-M HULL SYSTEM TECIISIC.M. JOIRSAL 



line niiKlitions wliirh ari' praciiiMlly irrtaiii to exist and which wouUl 

 sond back ri-tk-* iwl waves that would reversi- the [xUcntial across 

 the line east at the terminals of the reiK-ater, causing ini[nilscs to 

 reach the input of the west hound amplilier. These im|)ulses will be 

 amplitieil and returne<l to the line west where, if similar tondilions 

 exist, they will oncx- more enter the east bound amplifier. If the 

 gains are great enough to offset the losses caUM-d by the rectifiers, 

 the system will sing. 



It is, thereffjre, e\ ideiil that rectifiers offer no chance for improving 

 on the aciinn ..f ilu pri-ciii typc-s of repeaters because they cause 



Fig. 11 



serious distortion and do not pre\eiil singing exce|)i under certain 

 six'cial conditions not likely to l>e found under practical conditions. 



3. Circiiils usinn ] I ii^h- Frequency Sxcilfhint:,. Another device which 

 is fre<|uentl\ prr>iH>si'd in one form or another is illustrated in Fig. 11. 

 In this case an amplifier is pro\ided for each direction of transmission. 

 These amplifiers are so designed that their amplifying power can he 

 destro\ed and rest(»red |X'ri(Klically at high fre(|uency by currents 

 frtmi a suitable source, the amplifier in one direction being active 

 when the other is inactive. The frecpiency of the controlling cur- 

 rents is aUive the audible range. In a \arialion of this scheme a 

 single amplifier is use<l which is |xiinted first in one direction and 

 then in the other at a fre(|Uency alK)ve the au<lible range, it i^ 

 argued that since there is amplification in onl\- one direction al .ui\ 

 given instant the s>stem <-annot sing. 



Imagine such a rejiealer to Ik- inserted in the line .it R in Fig. 5, 

 .ind that voitx- waves are arriving over the line from /I i. Owing to 

 the nature of the re|iealer these waves will be cut up into a series 



