Complex sentence: A standard telegraph circuit connects all the keys, sounders and batteries in a single series loop so the sounders actuate only when both keys are down (closed or "marking", after the ink marks made on paper tape by early printing telegraphs).
Split into simpler sentences: A standard telegraph circuit connects all the keys, sounders and batteries in a single series loop. Thus the sounders actuate only when both keys are down (closed, also known as "marking"--after the ink marks made on paper tape by early printing telegraphs).