ON ALL WAVELENGTHS

Evening Star, Issue 20364, 21 December 1929, Page 8

ON ALL WAVELENGTHS

By

CHOOSING A SET. In these columns' Inst week several points to be noted in a receiver by the prospective buyer were outlined, and the desirable features in an a.c. receiver were left for this week. _ Good parts are especially beneficial in this typo of set, for in addition to the usual components there arc such things as filter chokes and condensers, transformers and voltage resistances, and it is therefore essential to avoid cheapness because of the consequences when a breakdown occurs. ' Because an a.c. set is low in price it does not always follow that the parts are not good. However, in any a.c. set make sure, if possible, that the various parts have been proved to bo capable of giving long service. Do not run away with the idea that an eight-valve set is better than, say, a five-valve set. Sometimes it is, and sometimes it isn’t. Some people would say that an eight-cylinder motor car is more powerful than a six-c.vlindcr car, without bothering to take into account the size of the cylinders, revolutions, or certain other factors that all enter into the question. However, there is one way for the new buyer to find out if the receiver has good tone, selectivity, and sensitivity. He can have a demonstration at his own home. What is known as “a.c. hum” must also be taken into consideration. It is generally desirable to choose a set with a minimum of a.c. hum, for, while the music from the local station will be loud enough to drown the hum, often enough the signals coming from a distant station will not be very loud, and the hum will seriouslv interfere. It grates on the ears. Hum can be reduced to such an extent that in an ordinary sized room it will be almost unnoticed: but to do this costs money, which partly explains the apparently high cost of some receivers. As a final word, it may be said that it is always advisable when buying a set to ask the dealer what has to be done in the way of maintenance and the estimated cost. It is sincerely hoped that these few remarks will help those who are contemplating joining the ranks_of the listener’s from eventually throwing good money after bad. THE MEANING OF WAVE LENGTH Light and wireless waves travel through the ether at exactly the same speed—lß6,ooo miles per second. But the two sets of waves have different properties. The question may naturally be asked: "What is meant by the length of a wave? and the reply is the distance between the crests of the waves. If a stone is dropped into a pond of water ripples on the surface of the pond will be seen, and the distance between each ripple is always the same. So it is in the case of wireless waves.

Every station .'-a'V’sthig Rands . : it. a certain length of wave, and the crests of two consecutive waves are always a certain distance apart, the distance varying according to the -wave length used. In view of its importance, perhaps it would be better to say a little more about the meaning of the term wave length. If we could see wireless waves in the air it would be an easy matter to measure the distance from the crest of one wave to the crest of the next, but as we cannot see them we are compelled to the length of the wave by the sound heard. Of course, there arc special instruments used for measuring the different wireless waves. Wireless waves commonly used vary in size from 300yds to ton miles, and the French measure of unit is used, the “ metre,” which is just a little longer than a yard. The above is just a short explanation of what is meant by the term for the benefit of the man working with wireless who does not really understand its importance. BROADCAST OF ‘THE MESSIAH.’ Station 4ZM was on the air earlier than usual last Sunday for the purpose of relaying the presentation of excerpts from Handel’s ‘ Messiah ’ from the Hanover Street Baptist Church. Tho relay was exceptionally well conducted by 4ZM. From the opening tenor solo, ‘Every Valley,’ to the final chorus, * Worthy is the Lamb.’ everything was fairly plainly caught by listeners. The soloists especially came through very well, their offering not being marred by any of the noises often heard in a broadcast relay. The organ, too, was very clear, but there was a slight mushiness about the choir, giving the impression that each member was straining to be heard -ni-rophone. How ever, despite this little defect, the choir’s singing u ••, mo l enjoyableAgain it may ho mentioned that it was a fine piece of work on the part of 4ZM, and listeners would surely appreciate anything further from the station in this direction.

