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Re: Sextant Accuracy and anomalous dip
From: George Huxtable
Date: 2003 Mar 23, 20:13 +0000
From: George Huxtable
Date: 2003 Mar 23, 20:13 +0000
Walter Guinon said, with regard to a telegraph cable in the era around 1900- >I think that ignoring the propagation time completely would introduce >negligible error in establishing GMT. e.g. the delay over 2000 nm would be on >the order of 20 milli-seconds. However, these cables were not used in the way that we use coax today. Walter is assuming that the signal propagated at the speed of light in the insulating material, I think. I do not think that was the case, for such telegraph cables, which because of their length had a very high electrical resistance between the two ends.. I understand that such cables operated by putting kilovots onto the cable at one end, and detecting microamps coming out at the other. The dielectric was the lossy combination of gutta-percha and sea-water. For signals to be regarded as travelling at or near the speed of light in the medium requires the cable to be fed from. and loaded by, its "characteristic impedance", which would be of the order of 100 ohms, at a guess, and for there not to be much dielectric loss in the insulator. This is far from the case in such a telegraph cable. Derek Howse, in "Greenwich Time and the Longitude" (1997), says that Airy reported a time of passage of 1/2 second from Greenwich to Paris in 1854, and a second (nearly) to Valentia [Ireland] in 1862. No doubt these times improved as technology advanced. But the bandwidth was so poor that text messages could be sent only slowly. Even so, telegraph cables provided the most accurate way of distributing Greenwich Time around the world, until the days of radio. Wherever a cable touched, the longitude there was established promptly and precisely. George Huxtable ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================
