RADIO NOTES

Dominion, Volume 24, Issue 187, 6 May 1931, Page 13

RADIO NOTES

By ‘

“ETHER ”

MAGNET MATERIALS

New Compounds Found EFFECT ON RADIO Those who have seen aud heard the recent cinema film in which Sir Oliver Lodge describes the remarkable properties of the new cobalt-steel magnets will remember his characteristic comment that “magnetism is merely a word used by scientists to cloak their ignorance,” writes J. C. Sevens in “Popular Wireless.”

It is unfortunately only too true that we know nothing whatever of the absolute or fundamental nature either of magnetism or electricity. Luckily, however, this does not prevent us from making good use of their Known properties. Nor does it discourage us from seeking for further new properties in the hope that they too can be utilised to good purpose.

Importance of Magnetism.

Magnetism, in particular, offers a very promising field of research at the present time. For some reason there is a general tendency to regard it as a sort of "maiden aunt" among the sciences—old-fashion-ed and of little interest apart from its association with electricity. Nothing could be further from the truth, In fact, at least two, important developments in modern radio practice are due entirely to recent discoveries in the field of magnetism. In the first place we have the introduction of nickel-iron-cored transformers for low-frequency amplifiers, and in the second, place the use of “permanent” magnets made of cobalt-steel for moving-coil loudspeakers.

The magnetic nroperties of iron are: profoundly affected both by the presence of various “foreign” substances in the pietal, and by change of temperature. Take, for instance, the difference between the. magnetic behaviour of soft iron and annealed steel, which after all is only soft iron mixed with carbon and other “Impurities” and subjected to a particular form of heat treatment.

Nickel-Iron Materials.

Still more remarkable are the changes thilt can be produced by mixing iron with different proportions of nickel or cobalt. For example nickel-steel, which contains about 25 per cent, of nickel, has practically no magnetic properties at all, although both steel and nickel are normally magnetic metals. On the other hand, simply by changing the proportions of nickel and iron, a number of different alloys can be produced which possess extraordinary magnetic permeability. Among these the most important, for the wireless constructor, is the particular series known as Permalloy, Mumetal, etc., all of whicli produce an intense magnetic flux in 'response to a compara-tively-l'eeble exciting-current. The first of this series—Permalloy— was discovered some years ago by G. W. Elmer of the Bell Telephone laboratories. It consists of a mixture of four parts of nickel with one of iron, the mixture being subjected in the course of preparation to a particular and complicated process of heating and cooling. More recently a similar combination of nickel and iron—known as Permalloy C —has been discovered which has even greater permeability. In addition, it requires a less elaborate form of heat treatment during manufacture. The permeability of ordinary Permalloy 1b between six and seveii times that of the best silicon steel. Roughly speaking a choke or transformer fitted with a core of Permalloy is equivalent to one three times its weight and bulk having a ,silicon,-steel core. ~ • . This represents a considerable saving where questions of size and weight must be carefully studied, as in the case of portable and transportable sets. For instance, one well-known firm makes a transformer weighing only 70z., having a primary inductance as high as 50 henries, and stepup ratio of 2i to 1. Few of the Advantages. “High-mu” cores are particularly suitable for L.F. transformers, where the main object in view is to give an effective step-up in voltage. Since fewer primary turns are required to create a given flux density in the core, the self-capacity of the windings is correspondingly reduced, and there is less tendency to by-pass the higher notes. Also since the ratio of secondary to primary windings can be increased, for a given size of transformer, a higher step-up amplification is obtainable. Another remarkable magnetic alloy is that known as cobalt-steel, which consists of a mixture of iron with cobalt, the latter being a silver-white metal harder than either nickel or steel. The outstanding property of cobalt steel is the power of retaining an intense magnetic flux indefinitely. Magnets of Colossal Strength. So strong is this field that when Sir Oliver Lodge in his screen picture drops a bar magnet about five inches long and’ as thick as his little huger dowji on to a second bar of similar size, the first bar rebounds and remains suspended, apparently in empty space. The force of repulsion between the two is obviously stronger than the gravitational’ pull on the suspended metal bar. This intense magnetic flux is permanently fixed in the alloy. It does not “evaporate”, either under mechanical vibration or with change of temperature. For this reason it is now being fitted to moving-coil speakers. In the ordinary way it is necessary to supply from 20 to 30 watts of energy, either from a special battery or from the mains, so long as the speaker is in operation. With a cobalt-stecl magnet all this is saved.

