Two clocks are being tested against a standard clock located in a national laboratory. At $12: 00: 00$ noon by the standard clock, the readings of the two clocks are 

$\begin{array}{ccc} & \text {Clock} 1 & \text {Clock} 2 \\ \text { Monday } & 12: 00: 05 & 10: 15: 06 \\ \text { Tuesday } & 12: 01: 15 & 10: 14: 59 \\ \text { Wednesday } & 11: 59: 08 & 10: 15: 18 \\ \text { Thursday } & 12: 01: 50 & 10: 15: 07 \\ \text { Friday } & 11: 59: 15 & 10: 14: 53 \\ \text { Saturday } & 12: 01: 30 & 10: 15: 24 \\ \text { Sunday } & 12: 01: 19 & 10: 15: 11\end{array}$

If you are doing an experiment that requires precision time interval measurements, which of the two clocks will you prefer?

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The range of variation over the seven days of observations is $162 \;s$ for clock $1$ , and $31 \,s$ for clock $2 .$ The average reading of clock $1$ is much closer to the standard time than the average reading of clock $2 .$ The important point is that a clock's zero error is not as significant for precision work as its vartation, because a zero-error can always be easily corrected. Hence clock $2$ is to be preferred to clock $1$

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