Hints for Christmas Buying

Radio Record, Volume I, Issue 22, 16 December 1927, Page 3

Hints for Christmas Buying

(By

M.I.R.

E.

Sound Advice for Prospective Purchasers

gy} AVING + dealt with the j) main considerations relative to the points to be cbhserved -in purchasing muses} and getting the best ont of a multi-valye xeceiver, and the principal accessories accompanying it, the time is opportune for a discussion of the salient factors to be observed in purchasing and running either erystal sets, crystal and valye combinations, or small valve receivers. A discussion of this class of apparatus is of patticular consequence at the present time because of the approach of the Christmas season, and of the intense interest which will be centred round tadio this year. A COMMUNITY GIFT. The many people who are realising that a radio set is a present acceptable to every member of the family, and that a present to each member may be made a community affair by such ai investment, will be eagerly scatining advertisements and seeking information regarding this new benefit to humanity. INFORMATION FOR THE PUR: "CHASER. Information for the convenience of, and of interest to, prospective pur-. chasers alid owners of more expensive equipment has been made available in past issues of this column. In considetation of the fact that the majority of folk are not in a position to purchase multi-valve equipments, the crystal and small valve set user will be much in evidence in the radio shops and stores in the next few weeks. Many peopie haye already learnt that real roadcast reception is best obtained by using at most a three-valve receiver when situated within, say, 50 to 75 miles of a transmitting station, and by listening solely to that station, When closer in still a crystal and two-valve combination will be quite successful, while within the radius of the city and outer suburban boundaries of the main centres, either a two-valve receiver, crystal, and oue-yalye, or evel a crystal itself, will give quite ~ respectable signals in a loudspeaker. Of course, all. these ranges just quoted refer to loudspeaker reception. The range of any set is very much extended when head telephones are in use, because the energy necessary to fill a londspeaker with really satisfactory volume will drive as many as twenty or more pairs of telephones, This does not mean that the range of the crystal set ‘is greatly extended when using headphones. because actually the strength of a transmitting station falls very rapidly in terms of distance so far as the crystal reception is concerned. Although at three miles from the transmitter a crystal set may work a loudspeaker quite satisfactorily, it may only work one pair of telephones at twenty smiles. | WORKING OF THE CRYSTAL. It should be realised that the crystal actually works on power picked up from the transmitter, and is quite dif-

ferent to a valve set,. which works on power supplied by ‘the local battery, and represents a "‘relaying" action, The possibility of one day being able to put up a power station capable of transmitting power to be picked up Wirelessly has always captivated the imagination of electrical people since the introduction of radio. It is not usually appreciated by folk who have just been introduced to radio in the form of valve sets having (visible) batteries for driving purposes, that actually a transference of energy is taking place between transmitter and regeiver. In the case of the valve receiver. the energy actually absorbed ‘out of that picked up by the aerial is infinitesimal, but in the case of the crystal set the actual -current which works the head telephones or speaker is current which has been made available in the receiver, and such energy has been transmitted throngh space. FULL LOUDSPEAKER RECEPTION. The more energy picked up, presuming a given receiver’ efficiency, the more current will be passed through the sound reproducer, and the louder the signals which will result. It may be said Quite definitely that there are only four localities in New Zealand where efficient reception of this nature can be carried out, and that is within two to three miles, at most, of Auckland, Christchurch, and Dunedin stations, and within five miles of Wellington station, As a matter of fact, such a statement is optimistic, because there will be areas where buildings are closely grouped together, and "shading" will take place, and where speaker reception will be quite impossible with any degree of satisfaction, Naturally there will be places outside of those mileages whiere the desired results will be forthcoming. As has been explained before, there is no rule whereby receiving results may be forecasted, except with a very approximate degree of accuracy. Again, what might be considered fair speaker reception to one person would be impossible to one who has tendencies towards being a ‘‘volume fiend.’?’ Full loudspeaker strength has been defined as equalling a gramophone, and any interfering or extraneous noises should not be any more apparent than the needle hiss of a gramophone. A further extension of this definition has been given, that ordinary speaker volume should be sufficient to cause a person chewing peppermints to hear! Weak speaker strength is such that the hearer has to stop eating in order to listen! Very crude, but remarkably to the point, as will be appreciated by users of radio sets. }