The permanent magnet is made in several sections, which are not bolted together, but are held fixedly in position by their own mutual attraction. In fact it requires a straight pull of over a ton to remove a “keeper” once it has been placed ovver the roles of the magnet. HAVE YOU HEARD XEW? Mexican Broadcast Addition Mexico made an important step in advancing the quality of its broadcasting structure recently with the erection of a new 5000 watt radio station, the most powerful in the country, at Mexico City. The new station, which will be known as “The Voice'of Latin America,” is owned and operated Ijy the Mexico Music Company. This prominent Mexican business firm erected the station to be the nucleus of what they hope will be a national network of radio stations operating along the same lines as the large radio networks in the United States.

If its plans materialise, it hopes to effect nn exchange of programmes witii the United States, and with Central America. The Mexico Music Company also operates station NET, which is rated at 500 watts power. The transmitting apparatus used by the latter station is the old WJZ, New York, equipment. Station NEW, Mexico’s jfewest and most powerful voice, has just been completed and turned over to its owners by engineers.

Embodying the most advanced radio broadcasting technique, such as 100 per cent, modulation and crystal control, the new station has already made a successful debut on the air, and reception reports have been received from many parts of the United States. It operates on a wavelength of 384.7 metres.

DISTORTION TROUBLE

Tracking the Cause NOT AN EASY TASK One or two people have complained lately that 2YA has been puttting out somewhat low quality. There has been a tendency, they say, toward distortion, roughness and shrillness. They complain that compared to other stations such as 3YA the quality is distinctly inferior. This may be so. Nevertheless before one rushes into a grousing letter to the authorities it is just as well to make sure that it is the station producing the distortion and not the receiver. It is by no means always easy to prove which is the culprit. Merely tuning to another station does not necessarily prove the point. In nine cases out of ten when the receiver is at fault the distortion comes from overloading in the aerial circuit. If the first valve is being asked to accommodate a grid swing of, say, one volt, when it can only deal with, say, half a volt distortion is inevitable. Tuning to some more distant station cures the trouble. The first valve is then worked well within its limits. When a valve starts to be overloaded there is usually a falling off iu amplification. This in its turn confuses the issue. Then it is impossible to compare results by sound intensity. Another test is to ask a friend what his set is doing with the local station. Even so his set may be overloaded. A ■ simple way to avoid overloading the set is to disconnect the aerial. It is possible to receive 2YA on any set not more than ten miles 'away using only a foot or two of wire as an aerial. There will be no overloading when this is done. On the other hand other factors arrive on the scene to upset the test. For one thing the aerial circuit becomes very sharply tuned, with a very low loss, when the aerial load is disconnected. This Un itself may cause distortion. For another thing when the aerial load is removed many sets come nearer the oscillation point. This will occur especially in sensitive low loss receivers even if they use screen grid valves or neutralised circuits. The result may or may not lead to distortion.

The only other way to prevent overloading a receiver tuned to a nearby station is to distune. This brings distortion of the worst kind. Possibly the only person in a real position to say whether a station is distorting or not is someone who lives some thirty mile away. People who live further away get into areas where night distortion may occur while the waves are en route. They easily sort out the two forms of distortion. It will be seen therefore that every local station may take refuge behind all manner of excuses- when it is accused of putting out bad quality. In the case of 2YA many of the loudspeakers are out of date. They belong to an era when a little bad quality passed unnoticed. Moreover, doubtful quality, unless it is too ghastly, passes unremarked by some people, while it makes others feel uncomfortable. Taking all things into consideration, it is no easy matter to bring home to any station that its quality is bad. Perhaps some day there will be some mechanical method of doing so from which there can be no loophole.

