41 

 TIMERS, COUNTERS AND RATE MEASUREMENT 



TIMERS 



In protracted experiments in what might be called the 'kymograph' time 

 scale the provision of some form of automatic time marker relieves the 

 worker of the necessity for repeatedly consulting a stopwatch and marking 

 the time of significant events by hand on the smoked paper. In high-speed 

 experiments of the cathode ray tube variety time marks are almost essential; 

 it is usually more satisfactory to record a separate time trace than to rely 

 upon knowledge of the time-base sweep velocity. Even if the latter pro- 

 cedure is adopted a master time standard is still necessary to set the time- 

 base generator up. 



Kymographic time markers commonly comprise a synchronous electric 

 clock which drives a bank of cams possessing various numbers of strikers. 

 These operate electrical contacts so that, by appropriate external connection, 

 the circuit is closed at intervals ranging from 1 second to 1 minute. The 

 circuit is completed by a battery and an electromagnetic event marker. 

 The accuracy depends on the degree to which the mains supply frequency 

 approximates to 50 cycles per second. Over a period of days the cumulative 

 error is held at zero, but at any particular instant the frequency may be in 

 error by up to ±2 per cent. If this is not sufficiently accurate the syn- 

 chronous clock may be replaced by an electrically maintained pendulum, 

 which rotates a cam system of a similar kind. 



(a) Figure 41.1 ( b) 



Cathode ray tube displays may also employ the 50 cycle mains as a time 

 marker, though if an electrical time base is used the method is not much use 

 for bases shorter than the time of one cycle — 20 milliseconds. For longer 

 bases, however, a 50 cycle sine wave is better than nothing. 



A difficulty with sine markers is that it is not easy to measure off from 

 the eventual record a distance corresponding exactly to one cycle because 

 the maxima of the wave are so ill-defined {Figure 41.1a). A much more 

 satisfactory timer waveform is a train of 'pips' {Figure 41.1b). The author 



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