We have already seen that a radiometer can be set in motion by invisible heat rays , and that the sensitized film on photographic paper is affected by invisible radiations of shorter wave length than those of violet light. We are now to consider invisible radiations set up by electrical means. If the discharge of a Leyden jar is sent through a metallic circuit and across a short air gap, it will set up a like discharge in the air gap in the circuit of a second similar jar, provided the circuits are parallel and the areas of the circuits are the same. This electrical resonance is analogous to the setting of a tuning fork in motion by the vibrations of a similar fork. If the forks do not vibrate at the same rate, the second fork will not be put in vibration, and if the area of the second circuit is changed, no spark will appear in the second air gap. The fork is put in motion by waves of air; but electrical resonance is caused by ether waves set up by the spark in the first circuit.

We have seen that the electric spark discharge or a Leyden jar is oscillatory. If the image of the spark is observed in a mirror revolving at high speed, it is seen to be a succession of flashes following each other at extremely short intervals.

The velocity of the ether waves set up by electrical oscillations was first determined by Heinrich Hertz in 1888, hence they are known as the Hertz waves. This velocity is the same as that of light, 300,000 km. per second. Hertz found that there were 10,000,000 oscillations per second, hence the wave length is 30 m. By reducing the size of the Leyden jar and the length of the circuit, and letting the discharge take place between two balls, the wave length can be made much shorter and the number of oscillations per second much greater.

Silver or nickel filings placed between two metal disks in a glass tube have an extremely high resistance. On the passage of an ether wave sent out by a Leyden jar spark, the filings cohere and offer little resistance. Marconi used this coherer as a detector of ether waves in wireless telegraphy, by putting it in a local circuit, containing a sounder or bell, which gave a signal on every passage of a wave from the sending station.

The sending station (Fig. 1) of a wireless telegraph system will, in general, include: a transformer, A, or a Ruhmkorff coil, the secondary of which is connected to the spark gap, B, the condenser C, and the primary helix D of an induction coil. The terminals of the spark gaр B are of zinc and  usually inclosed in a glass globe to reduce the noise of the discharge. The secondary helix E is grounded at one end and terminates at the other end in the antenna F, a wire or group of wires going high into the air. On closing. the key K, a spark jumps across B and by properly adjusting the size of the condenser C, the turns of wire in D and E, and the distance between D and E with a given area and elevation of the antenna, the two circuits BCD and EF may be so tuned that a maximum of electrical wave energy may be radiated into space from F.

These electric waves, traveling in all directions with the speed of light, may be roughly likened to the waves set up in still water when a stone is thrown into it. Gradually lessening in amplitude, they may be perceived in any direction at a distance which depends upon the sensitiveness of the detector.

The receiving station for wireless work (Fig. 2) consists of the antenna F, connected to one end of a coil H, the other end of which is grounded; a coil I, movable with respect to H; a detector J; a condenser C, and a telephone receiver.

The feeble electric waves reaching the antenna F, many million times per second, may be heard in the telephone, upon properly adjusting H to F and I to H, as a distinct musical tone every time the key K in the sending station is closed. The very rapid oscillations of the electric waves cannot be detected by the unaided telephone receiver because the diaphragm cannot vibrate fast enough. Hence it is necessary that the detector be so constructed that the train of waves sent out each time the sending key is closed shall be received as an individual signal the length of which depends upon the length of time the sending key is in contact.

Many devices have been developed that will do this with more or less efficiency. One which combines sensitiveness and convenience consists of a contact between two crystals or between a metal and a crystal. Silicon, zincite, bornite, carborundum, and molybdenite have been successfully used.

The action of this type of detector depends upon the fact that the contact between the crystals offers a much greater resistance to the passage of the current in one direction than in the other. Thus one part of the electric wave will produce current through the detector and the telephone, while that part of the wave which is in the opposite direction will produce no current and is unheard.

The form of antenna used depends upon the power of the station. For a small output the straight vertical form is generally sufficient. A flat top is sometimes used with the vertical, as in Fig. 545. For receiving, nearly any metal surface raised above the ground and insulated from it will serve as antenna. Signals from a neighboring sending station may frequently be heard in the receiver of an ordinary telephone system, the wires acting as antenna and the house fuse block as detector.

The invention of the audion for receiving wireless messages has made possible the use of a much smaller aërial than that shown in Fig. 3. The audion consists of a glass

bulb that looks much like an incandescent lamp. It contains, besides a lamp filament, a thin metallic plate and a wire grid, placed between the plate and the filament.

One method of coupling the audion to the receiving circuit is shown in Fig. 546. The coil antenna is from four to six feet across. Its sensitivity is at a maximum when the direction of the wireless wave is in the plane of the coil; hence, it can be used to locate the direction of the source of the wave if it is mounted on a vertical axis around which it can be turned.

A modified form of the audion serves to amplify the loudness of the signals. By coupling a number of audions in cascade arrangement accurate and clear results are obtained in extreme long distance wireless telephone transmission, such as that in transoceanic communication. The audion is also, for the physical investigator, a tool of the greatest  delicacy, applicable to investigations in many different fields.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يشرع بإدامة شباكين قديمين لمرقد الإمامين العسكريين (عليهما السلام)

فرقة العباس (عليه السلام) تختتم خدماتها المقدمة للزائرين في سامراء

موكب العتبتين المقدستين يحيي ذكرى استشهاد الإمام الهادي (عليه السلام) في سامراء

العتبة العسكرية المقدسة تكرّم وفد العتبة العباسية المقدسة لمشاركته في تقديم الخدمات للزائرين

المزيد

الآخبار الصحية

كنز غذائي على طبقك.. ماذا تفعل السبانخ بجسمك؟

ما لا يتوقعه أحد.. "سر غذائي" في الجبن الدسم

لتحسين الهضم والنوم.. هذا أفضل وقت لتناول العشاء

10 طرق بسيطة وغير مألوفة لحرق المزيد من السعرات يوميا

المزيد

الآخبار التكنلوجية

سابقة بعالم الطيران.. طائرة تقرر الهبوط بنفسها بعد ظرف طارئ

"الإنفلونزا الخارقة" تكتسح دول العالم.. وتثير قلقا كبيرا

مرحلة ما قبل الأعراض.. دواء جديد يستهدف "شرارة" الزهايمر

إزالة ثاني أكسيد الكربون من الغلاف الجوي.. ما دور المحيطات والبحار؟

المزيد

مواضيع ذات صلة

The Mercury Vapor Lamp

Alternating Current Transformer

The E. M. F. of a Dynamo

The Electric Motor

Lenzs Law

The Alternator

The Alternating Current

The Compound Dynamo

The Shunt Dynamo

The Series Dynamo

The Separately Excited Dynamo

The Field Magnet