1930-08-14 — Page 2

HONG KONG DAILY PRESS THURSDAY, AUGUST 14, 1930.

RADIO

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FEATURES

MORE ON THEORY OF THE VALVE.

AN EXPLANATION OF THE A.C. FILAMENT.

The valve playing such an im- portant part in the design of any receiver, its capabilities bearing directly on the choice of practic ally every other component in the set, it is but natural to assume that any amateur who is at all keen on

more of this article than the mere fact that it detects and amplifies.

As has been already mentioned in these columns, the term valve" was applied by the inventor, Sir Ambrose Fleming." At the time the

{BY COHERER."]

1.03 Valves.

It has been pointed out that the only service of the low tension bat tory is to heat. the filament; no electrical purpose is served.

*

A cheaper and sometimes more convenient, method of raising the his installation requires to know filament temperature has recently been put into practice whereby the energy required is obtained from AC apply mains, The field gen- crated around circuit carrying such a current would give rise to serious complications in such a de- licate instrument as the valve were A.C. current applied direct to the filament. Special valves have, there- foré to be used in which the elec- tron-emitting filament receives heat only from an element placed in close proximity to it, the latter being heated by A.C. supply but having no

Full particulars and terms on application. only function of the valve being to

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act as a non-return passage for an ̈electrical current, the term was a most obvious one. In view of the many uses a valve is now put to, bowever, perhaps the American

term

"Tube" is more correct.

The Electron Theory."

The basis of the electron theory is that all matter is made up of a number of atome each of which have a number of free electrons (negative electricity) associated with it. Heating of a certain material gives rise to agitation of the atoms, with the result that a number of these electrons are thrown off. A body having a de- ficiency of electrons would be posi- tively charged, whilst when in

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it becomes negatively charged. Such an electrically charged body

has a field of electrical energy, in its vicinity.

لولو

Already the action of the valve ran be followed, the heated filament giving off electrons which are at tracted to the plate or anode, the latter having a potential positive- relative to the filament,

The Space Chargë.

It follows that the space between. filament and anode will hold a large number of electrons, in other words, the space in itself will be electric- ally charged This is known as the. space charge" and the flow of electrons is termed the electropic stream or thermionic flow.

Such a space being negatively charged it follows that it will tend to drive the electrons back to their" source (the filament), like charges of electricity tending to repel.

The positive potential of the anode must therefore be sufficiently large to overcome this effect it all the electrons are to reach "the anode.

Efficiency Factors.

It is now obvious that the two main factors for obtaining an-elec- tronic flow through the vacuum in a valve are the temperature of the

filament and the degrees of posi

tive potential applied to the anode,

electrical connection

with the emitter. ".

This method of indirect heating known as an A.C. valve, is becom-" ing increasingly popular, especially where mains units are employed. An important advantage is that the emitting flament or cathode is at the same potential at all points of its surface, giving a far better emission characteristic than the majority of direct flament valves.

SOME USEFUL HINTS.

Two of the most common causes of poor quality are the use of in. efficient high tension and improper valves.

An old high tension battery should never be connected in series

with a new one.

*

Although grid-bias batteries do not supply current to a set, they deteriorate in time, and should not be expected to last longer than six months or so.

As the value of the grid lenk has na important effect upon reaction in a short wave set, it is always advisable to try the effect of differ- ent values of grid leak if these are available,

*

An ordinary neutrodyne conden- ser, connected in series with the aeris lead, makes an excellent "Loose coupling the means of aerial for short-wave work

+

.

Metal dust is one of the greatest. foes of good reception. Be sure to keep the set free from this when soldering terminals, etc."

*

Even soldered joints deteriorate rapidly in the open air in this coun try, so unless yours are large gener- ous blobs of solder, it is a good plan to examine them occasionally so as to make sure they are sound.

•

**

Tuning-coils should always be spaced well away from copper

or amilaz metal surfaces.

screens

*

The use of a counterpoise earth instead of a direct earth connec

often of assistance in tion is eliminating interference from out- side sources, such as electric rail- ways, tramcars, power-mains, etc.

*

One of the best forms of counter poise earth is to make a replica of the aerial erected at a height of ten or twelve feet from the ground, and well insulated.

The commonest cause of low fre queney howling is a run-down H.T. battery.

*

Do not use an L.F. amplifer which has no grid-bias battery in it. It is out of date, and can easily be altered.

Do not forget to keep the acid in the accumulator at the proper level.

HAVE YOU HEARD THESE?

Here are Hists of long-wayS and short-wave stations which should be picked up by anyone in Hong Kong who has a mode- rately good sot suitable for re- ceiving such signals. Success in picking up these stations also depends very largely upon fav ourable atmospheric conditions, Readers are invited to add to this list should they succeed in picking up any station not in. cluded in either of these liats.

LONG-WAVE STATIONS.

Call Kilo- Sign cycle KZLB. 1,153. K.S.M.S. 1,085 C.R.C. 1,070 KR.C..

Wave length (Metres) Station

200 Manila 277 Shanghai. 280 Tientain 310 Shanghai 320 Péping C.O.P.K. 945 343 Tokyo JO.A.K. 870 353 Hiroshima J.O.FK 850 355 Hong Kong Z.B.W. -848 357 Bombay V.U.B 361 Sapporo J.O.LE. 830 368 Keijo J.O.D.K. 370 Nagoya (J.0.0.K. 810 370.4 Calcutta V.U.C. 800.9 380 Kumamoto J.O.G.K 790 390 Sendai 395 Dairen 398: Rangoon 400 Osaka. 410 Canton 413 Manila

SHORT-WAVE - STATIONS.

