Looking for a needle in a haystack: fault-finding and repair on submarine telegraph cables in the Victorian era

I am an Arts and Humanities Research Council-funded doctoral student (one of about ten at the NMM) jointly supervised by the Museum and the University of Leeds. My PhD research concerns the work of submarine cable technicians from 1850 to 1914.
The first successful international submarine telegraph cable was laid between England and France in 1851. By the end of the 19th century over 250,000 nautical miles had been laid and were in operation. However, faults occurred and needed to be corrected as soon as possible as this means of communication, being the first 'instantaneous' method, had by then become vitally important.
At the first indication of the malfunction of a cable, precision electrical tests were carried out at the cable stations at each end of the faulty cable, the results of which enabled the location of the fault to be estimated. Comparing these results with the charts drawn up at the time the cable was laid, a cable repair ship would sail to the calculated location of the fault and grapple for the cable. Once the fault was located a piece of new cable would be spliced in to replace the faulty length. The picture below shows the electrical test bench in use in the Victorian era up to mid 20th century!
Electrical test bench
Sounds simple? The skills required were incredible: the accuracy of the electrician's measurements; the navigator's ability to find the cable even if the last sight of land may have been 1000 nautical miles away using only dead reckoning and celestial navigation; the cable engineer's skill in grappling and raising the cable and that of the cable jointer in making a perfect joint.
Cable Jointer and his assistant, courtesy of Porthcurno Telegraph Museum