The Science Book

186

RAYS WERE

COMING FROM

THE TUBE

WILHELM RÖNTGEN (1845–1923)

L

ike many scientific

discoveries, X-rays were

first observed by scientists

studying something else—in this

case, electricity. An artificially

produced electric arc (a glowing

discharge jumping between two

electrodes) was first observed in

1838 by Michael Faraday. He

passed an electrical current

through a glass tube that had

been partially evacuated of air.

The arc stretched from the negative

electrode (the cathode) to the

positive electrode (the anode).

Cathode rays

This arrangement of electrodes

inside a sealed container is called

a discharge tube. By the 1860s,

British physicist William Crookes

had developed discharge tubes

with hardly any air in them.

German physicist Johann Hittorf

used these tubes to measure the

electricity carrying capacity of

charged atoms and molecules.

There was no glowing arc between

the electrodes in Hittorf’s tubes,

but the glass tubes themselves

glowed. Hittorf concluded that the

IN CONTEXT

BRANCH

Physics

BEFORE

1838 Michael Faraday passes

an electrical current through

a partially evacuated glass

tube, producing a glowing

electric arc.

1869 Cathode rays are

observed by Johann Hittorf.

AFTER

1896 First clinical use of

X-rays in diagnosis, producing

an image of a bone fracture.

1896 First clinical use of

X-rays in cancer treatment.

1897 J. J. Thomson discovers

that cathode rays are in

fact streams of electrons.

X-rays are produced when

a stream of electrons hits a

metal target.

1953 Rosalind Franklin

uses X-rays to help her to

determine the structure

of DNA.

When an electric current

is passed through a sealed

glass tube, cathode rays

cause part of the tube

to glow.

Fluorescent screens

near the tube also glow,

even when it is covered

in black cardboard.

Invisible rays

are coming

from the tube.

Some unknown type of

ray must have passed

through the cardboard to

make the screen glow.