Many of ns have had the heartbreaking experience of inserting a new valve in our receiver, only to be greeted by a brilliant flash and a subsequently ruined valve. Without knowing the*cause, we blame it on a defective valve, and henceforth definitely condemn that particular brand. Yet the fault is our own, for we have doubtless broken a cardinal rule ot radio, which states that the set should be ‘‘off ” when removing or inserting valves. What actually takes place is that the valve is inserted at an angle while the current is left on. The plate prong of the valve makes contact before the grid prong, and an abnormal plate current is drained without the restraining influence of the requisite grid bias. This is particularly true in the case of power valves, which call for a considerable plate current under normal conditions and an enormous current, when operating without jjhe grid bias.

NOTES AND COMMENTS 7

MAKING OK SPEAKERS. DEALER VISITS FACTORY. Perhaps it is only natural that most of the dealers and almost all laymen take dynamic speakers more or less “for granted." The fact that there arc so many thousands of them manufactured every year veils the background of manufacturing problems that had to bo overcome before even one of them could bo offered for sale. A recent visit to one factory by a dealer was an oye-opener to him in this regard. He was fortunate in having one of the company’s engineers conduct him through the group of low-lying buildings which comprise the plant at Oakland. The dealer was -rst taken to the machine shop to watch the shaping of the field cases. A glance dispelled his former notions that this operation was simple. Big presses move their way through metal very easily. Asking why these cases were made in this manner, the dealer was referred back to the well-known phenomena of a current-carrying wire in a magnetic field. It was recalled that there is a force produced which tends to move the wire out of that field, as well as a resistance against the passage of tho current, an impedence, to use tho

technical term. It was recalled also that the currents in a speaker, varying according to the tones of the sound to bo reproduced, moves through a wire (in this ease a coil) lying in a magnetic field, and that the resultant force was used to drive or move the diaphragm or cone of the speaker, and so to reproduce the sound as it was originally. It was clear that even a slight variation in the field would produce a great variation in the driving force and so a tremendous distortion in the sound from the speaker. That was why the construction of the field cases must he so precise. To see the massive dieu hit the heavy iron and push it into shape hko modelling day was very impressive. The dealer watched the raw materials starting being made into a completed speaker part. He saw the cone housings pass through the many operations necessary—the blanking of the steel, the forming operation, and by many stages to the final, precise blow that make them, just the right height, the right diameter, and the correct angle.

There were over 200 inspections in the manufacture of a dynamic speaker. An inspection took place a Iter every machine operation before the part went on to tho next machine, and in addition there were many inspections in sub-assembly and of the raw materials. .Some of the inspections were automatically made by machinery, an inspection which precision handworkers could . never hope to equal. Others wore made painstakingly by hand.

One of tho most interesting phases of the manufacture lay in the automatic machines. These lathes turned, cut, drilled, tapped, and finished cores without a workman near. Tho question of why some speakers were better than others when all operated on the basic theory, puzzled tho dealer, and it was explained to him that in a theoretically perfect speaker tho diaphragm would bo massless, of infinite size, totally' rigid, and would operate without friction and with no load except the resistance of the air or “ air load.” T'his was Sabin’s specification. In addition, there was the problem of keeping tho electrical factors constant. A speaker should furnish a. pure resistance load equal to twice the output tube impcdenco. How these theoretical impossibilities wore overcome by making tho errors compensate for each other was a marvel of ingenuity, particularly when it was realised that a speaker must reproduce all tho notes of _the audible scale, and must, in addition, be sensitive and strong. The speakers employ a suspension of hgkt, flexible bakclitc, for which size, thickness, and other factors, have been carefully worked out by several rears of hard work.

.fn the speaker the weight of the moving system was carefully determined, the smallest, of course, consistent with strength. Then the suspension, both the bakelite spicier near the apex of the cone, , and the cone flange material, was designed to compensate for that weight. The diaphragm must vibrato like a piston and not like a sail in the wind. The cone material and the shape determined its action, and if the cone material was practically rigid other factors could be used to compensate and so insure actual reproduction of sound as it was in the beginning.

