Some operations - such as sending two messages on the same wire at the same time in opposite directions - need the help of an "artificial line." This is a set of components that duplicates the electrical properties of the telegraph circuit. Compare the artificial lines for overland and underwater circuits. You can see at a glance the complexity of the underwater circuit.
The primary electrical property of a telegraph circuit on land is resistance. The artificial line for such a circuit is a simple "resistance box."
Artificial line for land line, late 1800s
National Museum of American History, from Western Union
|An underwater circuit combines resistance and capacitance (and sometimes inductance). You need a careful arrangement of components to mimic its electrical properties. The icebox-like insulated cabinet kept the equipment at a constant temperature. Note the improvised changes. Every time the real cable was repaired, its electrical properties changed, and the artificial line had to be adjusted to match.|
Components from artificial line for underwater cable, 1928
National Museum of American History
|Explore Resistance and Capacitance|