INVISIBLE RADIATIONS
(1) ELECTRIC WAVES AND RADIO-ACTIVITY
=415. Oscillatory Nature of the Spark from a Leyden Jar.=--In studying
sound (Art. 339), the sympathetic vibration of two tuning forks having
the same rate of vibration was given as an illustration of resonance.
The conditions for obtaining _electrical resonance_ by the use of two
Leyden jars are given in the following experiment.
Join the two coats of a Leyden jar (Fig. 413) to a loop of wire
_L_, the sliding crosspiece _M_ being arranged so that the length
of the loop may be changed as desired. Also place a strip of
tinfoil in contact with the inner coating and bring it over to
within about a millimeter of the outer coating as indicated at _G_.
Now join the outer coating of another exactly similar jar _A_ to a
wire loop of fixed length, the end of the loop being separated from
the knob connected to the inner coating, a short distance at _P_.
Place the jars near each other with the wire loops parallel and
connect coatings of _A_ to the terminals of a static machine or an
induction coil. At each discharge between the knobs at _P_, a spark
will appear in the other jar at _G_, if the crosspiece _M_ is so
adjusted that the areas of the two loops are exactly equal. When
the wire _M_ is moved so as to make the areas of the two loops
quite unequal, the spark at _G_ disappears.
[Illustration: FIG. 413.]
The experiment just described shows that two electrical circuits can be
_tuned_ by adjusting their lengths, just as two tuning forks may be
made sympathetic by adjusting their lengths. This fact indicates that
the discharge of the Leyden jar is _oscillatory_, since resonance can
plainly not be secured except between bodies having natural periods of
vibration. This same fact is also shown by examining the discharge of a
Leyden jar as it appears when viewed in a rapidly revolving mirror. (See
Fig. 414.) The appearance in the mirror shows that the discharge is made
up of a number of sparks, often a dozen or more, vibrating back and
forth until they finally come to rest. The time of one vibration varies
from one millionth to one hundred millionth of a second, depending on
the space between the discharging balls and the size of the jars.
[Illustration: FIG. 414.--Photograph of the oscillatory discharge of a
Leyden jar.]
The discharge of a Leyden jar or of another condenser sets up ether
waves that have the speed of light. Heinrich Hertz in Germany first
proved this in 1888. These waves are now known as Hertzian waves. The
length of these varies from 3 cm. to several miles, depending upon the
size and conditions of the discharging circuit.
[Illustration: FIG. 415.--A coherer.]
=416. The Coherer.=--The coherer is a device for detecting electric
waves. It consists of a glass tube with metal filings loosely packed
between two metal plugs that fit the tube closely. (See Fig. 415.) These
filings offer a _high_ resistance to the passage of an electric current,
but when electric waves pass through the filings these _cohere_ and
allow a weak current to pass through. This current may be strong enough
to operate a relay connected with a sounder or bell that gives audible
signals. If the tube be tapped the filings will be disturbed and the
resistance again made so high that no current can pass through.
=417. Wireless Telegraphy.=--In 1894 Marconi, then a young man of
twenty, while making some experiments with electrical discharges
discovered that the coherer would detect electrical waves at a
considerable distance from their source and that by the use of a
telegraph key the "dots and dashes" of the telegraph code could be
reproduced by a sounder attached to a relay. At present the coherer is
used principally in laboratory apparatus, as much more sensitive
detectors are now available for commercial work. The essential parts of
a modern wireless telegraph apparatus as used in many commercial
stations are shown in Fig. 416.
Alternating current at 110 volts is sent into the primary, _P_, of
a transformer, the secondary, _S_, of which produces a potential of
5000 to 20,000 volts. The secondary charges a condenser until its
potential becomes high enough to produce a discharge across a spark
gap, _SG_. This discharge is oscillatory, the frequency being at
the rate of about one million a second, depending upon the capacity
of the condenser and the induction of the circuit.
These oscillations pass through the primary of the oscillation
transformer, inducing in the secondary, electric oscillations which
surge back and forth through the antennæ, or aerial wires, _A_.
These oscillations set up the "wireless waves." The production of
these waves is explained as follows: An electric current in a wire
sets up a magnetic field spreading out about the conductor; when
the current stops the field returns to the conductor and
disappears. The oscillations in the antennæ, however, have such a
high frequency, of the order of a million a second, that when one
surge of electricity sets up a magnetic field, the reverse surge
immediately following sets up an opposite magnetic field before the
first field can return to the wire. Under these conditions a
succession of oppositely directed magnetic fields are produced
which move out from the antennæ with the speed of light and induce
electric oscillations in any conductors cut by them.
[Illustration: FIG. 416.--Diagram of a commercial wireless telegraph
apparatus.]
While the electric waves are radiated in all directions from the
aerial, the _length_ of the waves set up is approximately four
times the combined length of the aerial wires and the "lead in"
connection to the oscillation transformer.
