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Using a cable twice

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Duplex working

I thought about trying to create diagrams that made this clever principle clear but thought better of it. Bright's book has some superb illustrations and few will want know the precise details, so here are some Bright diagrams and some very outline explanations. The basic idea is this - you have one cable which was very expensive to buy and even more expensive to lay across the Atlantic. If you can contrive to send messages each way at the same time then you get twice the revenue in return for your investment.

There were actually two principles in use originally: differential and "Wheatstone Bridge". The Wheatstone Bridge method got developed a lot further and was more successful, ultimately, than the differential.

A circuit as in the diagram was required at each end of the cable. On the face of it this looks simple. Two resistances on the keying side of the bridge - r and r' and two on the other - the cable itself (LINE) and a resistance R equivalent to that of the cable.

If the cable behaved like a simple resistance this would be fine - what happens is that current put in on "our" side by keying affects each side of the detector (G) equally - making no deflection - but currents coming from the other end do get detected.

Except that the cable does not behave like a simple resistor and the above does not therefore work. In fact there is a lot of capacitance in the cable - distributed all along it. The local resistor, R has to made to mimic this and that is where the developments to perfect this technique were mostly made. Fine tuning the "Artificial Line" to properly balance the bridge was an art that Cable Station staff had to get very proficient at. If the cable had a varying leakage to earth, maintaining the balance could become difficult or impossible, and the insulating material of the time - Gutta Percha - was nothing like as good as the plastics of today.
Many of the improvements and enhancements were made by the company of Muirhead and Taylor. Artificial lines became complex arrays of resistances and capacitances - but still much cheaper than actually laying another cable.
The differential principle.

This diagram is indicating that the galvanometer detecting the signal from the other end of the line has a winding connected from between the key and a "dummy line" resistor as with the Wheatstone method. Again signals from the home side balance out.

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