Man's physical universe




tubes. Crookes observed flickering streams of light shooting from the

negative electrode to the positive electrode, which he found would

cause diamonds, rubies, and other substances to become fluorescent

or phosphorescent. The rays would

not penetrate mica or relatively


layers of certain metals but cast a

shadow of the pattern in which they

were cut.

He found that these rays would heat

a target placed in their path or cause

a paddle wheel to

revolve and therefore

concluded that the rays consisted

of particles having mass.

He also found that a beam of these

rays, passed through a slit in a shutter

Fig. 267. Crookes tube for

showing the shadow

placed in front of the negative electrode

(cathode) and made visible by a

of a cross.

(Courtesy of the Central Scientific


fluorescent screen placed in its path,

was deflected by a magnet.

These discoveries aroused the interest of the whole scientific world

as few others have ever done.

Later experiments in which the cathode rays were deflected by

both electric and magnetic fields proved that they were negatively


Hertz demonstrated that these rays, which would not penetrate

mica or certain other substances, would pass through thin sheets of

certain metals, such as aluminum.

In 1894 Leonard showed that

these cathode rays would affect

a photographic plate.

He was

able to obtain the rays by passing

them through an aluminum

window at the end of the tube.

In 1895 W. R. Roentgen studied

these rays that had been passed

through the aluminum window

and found that rays of another

Fig. 268.

Rolling wheel tube for showing

the kinetic energy of cathode rays,

(Courtesy of the Central Scientific Company.)

kind were emitted by the

Crookes tube itself and were produced whenever the cathode rays

struck an obstacle, such as a metal or glass target, placed in their path.

These were X rays, or Roentgen rays, as they are often called in honor

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