Physics Part 66

[Ill.u.s.tration: FIG. 420.--The stream of cathode rays is deflected by a magnet.]

[Ill.u.s.tration: FIG. 421.--A fluoroscope.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 const.i.tuents 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.

[Ill.u.s.tration: FIG. 424.--A spinthariscope.]

=424. The discovery of radio-activity= has revolutionized the ideas of the const.i.tution of matter. Further, the results of experiments upon radio-active materials reveals the presence of immense quant.i.ties 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.

[Ill.u.s.tration: CONTINENTAL TELEGRAPH CODE

A . - J . - - - S ...

B - ... K - . - T - C - . - . L . - . . U . . - D - . . M - - V ... - E . N - . W . - - F . . - . O - - - X - . . - G - - . P .- - . Y - . - - H ... . Q - - . - Z - - . .

I . . R . - .

PERIOD INTERROGATION EXCLAMATION ... ... . . - - . . - - . . - -

1 . - - - - 2 . . - - - 3 ... - - 4 ... . - 5 ... . . 6 - ... . 7 - - ...

8 - - - . . 9 - - - - . 0 - - - - -]

CHAPTER XVIII

WIRELESS TELEPHONY AND ALTERNATING CURRENTS

The developments in wireless communication have been so rapid during recent years that a more extended account, than that given in Art. 417 of the apparatus and methods used at the present time, seems desirable. The study of Alternating Currents is also included with the idea that it will make the text more complete and of wider usefulness.

WIRELESS TELEPHONY

=425. The Wireless Telephone.=--One of the most important developments in wireless communication in recent years has been in wireless telephony. We realize its possibilities, when we hear of the achievements of talking across an ocean or between airplanes and the ground.

The wireless telephone can be best understood by comparing it with the common telephone. When the latter is in use, a direct current flows continually through the instrument. (See Arts. 312-316.) When a person speaks into the transmitter, the sound waves of the voice cause the diaphragm to vibrate, this action causes rapid changes in the _resistance_ of the transmitter, which in turn causes the direct current to fluctuate just in step with the pulses of the voice waves. This fluctuating direct current pa.s.ses through the primary of an induction coil, producing in the secondary an intensified alternating current.

This pa.s.ses over the line wires to the receiver where it produces variations in the magnetic field affecting the receiver diaphragm, causing the latter to reproduce the voice of the person speaking in the transmitter. Now to make the comparison clear, two facts must be noted with regard to the wire telephone: first, there must be an action in the transmitter which causes variations in a current through the instrument; second, this fluctuating current produces a more intense alternating current which flows over the line and affects the receiver diaphragm, producing there sound vibrations of greater intensity than those used at the transmitter. This added energy comes from the current flowing through the transmitter. The case is a.n.a.logous to that of an electric bell. The armature of the bell vibrates with greater energy than is required to push the b.u.t.ton, the extra energy being derived from the battery.

=426. The Action of the Wireless Telephone.=--In the wireless telephone we have a continuous stream of electric waves of high frequency. (See Fig. 425_A_.) This stream of electric waves corresponds to the current that flows through the transmitter in the wire telephone. These waves are of such high frequency that even though we had a receiver diaphragm vibrating in step with the waves, we could not hear the sound because the human ear cannot hear a sound which consists of more than about 40,000 vibrations per second. The sound waves act upon this stream of waves very much, as in the wire telephone, the transmitter acts to modify the line current. The impulses caused by the voice are much slower than the electric waves first mentioned and these slower impulses are reproduced in the receiver. Not only are these slower impulses reproduced but they are _amplified_, that is, produced with greater energy than the impulses impressed on the stream of waves. Fig. 425_A_ represents as nearly as is possible in a diagram the continuous stream of electric waves. Fig. 426_B_, represents the impulses produced by the sound alone, and Fig. 426_C_, shows how these voice impulses are impressed on the stream of waves.

[Ill.u.s.tration: FIG. 425.--_A_, unmodified high frequency waves; _B_, waves of voice frequency; _C_, high frequency waves modified by waves of voice frequency.]

