312 SEC. 10. ELECTRICITY. 



About 20 seconds before 1.0 p.m. a local current sent from the clock 

 actuates this switch, causing the levers to be shifted from the stops which are 

 in connexion with the ordinary instruments, and places them against the 

 opposite stops in connexion with the time relay ; the wire from the central 

 station on which the time signal is received being placed in connexion with 

 the coils. 



The stop on which this tongue rests is connected to a battery which sends 

 a negative current called the " preliminary," the object of this current being 

 to prevent, as far as possible, stray or contact currents from giving a false 

 signal. It also keeps the tongue of the relay at the distant station on its 

 " rest " stop, and in addition acts the part of a warning signal, which enables 

 the different offices at the proper time to " switch " the line wires into 

 connexion with their respective time relays. 



This preliminary current is not supplied from Greenwich Observatory, but 

 is adopted by the Post Office as an additional precaution to ensure the correct 

 transmission of the time signal, and emanates in the first instance from the 

 chronopher at the central station. This preliminary current does not pass 

 beyond the terminal office from which the time signal is distributed. 



The preliminary current from the central station keeps the tongue of the 

 relay at the clock station on its rest stop until the time- current proper passes 

 through the coils, when the tongue is carried over and placed in connexion 

 with the opposite stop, and through it with the positive pole of another 

 battery, from which at the moment of contact, a current is sent giving the true 

 time signal. 



About 20 seconds after the time current has passed, the local current 

 actuating the electro-magnetic switch is interrupted by the clock, and the 

 levers of the switch to which the line wires are attached are by means of a 

 spring again placed in connexion with the stop leading to their respective 

 instruments. 



The clock itself is corrected daily in the following way : 



It is kept at a slightly gaming rate, and is stopped automatically by a 

 detent which acts on a pin projecting from the escape-wheel when the hands 

 point to 1.0 p.m. The pendulum, however, continues swinging. 



When the time-current is received the outgoing current from the time-relay 

 passes through an electro-magnet inside the clock which liberates the detent, 

 and the clock is set to time. 



The clock is so arranged that a second electro-magnetic switch may be 

 actuated, and the time current distributed in a similar way at 10.0 a.m. 



1571. Automatic Time-Switch for switching line-wires from 

 their instruments and connecting them to the time-relay. 



1572. Time-Belay for relaying and distributing time-currents. 

 (Ordinary Siemens' form.) 



1573. Galvanometer for showing incoming time-current. 



1574. Galvanometer for showing outgoing time-current. 



1575. Lightning Protectors or Dischargers (various). 

 a. Varley's Lightning Protector (original form). 



