532 BELL SYSTEM TECHNICAL JOURNAL 



provided. Each of the group keys is set to a certain code call which 

 may be changed by resetting the key by means of a screw-dri\'er. 

 One of these group keys may be used as a call for the entire system so 

 that by the operation of this key, every receiver is called simultane- 

 ously. The other keys may be used for the four main group calls. 

 The master key is similar in appearance to a miniature cash register and 

 by setting its levers to the proper code combination, any desired 

 station may be called indi\idually and this key may also be used for 

 the sub-group combinations. 



The output terminals of the oscillator of the transmitter attach- 

 ment may be connected directly to the speech input equipment of a 

 standard radio telephone transmitter in place of the microphone. 

 The speech input amplifier is adjusted so that when the signaling 

 attachment is used the maximum possible modulation is obtained. 

 The sensitivity of the radio receiver is adjusted for reliable operation 

 of the signaling system, w^hich is sharply tuned to the frequency of 

 the transmitter attachment. This frequency is 3,000 cycles and is 

 so high that the volume of music or speech will be amply sufficient 

 for easy reception but yet insufficient to operate the signaling system 

 relays as only a relatively small proportion of the energy in normal 

 speech or music occurs at frequencies in the vicinity of 3,000 cycles. 

 Even should the relays be operated occasionally by excessive volume 

 of speech or music the receiver signal lamp will not light unless the 

 proper code call is sent. 



Receiving Apparatus 



Photographs of the receiving equipment are shown in Figs. 3, 4 and 

 5. At some of the outlying stations of the New York Police Depart- 

 ment 110- volt DC power supply is available while at others 110 volt 

 60 cycle AC is provided. The radio receivers are made in two differ- 

 ent types, one type for each kind of power supply. A schematic 

 circuit of the DC type of receiver is shown in Fig. 6 and that of the 

 AC type in Fig. 7. These two types are similar in all respects except 

 such modifications as the different sources of power supply necessitate. 



In the direct current type of receiver all of the vacuum tube fila- 

 ments are connected in series, current being taken directly from the 

 line through a filter to eliminate line noises due to generator hum and 

 other causes. In parallel with each filament is connected a small 

 switchboard lamp with a red cap mounted on the panel of the receiver 

 cabinet. The resistance of each lamp filament is sufficiently greater 

 than that of the \acuum tube filament so that the lamp will light 



