THE WIRELESS WORLD

Otago Daily Times, Issue 22367, 14 September 1934, Page 4

THE WIRELESS WORLD

News and notes By Magna Vox. Items of local interest are invited by “ Magna Vox” for publication in this column. It is necessary that such matter should reach this office by Tuesday or each week for insertion on the following Friday. 2BL, Sydney.—Bss K.C., 853 metres. 2FC, Sydney.—o6s K. 0., 422 metre*. 3AB, Melbourne.—62o K.C.. 492 metres 3LO. Melbourne. —830 K.C., 371 metres. SCL. Adelaide—73o K.C., 395 metres. 4QG, Brisbane—76o K.C., 385 metres, IVA, Auckland.—6so K.C., 461.3 metres. 2YA, Wellington.—s7o .K.C., 526 metres. SYA, Christchurch—72o K.C., 416.4 metres. 4YA Dunedin—79o K.C.. 379.5 metres. 4YO. Dunedin—ll4o K.C., 261 metres. ANSWERS TO CORRESPONDENTS. G. F. R.. Dunedin.—Are the tuningcondenser plates touching? Note whether the noises vary when you rotate the variable tuning condenser. If at certain points you hear scratchings, examine the condenser - plates and see that the moving plates are not touching the fixed ones, at any point. • Make this test with the aerial disconnected in case it is an external source from which the noise is coming. “ Val ve,” Outram— If you have adjusted your grid bias as carefully as possible in steps of not more than halt a volt, and are still unable to obtain a practically constant reading from thy plate milliamcter as the signal is receiver, it is obvious that, irrespective of grid potential, the signal is so strong as to cause the tube to saturate, and the remedy is to use a larger tube with greater handling capacity and (or) of a higher plate voltage. “Scratch,” Seacliff.—(l) When the anode connection to a metallised screengrid valve is screened by a metal covering it often happens that this covering \nakes intermittent contact with the valve. This sets up. background noise somewhat similar to atmospherics. The same effect can be obtained if the earth connection to the braiding becomes defective, particularly if it has passed through a metal chassis. (2) You will find a full description of it in the handbook I loaned you last November, and which you have not yet returned. NOT FOR AMATEURS. An American tube, type 57, which has been on the market for some time in America, has an amplification factor of 1500 theoretically. It has never been possible, however, to attain even one quarter of this amplification, because it would be necessary to employ a resistance of 13.000,000 ohms and a plate supply of 27,000 volts, which is slightly inconvenient. A physicist of the University of St. Louis, Mr Otto H. A. Schmitt, has found a solution which permits him not only to utilise the total amplification of the tube, but even to exceed it. He employs another tube of the same type as a high resistance load in the plate circuit. With this arrangement he needs the plate supply from 300 to 600 volts. He has succeeded in delivering undistorted signals of 450 volts. The circuit employed cancels all bad effects due to variations in the supply voltages. The amplification remains perfectly constant, which has been ascertained during a test lasting three weeks. The variation of only .2 per.cent. A STUDIO TRAGEDY. Soviet radio realism, which has thrilled millions of listeners with dramas of real life enacted before the microphone, reached a terrible climax recently when an actual murder was committed in a broadcasting studio. From liflis, in the Caucasus, the faint voice of a police official could be heard describing a daring bank robbery for which a bandit was being sought. The official asked the inhabitants of Tiflis to come, forward with information and help. “ The man is wanted for the robbery of. 30,000 roubles from the Tiflis bank " repeated the voice. He is tall, broad. . . .” 1 The speaker was about to continue his description when three loud detonations were heard, followed by the crash of glass as bullets fired from the street outside crashed through the window into the broadcasting studio. “I ajri hit,” cried the official, his voice trailing off into a cough. Before the microphone was switched off the horrified listeners heard the clatter of rushing feet and gruffly-given orders | as the chase of the assassin began, Later a Moscow wireless station announced that no trace of the murderer had been found. The bandit of Tiflis, wanted for bank robbery, had taken his revenge on the man who was working to track him down. TROUBLES OF ANNOUNCERS. When the two .senior announcers at Broadcasting House made 8.8. C. history recently by being interviewed for the first time in 11 years, they confessed they were still nervous of the microphone. “Nearly every time I get into the studio to speak I am a. little frightened,” said Mr Stuart Hibberd, the chief announcer. “ And so am I,” said Mr Frederick Gnsewood. “ Sometimes, just as I am about to speak, I look at the microphone and have a sudden unpleasant feeling- that it is going to ‘get me down;’ 1, don’t think anyone ever quite overcomes microphopefriffiit.” Mr Hibberd said the, big business men, professors, and many damous people were more nervous at the microphone than anyone else. “We have even tried hiding the microphone or putting it up above them,” he said. “ I think the secret is to 'try and imagine that you are talking, say, to some person by your own fireside. It is fatal to let your mind dwell upon the millions of listeners.” “What I loathe,” said Mr Hibberd,_ “is a revolution in Peru or a new Chinese general. If I can, I ring up an embassy before trying to pronounce a foreign statesman’s name —even then the pronunciation given me is sometimes beyond any announcer’s capabilities! ” » TUNING BY EYE. The time is now definitely in sight when wireless receivers will no longer be tuned by ear. Even tbe cheapest sets are now provided with calibrated dials permitting tuning with little trouble. Highly ingenious tuning-in mechanisms have been created, making such optical control both simple and exact. But despite the greatest precision, it is still necessary to rely on the human ear, for even with the most carefully calibrated scales, optical adjustment merely has the effect of bringing the desired station within hearing range. We have only to consider the closely calibrated dials of modern sets, necessary to embrace the large number of existing wireless stations, to comprehend why any mechanical adjustment is too coarse for accurate tuning. The greatest volume is an hypothesis for the best quality, of sound, which only people possessing a particularly good ear for music are qualified to do, whereas differences in .volume can he easily detected by the average person. Therefore if a receiving sot does not lose in volume when inaccurately tuned, the average person is denied the possibility of close tuning, as this is then entirely governed by musical talent. Most receivers are, however, fitted with, an automatic fading compensating arrangement which keeps the volume uniform within certain limits, irrespective of whether the station is weaker or stronger, or is tuned in accurately or otherwise. In other words, a weaker reception also signifies a weakening of the rectified current passing the audio circuit, and when this current flows through a resistance provided for this purpose it experiences a slight voltage drop. Since the grid of the input valve is connected to this resistance, its grid bias is consequently reduced. This valve is, however, not like the usual valves as it has the characteristic of greater amplification the smaller the grid bias imparted to it. This valve therefore automatically ensures reception at uniform volume, but at the same time obviates fine adjustment by ear. If the station has not' been accurately tuned in, the hexode will, for instance, be subjected to greater load than I if this were not the case. But greater loading means that a heavier current will i flow through it, and vice versa. The intensity of the current passing the hexode I consequently provides an accurate measure as to whether the optimum of the fine adjustment lias been attained. Hence, if a modern receiver of this kind is furnished with an additional device allowing it to bo known when the intensity of_ the current passing through the hexode is at its lowest value and the operating knob is actuated until this value is attained to a nicety, the optimum of fine adjustment will have been ascertained in a much more reliable way than is possible by car. The

