SOME NOTES ABOUT TELEPHONES

Radio Record, Volume I, Issue 21, 9 December 1927, Page 2

SOME NOTES ABOUT TELEPHONES

(By Waring S. Sholl, A.M.L#.E.) To all external appearances, the welknown headphone type of receivers are much atike, Roughly, the instruments may be divided into two types, viz., the simple diaphragm pattern, and the: armature or "reed" type, in which the magnetic impulses are imparted firstly to a small, delicately pivoted armature which is linked up to the diaphragm. This form is used in the well-known "reed pattern" telephone, and is developed further in the ‘balanced. armature" type of design employed in some makes of large loudspeakers. One great advantage of this design appears in the central pull upon the diaphragm, which avoids distortion to a considerable exent. . Resistance No Advantage. The ordinary diaphragm type of instrument, however, works very well, and the user whose means will not allow of the purchase of the more expensive type will get every satisfaction from the less costly article. High resistance and low resistance are factors: which need to be understood in the terms of electrical efficiency. , Resistance, as resistance, is a distinct. disadvantage. True high resistance *plhiones are wound with high conductivity wire of very fine gauge, about No. 47, S.W.G. copper, which enables a large number of turns to be got on the bobbins. This produces a comparatively great effect, as a smatl current wilt have the same effect if sent round the pole-pieces a large number of times as a Jarger current which only traverses a few turns. Some makers have gone so far as to produce a ‘‘high-resistance" ’phone indeed, but wound with comparatively few turns of high-resistance wire, a thoroughly, reprehensive swindle for which there is no extenuation whatever. ' Jn choosing a set of telephones a fair test may be made of sensitivity by placing the instruments over the ears and putting one tag into the mouth. The other tag is rubbed gently upon a ,small file or a key which is held in the hand. A distinct rasping sound will be heard in the earpieces if the set is reasonably sensitive and in good order. Telephones should have the leads clearly marked as to polarity, or continual use will tend to demagnetise the magnets; also the signals will not come. up to full strength when the instrument is new. Far too many makers neglect this important point, and in such case the careful worker will do well to conduct the foHowing test: Remove the ear-cap and diaphragm and place a compass near to the pole-pieces until the needle comes to rest. Arrange the ‘phone so that its magnetic puli sets the needle at N.E. Now place the ’phone leads on a battery and note if the compass needle comes over very slightly towards the telephone magnet. If so, it proves that the current is circulating the right way, and is tending to strengthen the magnet. If, on the other hand, the needle moves over towards north, it shows that the current is passing in the wrong direction, and that the magnets } are being reduced in strength. Having satisfied himself on this point, the user will do well tu slip a piece of red sleeving over the positive tag, if o ‘the pintype, or wind some red silk over the cord in the case of the spade type of terminal. After prolonged use, the eat-pieces should be wiped to avoid ‘moisture rusting the diaphragms, and the ’phones hung up with the cords preferably straight... The method of packing the ’phones with the cords tightly twisted round the head-bands is thoroughly bad for the eerds and displays a want of forethonght rpon the part of the makers. Some ’phones are much improved by fitting thinner diaphragms, of ferrotype, in place of the heavier stalloy. While this material is good in some cases, it is not always the best thing to use, although the word sounds very nice in advertisements. Buy the best *phones you can afford, and then take { care of them. —