anode),

pass through a small hole

at the center

of the anode,

travel in a straight line,

pass between

the plates, P₁

and P₂,

and finally arrive

at a screen

of fluorescent substance

to make a small spot

on it.

If the electric

potential is

so applied

that the upper plate

is negative

and the lower

is positive, then

the beam would

be curved downward

and the spot on the screen

would move downward.

J. J. Thomson

observed this phenomenon,

and he also found

that the spot neither

spreaded greatly

nor faded.

We can conclude

from these results

that J. J. Thomson's

presumption is correct;

namely, the cathode rays

are a beam

of the same kind of

"particles"

with a negative charge.

[Charge-to-Mass Ratio of the Cathode Rays]

Using the experimental

set up discussed above,

we can measure

the charge-to-mass ratio

e /m

of the "particle"

in the cathode rays.

Here e is

the charge

of the particle and

m is its mass.

To explain the

details

of the method

to evaluate the

charge-to-mass ratio,

we need some

mathematical expressions,

which we

will explain

on another page:

1-7-A: Measurement of the Charge-to-Mass Ratio of Cathode Rays

Using the method

explained in the other page

(1-7-A),

J. J. Thomson measured

the charge-to-mass ratio

of the particle

in the cathode rays

and got a value of

A more precise

estimate is

J. J. Thomson confirmed

that an almost

constant value

of e /m

was always obtained

under various experimental

conditions.

He therefore concluded

that cathode

rays are a collection

of the same kind

of particles.

[The True Nature of the Cathode Rays, the Electron]

Let us compare

the charge-to-mass ratio

of the particle

in the cathode rays

with that of

the hydrogen ion.

As seen in

Faraday's Law of Electrolysis

on Page 1-6,

a charge of

is necessary to separate

1 gram equivalent element

through the electrolysis.

Consider the case

of hydrogen for example.

Because the valence

of hydrogen is 1

and the atomic weight

of hydrogen is about 1,

1 gram of hydrogen ions

have a charge of .

Namely the charge-to-mass ratio

of the hydrogen ion

is about .

Accordingly,

This inplies either

that the mass of the

"particles" in cathode rays

is about 1/1800

of the mass

of a hydrogen ion,

or that the "particle"

can carry 1800 times

more charge than

a hydrogen ion.

J. J. Thomson thought

that the latter is

not plausible.

Therfore he assumed

the former

and named the particle

in the cathode rays:

electron (1897),

whose mass is

by a factor of

about 1/1800 than

the lightest atom,

hydrogen.

[The Mass of the Electron]

A little later than

J. J. Thomson's research

on the electron,

the value of

the elementary charge

was clarified

by the Millikan oil

drop experiment

as explained on Page 1-6;

Millikan's Experiment

Thus it is quite natural

that the charge of the electron

would be equal

to the elementary charge e.

Accordingly, the mass

of the electron

is determined.

Today, the data

on the electron are precisely

measured as

[Electron as a Common Constituent of Atoms]

Various experiments

showed that

the properties

of cathode rays

do not depend

on the kind of gas

in the discharge tube.

Moreover, when a metal

is heated to

a very high temperature

around

a large number of electrons are

emitted from it.

They are called

thermal electrons.

O. W. Richardson

(UK, 1879 - 1959)

investigated the details

of the thermal electrons.

He considered

that the electrons

in atoms in a highly

heated metal

were strongly shaken

and rushed out of the metal.

Thus he confirmed

that the electron

is one of the common

constituents of atoms.

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