54 Electric Fire Alarms. 



always be present. These stations would be connected by 

 the close circuit system with the central station. The station 

 instruments would consist of a polarised relay, switch and 

 key, telephone and transmitter, and a local battery and bell. 

 At various places on the line, inserted in a loop, would be 

 placed the fire alarm boxes, consisting of small iron cases 

 having a glass door, on which would be painted, "In case of 

 fire, break the glass." 



The mechanism inside is of a simple description. A small 

 polarised lever connects the loop-line; but which, on the glass 

 being broken, is moved back, and then rests on a stud, and 

 places the line to earth at that spot and through a known 

 artificial resistance. Every box on each line would have a 

 different resistance, increasing by 5, 10, or 15 ohms, accord- 

 ing to the length of the line. On the signal being received 

 at the central station a reversed current is sent through the 

 line, which, passing through a small electro-magnet in each 

 box, restores the lever to its original place, and at the same 

 time shows a small disc attached to it, thus showing to the 

 sender that the alarm has been received. In the alarm box 

 would also be two plug-holes, by which means a telephone 

 could be readily placed in circuit ; this would prove con- 

 venient if after the brigade arrived at the fire it required 

 extra assistance or apparatus. 



The instruments at the central station consist of a relay 

 and battery in circuit with each line ; in circuit with all the 

 relays an alarm bell and switch-board ; a circular set of resist- 

 ance coils, corresponding to those in the alarm boxes, and a 

 galvanometer, the whole being arranged as a Wheatstone 

 bridge. On the circuit being broken in any line the alarm 

 bell will ring, whilst a number shown by the relay in action 

 will at once indicate the particular line signalled. This line 

 is then switched on to the resistance coils ; the galvan- 

 ometer needle will then be deflected ; a handle attached to 

 the line is then turned round the resistance coils until the 

 needle returns to zero. The number corresponding to the 

 resistance found will be the box from which the alarm is sent. 

 The current is then switched off, and sent in a reverse direc- 

 tion, replacing the lever in the alarm box, as before described. 



If either of the district stations be required, a reversed 

 current sent will at once act on the polarised relay and cause 

 the bell to ring, after which telephonic communication is 

 established in the ordinary way. You will be able by means 

 of this diagram to understand the whole matter better, 



