6734 HYDROGRAPHIC MANUAL PaGE 634 



Each fifth offset is marked longer than the others and at each tenth offset (each second) 

 a figure is printed, starting with at the first offset after the receipt of the bomb signal. 

 These figm-es are printed to 9 and then repeated so that the time intervals may be 

 rapidly read. 



a. Operation. — The bomber is signaled in the regular manner to fire a bomb. 

 A switch on the clironograph amplifier is closed, so that the bomb explosion may be 

 received on the chronograph. When the bomber signals that the bomb has struck the 

 water, the signaling current also operates a relay in the amplifier which automatically 

 starts the 60-cycle motor of the cln-onograph. This sets the tape in motion and the 

 timing stylus begins marking tenth seconds. When the bomb explodes and the sound 

 reaches the hydrophone, the signal stylus immediately marks on the tape and at the 

 next tenth second a clutch releases the printing wheel so that the numeral is printed 

 opposite this tenth-second mark. Thereafter each fifth offset is made longer by an 

 index wheel, also released by the clutch. Another relay automatically switches the 

 amplifier from the bomb hydrophone to the radio receiver (see 6724). 



The R.A.R. time interval to be used in plottmg is then all of the numbered seconds, 

 plus the fraction of a tenth before the zero, plus the fraction of a second after the last 

 printed number, plus the ship's run correction (6853 (d)). This latter is found from the 

 elapsed time recorded on the tape prior to the bomb signal. The chronograph attendant 

 does not have to time this interval by clock or watch, as is the case with the Gaertner 

 chronograph (see 6843a). 



A switch on the chronograph amplifier cabinet is then opened and the clutch reset, 

 ready for the next bomb. 



6734. The Criftig 



In order for the Dorsey chronograph to function as a precision instrument it must 

 be supplied with controlled 60-cycle alternating current and tenth second impulses of 

 high accuracy. This is obtained from electric circuits starting with a piezoelectric 

 crystal with a frequency of 100,000 cycles per second, adjustable within a narrow 

 range. As developed by the Coast and Geodetic Survey, these circuits, by fractional 

 frequency division, give a crystal-controlled current for the time generator. The 

 arrangement is abbreviated criftig. In this the 100 kc is first divided by four, then by 

 five, and again by five, giving 25 kc, 5 kc, and 1 kc. The latter operates a 1,000-cycle 

 synchronous motor at a constant speed of 10 revolutions per second. The shaft of the 

 motor also carries the rotor of a 60-cycle generator and a single rotating arm which 

 passes near the pole pieces of a polarized electromagnet once each revolution, producing 

 the 10 pulses per second. 



Clocks on the ship's bridge and in the radio room are operated from the criftig 

 and, by daily comparison with the radio signals sent out by the National Bureau of 

 Standards, the rate of the crystal may be adjusted to a value so that the clocks give 

 time which is seldom in error by as much as 0.1 second. 



A special chronograph amplifier is required. It drives the 60-cycle synchronous 

 motor, amplifies the 0.1 second pulses to the timing magnet, and amplifies the bomb 

 signal. By the arrangement of certain thermionic tubes and circuits, this amplifier 

 produces a sequence of events permitting the reception of bomb signals and radio 

 returns almost automatically. No relay action is involved in the timing of the return 

 signals so that no lag in time is introduced. 