[Ill.u.s.tration: FIG. 426.--Vacuum tube, transmitting type. (_Western Electric Co._)]

[Ill.u.s.tration: FIG. 427.--Vacuum tube, receiving type. (_Western Electric Co._)]

[Ill.u.s.tration: FIG. 428.--Diagram of wireless telephone transmitting set.]

=427. The Vacuum Tube or Audion.=--The device by which all of this is accomplished is the _vacuum tube_. (See Fig. 426.) This tube contains three electrodes. _First_, a _filament_ (_F_, in Fig. 428) which is heated by a current from a battery (_B_{1}_, Fig. 428) and because it is heated, sends out a stream of electrons. _Second_, the _plate_ which forms the anode of the circuit from battery, _B_{2}_. This plate receives the electrons which are thrown off by the heated filament, hence a current flows through the circuit of _B_{2}_; the discharge through the tube depending on the e.m.f. between the filament and the plate. _Third_, a _grid_ is placed between the filament and the plate and is connected to the _secondary_ of the induction coil, the primary of which is connected to the transmitter. When the transmitter diaphragm is vibrating, the e.m.f. induced in the secondary of the induction coil causes a variation in the potential of the grid. This means a variation in the electric field between the filament and the plate. (See Fig.

428.) The changing electric field causes a variation in the discharge of electrons through the tube; the variation corresponds to the vibrations of the transmitter diaphragm. This produces a surging current of the frequency of the sound waves in the primary of the transformer (_T_, Fig. 428). The secondary of this transformer is connected to the antennae (_A_) and the earth (_E_). By means of the transformer, rapid surgings are set up in the antennae and these surgings produce a continuous stream of electromagnetic waves which goes out in s.p.a.ce. (Like Fig. 426_C_.) These electromagnetic waves produce oscillations in the antennae of a receiving station. The antennae transmit the impulses to a _tube_ (Fig.

427) which acts as a _detector_, and makes possible the reproduction of the sound by an ordinary telephone receiver.

[Ill.u.s.tration: FIG. 429.--View of wireless telephone set.]

The _vacuum tube_ in the transmitting circuit also _amplifies_ the impulses, that is, the energy of the waves given out is greater than that of the impulses which produce them, the additional energy being derived from the battery sending current through the plate and filament.

In operation, the filament and the plate are connected to a battery with a _condenser_ (_VC_) and an _inductance coil_ (_I_) in the circuit, as shown in Fig. 428. Photograph of a complete modern wireless telephone set is shown in Fig. 429.

ALTERNATING CURRENTS

=428. Alternating currents= are of interest to us because of their general commercial use. To understand the reason for the extensive application of alternating currents it is necessary to learn the fundamental principles which pertain to them. The production of such currents has already been explained in Arts. 300-304. It should be remembered that the current developed in the armature of a dynamo is alternating. A dynamo may _deliver_ a direct or an alternating current, depending on the method of collecting the current from the armature. If a _commutator_ is used, the machine delivers _direct_ current, if _slip rings_ are employed, an _alternating_ current is delivered.

=429. The Magnetic Field of an Alternating Current.=--The magnetic field of a direct current has been considered in Arts. 255-256. It has been shown to be arranged in circles about the conductor, according to the _Right Hand Rule_. (See Figs. 229 and 230.) These facts will help one to understand the following experiment:

If a number of magnetic compa.s.ses be arranged in a circle about a straight vertical wire carrying a direct current, the compa.s.s needles will point out a circle about the wire. (See Fig. 430, _A_.) If now the current be reversed the compa.s.s needles will reverse themselves and point in a direction just opposite to that taken at first. (See Fig. 430, _B_.) This will be clear if you imagine yourself walking around the wire in the direction the compa.s.s needles pointed at first, and then walking around the wire in the reverse direction. This ill.u.s.trates what happens in the field of an alternating current. The field reverses each time the current reverses.