Physics Part 65

1. Interference of light: evidence, reasoning involved, ill.u.s.tration.

2. Polarization of light: evidence, reasoning involved.

3. Nature of light, differences between sound and light.

Exercises

1. Make a list of the differences between sound and light and state briefly the evidence upon which the knowledge of these differences is based.

2. Why will a thickness of film that will produce interference of red light be different from that producing interference for green or blue?

3. Using the formula _n_ = _v_/_l_ compute the vibration rate for violet light if its wave length is considered as 0.00004 cm.

4. Explain how the fact of polarization affects the wave theory of light.

5. Show how it is possible by comparing the spectrum of the sun with that of a star to tell whether the star is approaching or receding from the earth.

Review Outline: Light

Light; speed, source, medium.

Straight Line Motion; shadow, umbra, penumbra, eclipse, image.

Photometry; Law of intensity, candle power, foot-candle.

Mirrors; Law of reflection; image--real, virtual; plane, curved, parabolic, mirrors.

Refraction; cause and effects; plate, prism, lens; total reflection.

Lenses; six forms, princ.i.p.al focus, center, lens equation, 1/_F_ = 1/_D_{o}_ + 1/_D_{i}_.

Optical instruments; eye, defects and correction, camera, microscope, etc.

Spectra; 3 kinds, dispersion, production of color effects, spectroscope, uses.

Nature of Light; wave theory, interference, polarization, significance.

CHAPTER XVII

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 ill.u.s.tration 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 k.n.o.b 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 k.n.o.bs 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.

[Ill.u.s.tration: 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 s.p.a.ce between the discharging b.a.l.l.s and the size of the jars.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 415.--A coherer.]

=416. The Coherer.=--The coherer is a device for detecting electric waves. It consists of a gla.s.s 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 pa.s.sage of an electric current, but when electric waves pa.s.s through the filings these _cohere_ and allow a weak current to pa.s.s 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 pa.s.s 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 princ.i.p.ally 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 pa.s.s through the primary of the oscillation transformer, inducing in the secondary, electric oscillations which surge back and forth through the antennae, 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 antennae, 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 antennae with the speed of light and induce electric oscillations in any conductors cut by them.

[Ill.u.s.tration: 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 pa.s.s 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 pa.s.ses at _SG_, an intermittent current pa.s.ses through the receiver in one direction. Since some 300 to 1200 sparks pa.s.s 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, _s.p.a.ces_ 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 gla.s.s 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 pa.s.s 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 pa.s.s through it instead of between _a_ and _b_. This shows that an electrical discharge will pa.s.s 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.

[Ill.u.s.tration: FIG. 417.--An Aurora tube.]

_Geissler tubes_ are tubes like the above. They are usually made of different kinds of gla.s.s 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.)

[Ill.u.s.tration: 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 phosph.o.r.escent light. This is produced by what are called cathode rays striking the gla.s.s 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).

[Ill.u.s.tration: FIG. 419.--A cathode ray tube.]

=420. "X" Rays.=--In 1895, Professor Rontgen 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 Rontgen 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 pa.s.s through gla.s.s 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.)