The Story of the Atlantic Telegraph

The method used by the electricians to discover a fault is one of such delicacy and beauty as shows the marvellous perfection of the instruments which science employs to learn the secrets of nature. The galvanometer is an invention of Professor Thomson, by which "a ray of light reflected from a tiny mirror suspended to a magnet travels along a scale, and indicates the resistance to the passage of the current through the cable by the deflection of the magnet, which is marked by the course of this speck of light. If the light of the mirror travels beyond the index, or out of bounds, an escape of the current is taking place, and what is technically called a fault has occurred." Such was the discovery on Monday morning. At a quarter past three o'clock the electrician on duty saw the light suddenly glide to the end of the scale and vanish. Fortunately it was not a fatal injury. It did not prevent signalling through the cable, and a message was at once sent back to the shore, giving notice of the check that had been received. But the electric current did not flow freely. There was a leak at some point of the line which it would not be prudent to pass over. They were now seventy-three miles from shore, having run out eighty-four miles of cable. The tests of the electricians indicated the fault to be ten or a dozen miles from the stern of the ship. The only safe course was to go back and get this on board, and cut out the defective portion. It was a most ungrateful operation thus to be undoing their own work, but there was no help for it.