intensity of an electric current can, for instance, be determined by a measuring instrument by passing through it the current to be measured. In this case, the fine-adjustment knob would be operated until the merest pointer deflection is obtained. This method of fine adjustment has been adopted for the latest sets of the leading wireless firms. Other large firms in the radio trade allow the hexode current to flow through a small glow lamp of the resonance type. Another possibility of fine adjustment is the shadow indicator. Here, the indicating current traverses a magnetic coil ami by the co-operation of its magnetic field and the field of the permanent magnet surrounding it, an armature moving inside the coil is more or less deflected. A small lamp illuminates the armature in such a way that it throws a band of shadow in the small window'. Adjustment is such that this band appears smallest at lowest current intensity, and therefore at best fine adjustment. With these modern seta, fine adjustment is effected optically. The volume regulator is set to zero so that no sound can be heard from the loud-speaker. This is followed by coarse adjustment to the desired station, then fine adjustment by one of the three methods described, finally followed by volume control. Hence perfect reception can be obtained from any of the stations without producing irritating noises when tuning in. Moreover, optical control also provides a means of realising greater selectivity between stations. With the shadow indicator it will, for instance, lee found that stations whose shadow bands cannot be brought within a certain breadth, do not come into consideration for the best reception qualities. HERE AND THERE. The League of Nations has erected the “Asiatic Health Bureau” in Singapore. A health report is transmitted daily over the broadcast stations in the Far East, which include 163 harbour cities of the Pacific Ocean. As soon as the first case of ' contagious disease is discovered, a report is, scut to Geneva and Paris, and this is distributed to health departments throughout the world. Special attention is given to the reports of harbours, since this is of great importance to steamship lines. It is thus possible _to know at what harbours quarantine is required. V The 8.8. C. pays Catterall, leader of its symphony orchestra, £3OOO a year, and several other leading instrumentalists draw £IOOO a year each. As there are 110 players this orchestra must be nearly the most costly in the world, and with its dance band (conducted by a £3OOO a year man) and military band, the 8.8. C. must be the most liberal supporter of music on earth. A recent experiment was carried on on the air-line between the Netherlands and the island of Java, whereby messages were sent out simultaneously, on 900 metres and on 31 metres, with the intention of enabling distant land stations to receive these reports. The experiments were declared satisfactory, and this system is believed to increase the safety of passengers on the longest airplane line in the world. * * * In powerful stations the cost of power is a considerable item in the operating expenses and means for increasing efficiency are always sought for. Recently it has become the practice in Europe to make use of what is known as the “floating carrier” system. The scheme, which has been developed by the Marconi Company, effects an automatic control in the strength of the carrier wave, the result being that no more power is utilised than is necessary for !he requirements of the signal at any particular instant. V « * The town of Ruysselede in West Flanders, Belgium, which has been long known for its eight-antenna towers the highest in the world—has suddenly become known throughout the world as a result of a recent aeroplane accident. An aeroplane of ; the .Imperial Airways was flying low because ice had been forming on its wing, increasing the weight and preventing the ship from rising. As a result of the fog, the pilot saw the towers too late and the aeroplane crashed into the metal cage of wires; .10 persons were killed. The public was aroused and claimed that insufficient safety measures had been taken to prevent such disaster. Unfortunately, the technique has not yet advanced far enough to. give the pilot a beason which he can see in time in foggy weather. It is hoped that such a system can he worked out in the near future.