Man's physical universe

xanabras

548 MAGNETISM AND KLKCTRICITY

that of the hydrogen ion. The fact, discovered by Thomson, that

these particles travel with a velocity nearly a third that of light in a

high vacuum when the proper voltage is used suggests that the mass of

these particles is very small. Millikan's experiment showed that the

charge on these cathode particles is exactly equal, though different in

sign, to that of the nucleus of the hydrogen atom. It is now known

that these small, negatively charged particles in the cathode rays are

the identical electrons mentioned so often in previous sections.

In the Crookes tube there is also a stream of positively charged

ions, which travel from the anode to the cathode. These rays have

little practical significance, although they have furnished some evidence

concerning the structure of atoms, which will be discussed in the

next Unit.

X Rays Have Unusual Properties.

Roentgen first observed the evidence of X rays when he shielded a

Crookes tube with black paper in a darkened room. A piece of paper

covered on one side wath a phosphorescent substance became luminous

in spite of the fact that the light was covered.

This experiment has

been described as an accident, but such a combination of circumstances

would not occur for many men.

Fig. 271. A shockproof 200,000-volt X-ray tube used for cancer treatment.

(Courtesy of the Westinghouse Electric and Manufacturing Company.)

From this experiment Roentgen concluded that those

rays would

pass through opaque substances. He found that they would pass

through a thousand-page book, two packs of cards, and thick blocks

of wood. Thin sheets of metal also permitted the rays to pass, but

thick layers prevented their passage. When the hand was held between

the Crookes tube and the fluorescent screen, shadows of the bones were

seen. He then tried taking pictures with X rays and was delighted to

find that it could be done. On Christmas Eve, 1895, Roentgen showed

his first X-ray pictures to an astonished group of German physicists.

Among these pictures were photographs of the bones of his hand and

of keys contained in a purse.

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