RHi^tys IX rill-: pell system 37 



riu- rli.iin of rolays oporalinR from the lii^^li frwiuency signals 

 consists of relays /), E and F. Ri-lay "D" imisl be a very sensitive 

 structure in this case and a pohirized relay with a vibrating contact 

 has commonly been used. The circuit requirements are such that 

 the energy a\ailabie for the operation of this relay is seldom more 

 than a few hundred microwatts and m.w be iiuich less. The cir- 



Fig. 2f>— Compiisito riiigcr circuit 



cuits are being designetl on the basis of giving reliable operation on 

 20 microwatts. The operation of relay "/?" releases relay "E" 

 which in turn operates relay F. 



Where such a circuit depends on the operation of a vibrating con- 

 tact relay, the current through this contact is of vital importance. 

 Whenever the contact is clo.sed, current tends to flow through the 

 winding of relay "£." Fig. 27 illustrates the effect of very weak 

 signaling currents and of currents sufficient to give proper operation. 

 The current values through the vibrating relay winding and through 

 the winding of the secondary relay are shown for two different typical 

 conditions. Also, the average or effective value in winding "£" 

 is shown. 



A circuit feature which has recently been introduced to increase 

 the sensitivity of relay "D" and to improve the operation of the 

 secontlary relay consists in the introduction of a condenser and the 

 operation of the vibrating contact as a normally open contact. The 

 closing of the contact charges a condenser which tends to operate 

 the secondary relay by its discharge as soon as the contact op)ens. 

 By this combination, the effect noted in Fig. 27 is eliminated and 

 pt)sitive operation of the secondary' relay is secured as soon as the 

 armature vibrates sufficiently to make contact. The local circuit 

 embodying this feature is shown in Fig. 28. Referring to this figure 

 and to Fig. 2(5, relays "£)" and "E" represent the alternating current 