WHAT A SET CONTAINS A standard make of 1931 radio broadcast receiver having four —24 type screengride tubes, four —27 tubes, two —45 tubes and. an —BO rectifier, tube, contains in its makeup also the following parts: Nineteen resistors, thirty-two condensers, four transformers, three i.f. coils, one oscillator coil, two r.f. coils, one detector coil, one choke, one volume control, one tone control, one voice coil, one field coil, two switches, one pilot lamps, a chassis and various assembly shackle bolts and nuts.

PENTODE VALVES

How They Amplify In a uoruial three-electrode valve the electrons liberated from the heated filament are attracted by the anode and move with great velocity reward it, till they strike the anode. The thermionic current, if no grid current is allowed to flow, equals the anode current and therefore exists only between the filament and the plate, says Philips Radio in "The Story of the Pentode.” Now suppose the anode is not a plateshaped piece of metal as usual, but is constructed as a grid. Also in this case the grid-shaped anode will attract the electrons, but its attractive effect is not large enough to gather all available electrons at once on its surface; in fact, a number of them are shot through the meshes into the space behind the gridshaped “anode.” Suppose further that there is in this particular space a positively charged plate connected to the same anode voltage, this anode will then collect the electrons shot through the meshes of the grid-shaped "anode.” As the latter is closer to the filament than the real anode, its effect upon the thermionic current can be made much larger than that of the anode and by special construction of the "screen grid” (because it really “screens” the anode) the influence of the anode on the thermionic current can be made sd small as to be practically negligible. As the so-called amplification factor of a valve can be considered as giving the relation between the effects exercised by the grid voltage and by the plate voltage upon the thermionic current, and as a small effect.of the plate voltage compared with that of the grid voltage indicates a large amplification factor, it will at once be obvious that such a valve must have a very high amplification factor. This will be still more clear when considering the German technical term for amplification factor, which is “Durchgriff,” the degree by which the lines of force emerging from the anode get a grip, through the grid meshes, on the electrons. It is the reciprocal value of the amplification factor and is expressed as a percentage, so that an amplification factor of 10 corresponds to a “Durchgriff’’ of l/10th or 10 per cent. As far as yet explained, the operation of a pentode is comparatively simple. In reality, however, there occurs inside the valve an unfavourable effect which, if not remedied, would entirely spoil the quality of .reproduction. The electrons pass with great velocity through the grids and strike the anode. There the greater part of their kinetic energy, is converted into heat, but the effect of the' collision with the metnl anode is that other electrons are liberated, the so-called secondary electrons. This effect, which also occurs in normal valves, would do no harm, were it not that at certain instants the screen grid (connected directly to the H.T. supply) is at a higher potential than the anode, so that there would flow a stream of secondary electrons from the anode to the screen grid, and this would spoil the operation entirely. In normal valves the grid is always at a much lower potential than the anode, so one need not trouble about these secondary electrons. In the pentode valve this effect of the secondary electrons is eliminated by placing between the plate and the screen grid another grid,’ connected to the point of lowest potential in the tube, i.e., to the filament. TALKING PILLOWS A “singing” and “talking” radio pillow for use in hospitals and Pullman cars has just been developed in America. The radio pillow is of regulation hospital size, and is made of specially selected sponge rubber in which a sensitive radio reproducing unit is concealed. It is so constructed that although the sound permeates the pillow it cannot be heard except by resting the head on the pillow. The radio pillow was designed especially for use in hospitals to replace ordinary earphones which become irksome and chafe when worn for any length of time. The loudspeaker is often impracticable, too, because the radio programmes may be disturbing to other patients in various stages of illness.

The new pillow, according to its sponsors, may be sterilized like an ordinary pillow, and the pillow-cases changed at will. The patient lies in a natural reclining ot sitting-up position to hear the radio. A connecting cord of ample length is plugged into the centralised radio receiving system.