The electric waves induce effective electrical oscillations in the
aerial of the receiving station, even at distances of hundreds of
miles, provided the receiving transformer, _RT_, is "tuned" in
resonance with the transmitting apparatus by adjustments of the
variable condenser, _VC_, and the loading coil, _L_. The _detector_
of these oscillations in the receiving transformer is simply a
crystal of silicon or carborundum, _D_, in series with two
telephone receivers, _Ph_. The crystal detector permits the
electric oscillations to pass through it in one direction only. If
the crystal did not possess this property, the telephone could not
be used as a receiver as it cannot respond to high frequency
oscillations. While one spark passes at _SG_, an intermittent
current passes through the receiver in one direction. Since some
300 to 1200 sparks pass each second at _SG_ while the key, _K_, is
closed, the operator at _Ph_ hears a musical note of this frequency
as long as _K_ is depressed. Short and long tones then correspond
to the dots and dashes of ordinary telegraphy. In order to maintain
a _uniform tone a rotary spark gap_, as shown, is often used. This
insures a tone of fixed pitch by making uniform the rate of
producing sparks.
The _Continental_ instead of the _Morse_ code of signals is generally
employed in wireless telegraphy, since the former employs only _dots_
and _dashes_. The latter code employs, in addition to dots and dashes,
_spaces_ which have sometimes caused confusion in receiving wireless
messages. The United States government has adopted the regulations of
the _International Radio Congress_ which directs that commercial
companies shall use wave lengths between 300 and 600 or above 1600
meters. Amateurs may use wave lengths less than 200 meters and no
others, while the government reserves the right to wave lengths of 600
to 1600 meters. See p. 459 for Continental telegraph code.
=418. Discharges in Rarefied Air.=--Fig. 417 represents a glass tube 60
or more centimeters long, attached to an air pump. Connect the ends of
the tube to the terminals of a static machine or of an induction coil,
_a-b_. At first no sparks will pass between _a_ and _f_, because of the
high resistance of the air in the tube. Upon exhausting the air in the
tube, however, the discharge begins to pass through it instead of
between _a_ and _b_. This shows that an electrical discharge will pass
more readily through a partial vacuum than through air at ordinary
pressure. As the air becomes more and more exhausted, the character of
the discharge changes. At first it is a faint spark, gradually changing
until it becomes a glow extending from one terminal to the other and
nearly filling the tube.
[Illustration: FIG. 417.--An Aurora tube.]
_Geissler tubes_ are tubes like the above. They are usually made of
different kinds of glass twisted into various shapes to produce
beautiful color effects. The _aurora borealis_ or northern light is
supposed to be electric discharges through rarefied air at the height of
from 60 to 100 miles above the earth's magnetic poles. (See Fig. 418.)
[Illustration: FIG. 418.--Aurora Borealis.]
=419. Cathode Rays.=--When the tube in Art. 420 is exhausted to a
pressure of 0.001 mm., or a little less than one millionth of an
atmosphere, the character of the discharge is entirely changed. The
tube becomes filled with a yellowish green phosphorescent light. This is
produced by what are called cathode rays striking the glass walls of the
tube. These rays are called cathode rays because they come from the
cathode of the tube. They are invisible and that they travel in straight
lines is shown by the shadow obtained by using a tube with a screen
(Fig. 419).
[Illustration: FIG. 419.--A cathode ray tube.]
=420. "X" Rays.=--In 1895, Professor Röntgen of Wurtzburg, Germany,
discovered that when the cathode rays strike the walls of the tube or
any solid within it they excite a form of invisible radiation. This
radiation is called Röntgen rays, or more commonly, "X" rays. Careful
experiments show that they travel in straight lines, and that they can
not be reflected or refracted as light waves are. They pass through
glass and opaque objects such as flesh, cardboard, cloth, leather, etc.,
but not through metallic substances. The tube in Fig. 420 has a screen
covered with crystals which become luminous when struck by the cathode
rays. On bringing a magnet near the tube the luminous line is raised or
lowered showing that the magnetic field affects the stream of cathode
rays, attracting it when in one position but repelling it when in the
reverse direction. The cathode rays which cause the bright line possess
a negative charge of electricity. They are now believed to be electrons
shot off from the surface of the cathode with speeds that may reach
100,000 miles a second. "X" rays possess no electrical charge whatever
and cannot be deflected by a magnet. They produce the same effect on a
photograph plate as light does, only more slowly. Hence, they can be
used in taking "X" ray photographs. Certain crystals, like barium
platinum cyanide, fluoresce when struck by the "X" rays. The
_fluoroscope_ is the name given to a light-tight box closed at one end
by a cardboard covered with these crystals (Fig. 421). On looking into
the fluoroscope with an opaque object such as the hand placed between
the screen and the "X" ray tube, a shadow of the bones of the hand can
be seen upon the screen of the fluoroscope. (See Fig. 422.)