AN IMPORTANT POINT. Now before passing to the question of design of apparatus it would be of interest to stress a point of definite importance in the operation of sets. If a speaker is to be operated from a crystal set it should be one of a sensitive type, and the majority of sensitive types are horn types. As a tule the best cone types require a fair volume to drive them, but this does not mean that cone types will be unsuitable. When choosing the speaker do not overlook the fact that the sound reproducer is being purchased and this is equivalent to the sound box of a gramophone. Cheapness is false economy. A cheap gramophone may have quite a respectable sound box, but invariably the motor will be cheap and have a limited life A cheap crystal set is not likely to give trouble and can be easily fixel up if it does,

AQ ROR) ER a but a cheap speaker cannot be expected to give satisfactory results, and it is practically impossible to find one which will ‘fluke’’ them. Spend three-quarters of your money, or more if necessary, on a good speaker and ‘the balance on crystal set and aerial equipment. CHOICE OF CRYSTAL, The crystal itself should be of carborundum if the recciver is to drive a speaker. An ordinary crystal quickly "burns out’? and becomes insensitive when it is called upon to pass too much current. Galena, iron pyrites, etc., are more sensitive than carborundum, but are not as reliable or longlasting. Of course a new crystal may be obtained for 2s 6d. at the most, and if such a crystal burns out in a few weeks, the cost of crystals throughout the year doesn’t add much to the annual amusement bill Nevertheless, a really good carborundum crystal with attached potentiometer and battery costing up to £1 may be considered a good investment because it will be always ‘‘on the job’ and will last for a year or two anyway. When working two loudspeakers or more than one pair of telephones always connect thei in series, that is to say, they should be so connected

that the current from the output terminals of the receiver goes successively through each reproducing unit in turn, ‘this connection is in contradistinction to a parallel connection, where all the reproducing units are joined to the receiver terminals and the current divides between them. Very handy connecting pins or blocks may now be purchased very reasonably from dealers for the purpose of series-con-necting speakers and headphones. It is wise to have a switch to either disconnect the aerial from the set, or else put the crystal out of action, while the set is in disuse. With the "catswhisker’’ type, this wire should be lifted off the crystal, and if the crystal itself is not glass enclosed, but is exposed to the air for dust, ete., to collect on it, it should be covered over. TYPES OF CRYSTAL SETS, Crystal sets are mainly designed under four headings. 1. Variometer type. ‘That is the tuning of the aerial to the correct wavelength is achieved by turning a dial which rotates one’ coil of wire inside another. Such a design is undoubtedly very etiicient, and there is little to go’ wrong providing the rotor has good bearings, and the electrical windings on the rotor are connected by "‘pig-tails," and uot through the bearings themselves, where discontinuities are likely to develop through Inbrication, ete. 2. Variable tuning coudenser type. In this case a coil having one or two taps going to terminals is usually provided, and tuning is accomplished by a standard tuning condenser. ‘This is an excellent arrangement, aud providing the condenser is of a gcod make no trouble should be. experienced. The coil should be wornd on a "former," which is not likely to warp with heat. 8. A Multi-tapped cpil and switch. ‘There are very few sets of this design offering, and though no serious quarrel can be picked with them; it may be said that the efficiency is likely to Le less than types 1 and 2. Furthermore, the cost of switches is likely to make the price excessive. Naturally, unless the switches are well made, there is always a possibility of trouble develop-. ing. 4, Slider type. This is by far the most popular type, becaise of its simplicity and consequent reasonableisess of price. A coil of wire wound "‘flat" on a round former has a metallic arm traversing its length and picking up individual turns in its movement. The cheapest forms leave the coil and arm exposed, but the more prepossesstng types enclose the whole in a metal or wooden container, and the movement of the arm is indicated by a dial gradualed with respect to the usual pointer.

Providing the or@ft has a worktmanlike appearence, nothing better should be* designed, Ihe movement’ of the arm should be free, and not feel like a hacksaw. *Reject any set which does not come up to these specifications, .because it invariably means that the contact on the end of the arm is tog sharp, and it is cutting into the wire forming the tuning coil, and not only will it very soon eat through the wire; but’ while it is doing it, it will file off pieces of metal which will. collect we! tween the turns of wire and destro the efficiency of the outfit. * As an introduction to wireless the crystal set is ideal, and the’ prospact who wants to hear what is going of and get to learn something of this new business, should unhesitatingly spend £5 at least this Christmas, and joitt are enjoying the excellent programmes the ranks of 80,000 other listeners, who now available. ‘ .

BUY RADIO for CHRISTMAS" eee gee: