Useful Hints

Radio Record, Volume III, Issue 27, 17 January 1930, Page 11

Useful Hints

HEN purchasing a soldering: iron get one with a good heavy head and not oné of the very light models, as these will not keep hot long enough for satisfactory use. ETWEEN .0002 or .0003 mfd. is a good average capacity for an ordinary outdoor aerial, which, together with the lead-in and earth wire, does: not exceed 100ft. in length. audio-frequency transformer deign in the last two years or so that if you are using an 91d audio-frequency transformer on the score of economy you are missing a great deal of real~ ism and volume which you might be enjoying. F a pair of head-phones has no indication as to which is the positive and which the negative tag, the polarity can be determined in the following manner. Carefully unscrew one of the ear caps and remove the diaphragm, leaving the magnets exposed to view. Then suspend the telephones in some convenient position where they are perfectly rigid and load up one of the magnets with small pins, gramophone needles or similar objects, until its magnetism is supporting the full. load it can hold. Now arrange a dry cell so that it sends a small current through the telephone cords, first in @ne direction, and then in the other. Tf the loading of the magnet has been very carefully done, it is possible to tell when the current of the cell is flowing in the right direction because it will then tend to hold the load of pins more firmly. If, on the other hand, the cell are reversed SO that the current opposes the magnetism of the telephones, the load will fall off as soon as the current starts to flow. When the battery is placed so that it assists the magnetism to hold the load, that end of the lead which goes to the positive of the battery should be marked positive and the other marked negative, when using the ones with a valve set the positive terminal of the ’phones should always be connected towards the loudspeaker terminal which is joined to the B battery positive, and the negative side of the ’phones to that loudspeaker terminal which goes to the plate of the valve, great has been. the advance in s