RELAYS IX nir. PF.i.t. sysTr..\r 39 



till- fri"(iuriu-\- ul till- >i^n.iliiii; im|)ulst's. Tin- iin'iliod of ri'lcMsiiij; 

 relay "E" l)y short-ciroiiitinn its winding lias .ulvanla^;es over open- 

 ing the circuit for the purpose under consideration. The arcing at 

 the contact of relay E' is less se\ere ili.iii wouM he the case if an 

 inductive circuit were broken. 



.\n addetl feature which has been incorpor.Ued in the mechanical 

 design of relay "D" and which h.is an imp;)rtant bearing on its per- 

 formance electrically, is an atljustment limiting the armature travel. 

 This limilatiun of movement prevents a wide deflection when the 

 relay receives excessive current. Such deflection would tend to 

 set the armature into vibration ami woukl result in a sufficient num- 

 ber of impulses to operate rela\' E' and cause false signals. 



TnK V.vcif.M TinE 



The vacuum tube is used for the relating of energy in a number 

 of ways. It may be connected in circuit to amplify the received 

 impulses in which case it sends out energy from a local source with 

 the same wave shape as that of the received current. In this case 

 the tube serves to relay the impulses with as little distortion as pos- 

 sible. In the case of a tube used as a modulator or a demodulator 

 it is required to combine or separate impulses of different character, 

 the two operating together to preserve the same impulses at the out- 

 put of the demcKlulator tube as is received at the input of the modu- 

 lator. The impulses which are transmitted between the two tubes 

 have an entirely different wave form and may be amplified any num- 

 ber of times by means of amplifier tubes without affecting the action 

 of the modulator and demodulator. 



The \acuum tube may also be used as a rectifier to convert alter- 

 nating current to direct or pulsating current or it may be used as an 

 oscillator to produce alternating current from a local source of direct 

 current. In all of these applications of vacuum tubes, the tubes serve 

 as relays to introduce a fresh supply of energy or a desired wave 

 form or a combination of the two to serve their purposes in the com- 

 munication system. 



Rel.\vs for TelegR(\ph Circuits 



The use of relays for telegraph circuits presents an entirely dif- 

 ferent set of problems than those usually encountered in the con- 

 sideration of telephone circuits. Most telegraph relays are used for 

 repeating signals from one circuit to another rather than for switch- 

 ing local circuits. While some marginal operating conditions are 