[Illustration: FIG. 420.--The stream of cathode rays is deflected by a
magnet.]
[Illustration: FIG. 421.--A fluoroscope.]
[Illustration: FIG. 422.--A view of the "shadow" of a hand as seen in a
fluoroscope.]
A special form of the tube is used. (See Fig. 423.) In this tube a
platinum disc is placed at the focus of the concave cathode. This
concentrates the "X" rays in one direction. It is now generally believed
that "X" rays are waves in the ether set up by the sudden stoppage of
the cathode rays at the platinum anode.
[Illustration: FIG. 423.--An "X" ray tube.]
=421. The Electromagnetic Theory of Light.=--The study of electric waves
has shown that they are similar to light waves in many respects: (a)
they have the same velocity; (b) they can be reflected and refracted.
The main difference is in their length, light waves being very much
shorter. In 1864 James Clerk Maxwell, an English physicist, proposed the
theory that ether waves could be produced by electrical means and that
light waves are electromagnetic. In 1888 Hertz proved by his experiments
that ether waves having the same velocity as light could be produced in
this way. It is now the general belief that light waves are ether waves
produced by the vibrations of the electrons within the atoms and that
they consist of electromagnetic waves in the ether.
=422. Radio-activity.=--In 1896 Henri Becquerel of Paris discovered that
uranium and its compounds emit a form of radiation that produces an
effect upon a photographic plate that is similar to that resulting from
the action of "X" rays. These rays are often called _Becquerel_ rays in
honor of their discoverer. The property of emitting such rays is called
=radio-activity=, and the substances producing them are called
=radio-active=.
In 1898, Professor and Mme. Curie after an investigation of all the
elements found that _thorium_, one of the chief constituents of
incandescent gas mantles, together with its compounds, was also
radio-active. This may be shown by the following experiment:
Place a flattened gas mantle upon a photographic plate and leave in
a light tight-box for several days. Upon developing the plate in
the usual way a distinct image of the mantle will be found upon the
plate.
=423. Radium.=--Mme. Curie discovered also that pitch-blende possessed
much greater radio-active power than either thorium or uranium. After
prolonged chemical experiments she obtained from several tons of the ore
a few milligrams of a substance more than a million times as active as
thorium or uranium. She called this new substance _radium_. Radium is
continually being decomposed, this decomposition being accompanied by
the production of a great deal of heat. It has been calculated that it
will take about 300 years for a particle of radium to be entirely
decomposed and separated into other substances. It is also believed that
radium itself is the product of the decomposition of uranium, atomic
weight 238, and that the final product of successive decompositions may
be some inert metal, like lead, atomic weight 207.
The radiation given off by radio-active substances consists of three
kinds: (A) Positively charged particles of helium called _alpha_ rays:
(B) negatively charged particles called _beta_ rays: (C) _gamma_ rays.
The alpha rays have little penetrating power, a sheet of paper or a
sheet of aluminum 0.05 mm. stopping them. Upon losing their charges they
become atoms of helium. Their velocity is about 1/10 of that of light or
18,000 miles a second. The _spinthariscope_ is a little instrument
devised by Sir Williams Crookes in 1903 to show direct evidence that
particles are continually being shot off from radium. In this instrument
(Fig. 424), a speck of radium _R_ is placed on the under side of a wire
placed a few millimeters above a screen _S_ covered with crystals of
zinc sulphide. Looking in the dark at this screen through the lens _L_,
a continuous succession of sparks is seen like a swarm of fireflies on a
warm summer night. Each flash is due to an alpha particle striking the
screen. The beta rays are supposed to be cathode rays or electrons with
velocities of from 40,000 to 170,000 miles a second. The gamma rays are
supposed to be "X" rays produced by the beta rays striking solid
objects.
[Illustration: FIG. 424.--A spinthariscope.]
=424. The discovery of radio-activity= has revolutionized the ideas of
the constitution of matter. Further, the results of experiments upon
radio-active materials reveals the presence of immense quantities of
sub-atomic energy. If man ever discovers a means of utilizing this, he
will enter a storehouse of energy of far greater extent and value than
any of which he has as yet made use. A consideration of this unexplored
region gives zest to the work of those who day by day are striving to
understand and control forces of nature.
Important Topics
1. Oscillatory nature of discharge of Leyden jar. Proofs.
2. Wireless telegraphy and telephony.
3. Electrical discharges in rarefied gases.
4. Cathode and "X" rays.
5. Electromagnetic theory of light.
6. Radio activity and radium.
[Illustration: CONTINENTAL TELEGRAPH CODE
A . - J . - - - S . . .
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C - . - . L . - . . U . . -
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F . . - . O - - - X - . . -
G - - . P .- - . Y - . - -
H . . . . Q - - . - Z - - . .
I . . R . - .
PERIOD INTERROGATION EXCLAMATION
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