STATION

CALL SIGH

in other words the value of the €7.65 | Dobintz (Germany)

66.13 Khabaroval (Bussia)RA.97. high and low tension batteries and 68.7 Nanob

38.8

•A.F.K...

[IGJ.

[RYN:

IMA.

V.S.I.A.B. P.C.L U.S.4PJ 2.B.J. 3.1.0. WAXAF.

the condition of the flament. " "60. Moscow

The degros of temperature on the 38.5

Komé Forth" fisment depends on the material of

Singapore which it is made. Purc. tungsten,

Kootwijk (Holland) of which the first flaments were made, required a white heat, about. 2,400 degrees centigrade. With the 31.30 Eindhoven (Holland) PCJ modern practice of thoriating the filament a much lower heat is auth- cient, about 1,250 degrees.

Valve Emission.

The emission factor of a valve of which one hears so much will ob

Dekok (Biem), 8166 Melbourne 81.13 Schenectady

31.28 Sydney

26.53 Chelmsford (England) 5.5.

245 Manlia 23.38

vionsly, depend on the heat passed 18,89 tectady

to the filament, the material used in ita construction and its over-all Kred..

having a very high emission at com- paratively low temperatures, the modern practice to cost a tung- sten filament with one of these elements. This class of valve is known as the dull-emitter, and enables the use of two or four volt batteries for filament supply.

Kito

CTCLEA

840

890

J.O.H.K 770 J.D.A.K 760. V.U.R.754 J.O.B.K. 750 C.M.B. 782 KZ.B.M. 796

TIKE (HONG KONG)

от Подицке

4,434 Mon., Wed. & Fri, 6 p.m. & 2 a.m. 1000 8-10 p.m.

6,291 Not regular

0,000 Tues., Thurs, Sat, 8 p.m..

6,896 Sunday midnight

7142 Daily 6.00 pm. & 11 pm."

1,310 Not regular per

7,730 Dally 11 pm.

8,108 Tues & Fri, 0 p.m.-1 a.m.

9,230 Not regular

0,503 Not regular

9,620 || Fri, 8 am, Sat. 3 a.m. & 10 km.

4,690 Not regular

9,677 Midnight daily

10,620 Not regular

Midnight-3 a

daily

7.30 pan. & 8 am, daily, except

Saturday and Sunday.

13,850 41m.

Im. Wed, Fri. Sat.

16,102 Daily 0.80p.m.

2.F.C.

81

Nairobi (Kenya)

7.L.0.

28.5 Bydney

2.M.E.

Fandoeng

PL K

11,020

11,751

KLXE.

12,240 Nightly

W2LO.

PLO.. P.CL

P.L.F.

16.88 Enian (Holland)

H.8.1PJ. FEL

STILBO

WALK,

18.4.

Kootwijk (Holland) 37,4 Bandoong 16.9 Bangkok

13.74 Bandoong

Edy (Tace 18.08 Fittsburg

16,304 Dally 7 pm.

to midnight

17.280 Daily 8 pm to midnight

17.751 Sunday 7 p.m, & midnight 17,750 Daily 10 pm,

19220Daily 2.00 p

21,500 Not regular

(Allowance must be made for summer-time" in- pean countries, which is one hour ahead of true time.]

TALKS ON RADIO.

SOME RULES FOR SPEAKERS.

has made the development of an accurate directional aid to aircraft during the darkness a matter of urgent necessity, and a special re esiving aerial system for use of ground stations has been devised and successfully demonstrated to representatives of the Admiralty, Ideas, not merely words, must be Air Ministry, and Post Office which communicated to the audience in eliminates the unreliability of tak speeches that are broadcast on the jag bearings during darkness. The radio, says one of the periodic re importance of this result can be ports of the Advisory Committee understood, as it hastens the time on Education by Radio to the when regular night flights on com United States Ofice of Education.mercial routes can be carried out." Teaching by radio, the report adds, requires a particular kind of com- petenca. The right kind of voice is much more important than the right. kind of a degree.

The latest report incorporates 18" rules for preparing and delivering a radio speech which were propound- ed by Dr. A. S. Burrows. These are Dr. Burrows' suggestions for handling radio material:

Write out your exact wording. "Begin with one or more striking statements

"Present your specialty on the level of 13-year-olds.

Do not overrate the intelligence of your listeners.

Use a few pertinent historical allusions.

"Give informational details of your subject even though these seem too simple to you.

Anecdotes, short and clearly to the point, are good...

"Avoid too much generalization.

Minimize preachment and ad-

vice.

"Close with three or four short sentences of five or six words each, clinching the main points present ed.

"Revise for clearness and de- livery.

"Reread aloud several times for time and practice-or, better "still, memorise the talk..

"Speak distinctly directly in front of the microphone

"Do not change position while: speaking.

Sitting position is preferable."

WIRELESS AID TO DIRECTION FINDING.

Presiding at the meeting of Mar- coni's Wireless. Telegraph Company in London recently, Lord Inverforth said:-

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