TIMING STERILISATION PERIODS 39 



holes at equal distances apart, corresponding to the 

 minutes on the dial. This plate is connected with 

 one of the poles of a dry battery, the other pole of 

 which is connected to the metal case of the clock for 

 the purpose of actuating an ordinary magnet alarm 

 bell. In the centre of each of the holes in the plate 

 a metal rod is fixed, which then passes through an 



FIG. 31. Electric signal timing clock. 



insulating ring and projects inside the clock face, 

 where it makes contact with the hour hand. The 

 clock is mounted on a heavy base, with a key-board 

 containing 20 numbered plugs. If one of the plugs is 

 inserted in a hole in the plate it makes contact with 

 the rod, and when the hour hand of the clock touches 

 the other end the circuit is completed and the bell 

 starts ringing. The period of this friction contact is 

 approximately 20 seconds. The clock can therefore 

 be used for electrically noting the periods of time 

 from one minute by multiples of one minute up to one 

 hour. 



