AMATEUR RADIO

Gisborne Times, Volume LX, Issue 9723, 5 June 1924, Page 3

AMATEUR RADIO

LOW RESISTANCE PHONES^

USE A TRANSFORMER!

Specially Written for the Gisborne Times.

(By “Oscillator.) (3) The use of a telephone transformer in valve sets is always to be recommended. as not only does it protect the delicate windings of head-sets and prevent demagnetisation, but also it helps to cut out extraneous noises. It is not generally known, however, that, by using a telephone transformer Iti a crystal circuit, the ordinary compiercial watchcase telephones can bet usedw ith great success, and so render unnecessary the purchase of expensive high-resistance phones.

MATERIALS. The materials required to make this instrument are: — 120vds of No. 36 D.C.O. copper wire, 2Jozs. of No. 44 S.S.C. copper wire, a few feet of No. 22 soft iron wire cut up into lengths of 4Jin., and four terminals. CORE.The core of the transformer should be made first. This consists of a bundle of short lengths of No. 22 gauge soft iron wire, being 4J inches long, sufficient lengths being used to make a bundle about -Jin. in diameter A strip of strong brown paper about lOin. long and 4Jin. wide should be tightly bound round the wires, making them into a neat round bundle, the end of the paper being securely fastened with glue. When finished the whole should be dipped in hot paraffin wax. Two pieces of good dry wood (preferably mahogany) are now required to fit securely on each end of the core, to form a bobbin. These should be about 2JinV square and Jin. thick, with a hole bored through the middle of each to allow them to fit on each end of the core.

PRIMARY WINDING. The bobbin thus made has to be wound with 2joz. of No. 44 gauge S.S.C. copper wire. This is a, tedious job, but must be done carefully, as the efficiency of the instrument will Upends upon this winding. Anyone with a little ingenuity can soon put together a small winding machine which will save much time and 1 trouble.

Before commencing the winding a small hole should be bored with a needle through each of the wooden blocks, in one block as near to the core as posible and in the other about Jin. up from the core. The winding can now be commencel leaving about Gin. at the beginning for connections later. This should be threaled through the small needle hole nearest the core and tucked away somewhere outside for the time being. The winding should commence at the end of the bobbin, where the spare 6in. was threaded through the ole, and should be wound as evenly and closely as possible to the other end of the core, and then wound back again, continuing in this way until the whole 2’iOz. have been wound on. Great care should be taken not to break the wire, which is extremly thin, hut should a breakage oc-eur the join must be neatly soldered and dipped in hot wax. When the whole of the wire Ims been wound the last 6in. should be carefully threaded through the other needle hole and carefully tucked away. The winding should now be tested for breakages with a battery and telephone. In this case a battery and bulb would he of no use.

SECONDARY ,WINDING. If the winding appears to be continuous it should be neatly covered with two layers of waxed paper, and it is then ready to receive the secondary winding. This is a much simpler matter. About 120yds of No. 36 S.C.C. copper wire should be wound on in exactly the same wav as the thin wire, a few inches being left at- the beginning and end of the wire for taking to terminals later. This wire must not be joined in any way to the thin wire.

TESTING. When finished another test should he made to ensure that r.o break has occurred', the test being made with a small 4-volt battery ar.cl bulb. In this easQ the bulb should light. It no break has occurred the wire should be covered with a few layers of waxed paper and the whole instrument fixed to a. small mahogany baseboard. Four terminals should be fitted on the latter, preferably two at each end, and the loose wires which were left over at the beginning and ending of each finding, taken to them. It is important that the terminals should be carefully labelled, the two with the tin (44 gauge) wires attached being marked “P” (primary)'and the other two, with the thicker wires marked (secondary’. The instrument is now complete and readv for use, aud will he found to repay 'well the time, and trouble spent on it. If desired it may be given a coat of shellac varnish.

CONNECTIONS. In u c e, the two wires of the receiving set which would normally liaye been connected to the telephones should be connected to the teirninnls S.. The two ear-pieces should be wired up in series.

“TIMES” QUESTION DEPARTMENT.

Will the amateur who wrote and said that he received music from Dunedin on a. crystal set, come and have dinner with mo one day r 1 Ido like a fellow with a sense of great imagination. R. D. states that he can make toast by holding a piece of bread over bis loudspeaker. “Oscillator” 'says that is nothing. He can spray off wallpaper when 2 Y.M. ' broadcasts “Beneath they Window.” •

NOTES AND NEWS. Concert items, from a broadcasting station were recently given in a tunnel under the Hudson (U.S.A.) River. To issue from the loud-speaker in the form of sound, the waves had to travel through 500 miles of air, 70 feet of water, 30 feet of mud, and the heavy steel casting of the tunnel. Such an experiment demonstrates the penetration of radio .waves. Miners equipped with small ' portable sets will, in future work with less danger. Operating the three-coil set: Assuming that it is connected in the ordinary way and that standard parts are used, , remove both aerial and earth connections. Set both variable condensers at half capacity. Plug in the ticker coil and turn primary and’ tickler coils hack 90 degrees. With the rheostat and potentionmeter (where used) determine the lowest value of filament current to make a slight hissing in the phones* Leave the rheostat at this adjustment. Push priihnry coil up to about 10 degrees from the secondary and set the tickler coil at about 45 ‘degrees. Tune m the primary condenser and a fine adjustment of the secondary. Vary the primary condenser for maximum loudness. If the primary is close to to the secondary condenser requires critical adjustment. A loose coupling of the primary consequent finer tuning will make the set more selective. Determine whether the*tickler coil is connected in the right, way round. If correct- the signal will increase in intensity as the tickler is brought nearer the secondary.