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Free Iowa Assessments (ITBS) DiagnosticReading
Read the passage, then answer the question.
The Clock That Ran on Air
In 1928, a Swiss engineer named Jean-Léon Reutter built a clock that never needed winding and had no battery. Inside its glass case sat a sealed capsule filled with a gas and a liquid that expanded when the room grew slightly warmer and contracted when it cooled. That tiny swelling and shrinking pushed a bellows, which pulled a chain, which wound a spring. A change of a single degree in room temperature stored enough energy to run the clock for two days. The device was named the Atmos, after the atmosphere that powered it.
The trick was not the capsule. It was the rest of the mechanism, which had to waste almost nothing. The balance wheel that keeps time in an ordinary clock swings several times a second; the Atmos balance is a heavy disk hanging from a thin wire, and it turns once every thirty seconds. Slow motion means little friction, and little friction means the feeble energy of a warming room is enough. Sixty million Atmos clocks running together, one company calculated, would use about as much energy as a single light bulb.
The design was a lesson that engineers still repeat: a machine can be made to run on a nearly invisible source of power if every part of it is built to spend that power carefully. The Atmos is still made today, essentially unchanged, and some of the earliest ones are still keeping time.
According to the author, what was the real achievement of the Atmos design?