TELEPHONE EQUIPMENT FOR LONG CABLE CIRCUITS 133 



and accurate adjustment. Roth the magn?tic air gap and the con- 

 tact spacing may be adjusted, about 0.0015 inch or 0.038 millimeter 

 being used ordinarily for the latter. The relay is mechanically 

 tuned, the natural period of the reed with its adjustable weight cor- 

 responding closely to the frequency of the signaling current. A stop 



Fig. 20 — Assembly of 135-Cycle Relay 



pin is provided which prevents undue vibration of the reed due to 

 transient impulses or excessive currents. Also, the relay circuit is 

 electrically tuned by a shunt capacity and a series inductance and 

 capacity. It is thus very selective and is relatively free from the 

 ordinary sources of interference such as those caused by mechanical 

 vibration, telegraph signals, switchhook impulses, voice currents, 

 etc. The sensitivity of the relay is such that it will operate on as 

 little power as 30 microwatts, corresponding to a current in the neigh- 

 borhood of 0.25 milliampere. 



This relay is to be mounted so that it may be inserted in the circuit 

 or removed from it in the manner of a plug and jack, without requiring 

 changes in the permanent wiring and without affecting the circuit 

 operation, except to interrupt the signal-receiving system, while the 

 relay is removed. Thus it will be very convenient to make the neces- 

 sary adjustments of the relay separate from the circuit with which 

 it may be associated in service. The relay will be well protected from 

 mechanical interference, such as building vibration, by padding in the 

 mounting which prevents rigid mechanical connection between the 

 relay and its external support. 



The assembly arrangements proposed for the new signaling ap- 

 paratus are such as will lit in closely with the panel mounting meth- 

 ods designed lor the remainder of the toll equipment in cable in- 



