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To measure the clock offset and propagation delay, the break-circuit
chronometers at both stations were connected to the telegraph circuit
and their pulses compared.

Suppose the chronometer at station A is ten seconds behind B, and
there's zero propagation delay. From the point of view of A, the B
pulses arrive ten seconds early. But B observes A's pulses arriving 10
seconds late.

This symmetry doesn't occur if there's a propagation delay. Let's say
it's 0.1 second. Now A sees the pulses from B arrive 9.9 seconds
early. On the other hand, B sees the A pulses arrive 10.1 seconds
late.

In reality, it was probably not technically feasible to put both
chronometers on line simultaneously. Instead, chronographs at A and B
would simultaneously record the pulses on rotating paper drums, but
only one chronometer at a time would be on line. By analyzing the
graphs in the office, the true clock offset could be recovered.

I think the chronometers produced one-second pulses, with the pulse at
the top of each minute inhibited to allow specific seconds to be
identified in the chronograph trace. I've seen one of these
chronometers for sale on eBay. It had been used by the U.S. Coast &
Geodetic Survey.

In later years, the chronograph and break-circuit chronometer
continued in use, but you didn't need the telegraph link to a second
observing station anymore. Radio time ticks recorded on the
chronograph provided the time reference.

   

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