THROWDOWN MACHINE FOR TRAFFIC STUDIES 321 



are timed in less detail. For this purpose the clock is also arranged to 

 deliver pulses at one tenth and at one hundredth of the basic pulse rate. 



The clock, circuit wise, is a form of free running relay pulse genera- 

 tor. It consists of a series of relays in which the first, in a released con- 

 dition, causes the remainder of the series to operate in sequence. When 

 the last relay of the series operates, it causes the first relay to operate. 

 The remainder then release in sequence. When the last relay releases, 

 it causes the first relay to release. This cycle, if not interfered with, is 

 repeated continuously to produce pulses representing units of time. 

 Time, thus, can be stopped by the simple expedient of allowing the 

 stop time signals to hold or ''lock" the first relay in the operated state. 



Time is visually indicated in units and hundreds of units at the opera- 

 tors' positions by a group of telephone type message registers termed 

 the time counters. Certain of the registers indicate present machine time 

 for action recording purposes. Others are set at a specified number of 

 units ahead of present time to indicate future times at which held items 

 will be released or re-entered into the system: 



To drive these counters and to safeguard their integrity, the counters 

 are substituted for the last relay in the clock pulser relay series. The 

 operating windings of all units counters are connected in parallel. Con- 

 tacts, which make on each counter when the individual coimter is ad- 

 vanced, are all connected in a series circuit to form the last relay con- 

 tact. Failure of any units indicator to advance ^^dll, therefore, interrupt 

 the pulse generator cycle and stop time until the trouble is cleared. 



The integrity of the hundred units time counters is guarded in the 

 same manner. On each hundred pulse when these counters are advanced, 

 their windings and contacts also form a part of the pulser circuit. 



The basic pulse repetition rate of the clock is approximately four 

 pulses per second, being determined by the acting time of the counters 

 and the various circuit elements which the clock pulses must drive. 

 Since each pulse represents 0.0036 seconds, the ratio of basic machine 

 time to real time is in the order of 70 to 1. 



The clock pulses, Fig. 11, are counted by two types of time switches. 

 One type, the control circuit time switch, is associated directly with 

 the component control circuits of the throwdo^vn machine. These time 

 switches are not continuously driven but are automatically connected to 

 the clock as required to time events in the progress of a call. The con- 

 trol circuit time s^vitches, as discussed in more detail later, are returned to 

 zero after each usage in preparation for timing the next event. 



The secoud type of time switch, designated the master time counter, 