Many other factors in the make up of the speakers were explained to the dealer. And, finally, as he walked away he realised that all too seldom do we appreciate the wonderful background of inventive and creative reasoning that lies behind our everyday appliances which we buy in a store, set on the table, and forget about except to enjoy their performance.

“ Be sure, then, to turn off the set before changing valves, and keep the valve upright, so that all prongs make contact at the same time.”-

ELECTROSTATIC SPEAKERS. The newly-perfected electrostatic or condenser typo loud speaker, -which promises to become a formidable rival to tho moving coil instrument for volume and purity of tone, appears to bo an example of the old saying that there is nothing new under the sum A recent survey of the world’s scientific literature discloses the, socalled “ singing condenser,” first discovered in Europe approximately sixtysix years ago. This was nothing but a lightly-rolled condenser made by placing a paper dialectric between two sheets of tinfoil and using one sheet as an electrode opposed to the other sheet. When an alternating or pulsating voltage was impressed between both electrodes the condenser would give off a musical note dependent upon the frequency of the impressed voltages. Many radio fans Have noticed this “ghostly” phenomenon when they disconnected their loud speaker from the set and tho set kept on “playing,’ 1 although weakly. They could trace tho origin of tho sound to tlie small black box that housed the condenser in the output filter. The reason for the condenser giving off sounds, and the condenser speaker also, lies in tho fact that there is a mechanical stress generated between the two bodies of electrodes charged

electrically, and as the charges vary, the bodies or electrodes arc moved to and fro in accordance with the variations. If the electrodes have any considerable area the sounds that are given off may be of a perceptible order, and a specially constructed device built along these lines might servo as a reproducer for radio reception. Until recently, however, .few of the condenser speakers were sufficiently efficient or accurate in their reproduction to be very successful commercially. it is said that the reason for this is that no one Uiougut of introducing the third element, the flexible dialcctric, between the two plates or electrodes of the speaker, as is done by Mr Colin Kyle, the inventor of the type of condenser speaker which has been named alter him—the Kylectron. When Mr Kyle, a former Californian school teacher, who specialised in physics and electricity while in college, began work on reproducers of the condenser type, ho was told by the wiseacres that he was attempting the impossible. That ho “ accomplished the impossible ” was proved at a recent demonstration in New York before a critical audience of several hundred metropolitan radio men. Because articulation with the Kyleetron type -of condenser speaker is,, so nearly perfect, it is said to he applicable to the talking “ movies ” as well ns for home reproduction. A HANDY AERIAL. The greatest difficulty in operation of a portab l © radio receiver is finding a suitable aerial and ground. While some commercial portable receivers use a small loop, the majority of radio experimenters are inclined to use a simple set that will require an aerial. The well-known aerial consisting of a short length of flexible insulated wire thrown over the limb of a tree probably has been tried by many. This method is anything but convenient, and signals are not any too strong. Likewise the ground for a portable set is also a problem, probably the most common being a counterpoise formed by running a length of wire along the surface of the ground beneath the aerial hung from the tree branch.

A very satisfactory aerial and counterpoise is available in every rural locality. Next time you take your portable on a week-encT trip stop at a field surrounded with barked wire fence. Connect the aerial post of your sot to the top fence wire with n battery clip. The ground terminal of the sot is connected to the bottom fence wire. You will be surprised at the results.

STRAY SPARKS. ■ On quo occasion recently twontyscven loud speakers were used in Dublin to cope with 500,000 people. The occasion was the opening ceremony in connection with the Catholic centenary celebrations. * ♦ # • Victoria still holds the world’s record ratio of radio licenses in proportion to population. The total is now over 144,000. * » * * Listeners will naturally know that 4YA has been on the air from noon for about a week now. This continues until next Tuesday. The little orchestral combinations we have hftnrd from 4YA lately have been very acceptable to most of us. Quito a number of listeners have expressed satisfaction in hearing the quintets and sextet, but many are still hoping to hear a full orchestta such as the other stations broadcast. * * * » Long-distance reception upon simple sets is not so much a question of the circuit emploj’ed as of location, the skill of the operator, and of various other local conditions.