In Jaynes' Probability Theory, he states:
There are many more analogies. In physics we are accustomed to finding that any advance in knowledge leads to consequences of great practical value, but of an unpredictable nature. Röntgen’s discovery of X-rays led to important new possibilities of medical diagnosis; Maxwell’s discovery of one more term in the equation for curl H led to practically instantaneous communication all over the Earth.
Does anyone know what is being referred to here? I think it is related to fibre optics but am not sure. More generally resources for understanding Maxwell's equations would be great.