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As regards electrical properties, the trend is towards higher frequencies andhigher powers. This trend may be expected to continue. Electrical propertiescan clearly be built into semiconductors almost at will. But special types canhardly be expected to be evolved for telephony purposes apart from exceptionalapplications - the market would be too small.Within the component field the ferrites have attained very wide usageduring the fifties for inductance elements. Their permeability has been successivelyraised to a very high level without increasing their losses. A continuationof this trend may be expected, although hardly to the same extent asduring the past decade.For electronic exchanges the memory technique has come into the foreground.Several new memory elements have been evolved. New memory tubes,magnetic memories of different kinds and dielectric memories have beensuggested. During the sixties it is likely that technology will take a definitivedirection, with only a few types of memory as realistic alternatives. For certainrequirements, especially for semipermanent memories, there is at present nosaticfactory solution, and we look to new inventions in this sphere.TelephonesThe main evolution in telephone sets has been the successive improvementsin reference equivalents for sending and receiving. This has permitted higherattenuation in local networks, so longer local lines and smaller diameters ofwire. As regards the transmitter, the type that was patented in 1890 still stands,though, of course, improved in all details. The carbon granule transmitter hasthe advantage that it is itself an amplifier-about 25 db-and that it is cheap.This must be paid for in terms of high harmonic distortion, which gives thetelephone voice another timbre. The output of the carbon granule transmitterhas been increased in recent years by about 5 db, and a few additionaldecibels are perhaps within reach in the near future.The electromagnetic receiver principle originally evolved by Bell is stillused to a large extent. By the use of improved materials the efficiency ofreceivers has been increased by about 8 db within the past few years. Theelectromagnetic receivers of balanced type add a further 2 to 3 db. Noparticular improvement over these figures is likely in the near future.Thus, all in all, the efficiency of telephones has been increased by some15 db, but differently distributed between sending and receiving, so that aredistribution of the attenuation must be made in the differential coupling ofthe telephone from sending to receiving.But the greater efficiency in sending and receiving has had certain drawbacks.On short lines the volume has become unpleasantly high, severerrequirements must be placed on sidetone attenuation, and the requirements oncrosstalk attenuation in exchange and cables have also had to be accentuated.With modern telephone sets the system reference equivalent may lie withinthe range - 10 to + 40 db. The most agreeable range as regards volume isbetween + 10 and +25 db. For system reference equivalents below 0 thevolume is definitely annoying. Measures should therefore be taken to dampthe signal. In the U.S.A. (Bell System) and in the U.K., non-linear elements,whose attenuation of the signal is a function of the line current, have beenintroduced into the transmission circuit of telephone sets. Bell System employs