The ultimate pendulum clock, indeed the ultimate mechanical clock of any kind, was invented by a British engineer, William Shortt....

GMAT Reading Comprehension : (RC) Questions

Source: Official Guide

Reading Comprehension

Physical Sciences

HARD

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Notes

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The ultimate pendulum clock, indeed the ultimate mechanical clock of any kind, was invented by a British engineer, William Shortt. The first was installed in the Royal Observatory in Edinburgh in 1921. The Shortt clock had two pendulums, primary and secondary. The primary pendulum swung freely in a vacuum chamber. Its only job was to synchronize the swing of the secondary pendulum, which was housed in a neighboring cabinet and drove the time-indicating mechanism. Every 30 seconds the secondary pendulum sent an electrical signal to give a nudge to the primary pendulum. In return, via an elaborate electromechanical linkage, the primary pendulum ensured that the secondary pendulum never got out of step.

Shortt clocks were standard provision in astronomical observatories of the 1920s and 1930s, and are credited with keeping time to better than two milliseconds in a day. Many were on record as losing or gaining no more than one second in a year—a stability of one part in 30 million. The first indications of seasonal variations in the earth's rotation were gleaned by the use of Shortt clocks.

In 1984 Pierre Boucheron carried out a study of a Shortt clock which had survived in the basement of the United States Naval Observatory since 1932. After replacing the electromechanical linkage with modern optical sensing equipment, he measured the Shortt clock's rate against the observatory's atomic clocks for a month. He found that it was stable to 200 microseconds a day over this period, equivalent to two to three parts in a billion. What is more, the data also revealed that the clock was responding to the slight tidal distortion of the earth due to the gravitational pull of the moon and the sun.

In addition to causing the familiar ocean tides, both the sun and the moon raise tides in the solid body of the earth. The effect is to raise and lower the surface of the earth by about 30 centimeters. Since the acceleration due to gravity depends on distance from the center of the earth, this slight tidal movement affects the period of swing of a pendulum. In each case the cycle of the tides caused the clock to gain or lose up to 140 microseconds.

Ques. 1/4

The passage most strongly suggests that its author would agree with which of the following statements about clocks?

Before 1921 no one had designed a clock that used electricity to aid in its timekeeping functions.

Atomic clocks depend on the operation of mechanisms that were invented by William Shortt and first used in the Shortt clock.

No type of clock that keeps time more stably and accurately than a Shortt clock relies fundamentally on the operation of a pendulum.

Subtle changes in the earth's rotation slightly reduce the accuracy of all clocks used in observatories after 1921.

At least some mechanical clocks that do not have pendulums are almost identical to Shortt clocks in their mode of operation.

Solution

Solution data from serial 354

Answer Choices Explained

Before 1921 no one had designed a clock that used electricity to aid in its timekeeping functions.

Why It's Wrong: The passage describes electrical signals between the primary and secondary pendulums but doesn't make any claims about what existed before 1921. The passage only discusses the Shortt clock's innovations, not the entire history of electrical timekeeping. This choice makes an absolute claim about pre-1921 technology that isn't supported by the passage.

Atomic clocks depend on the operation of mechanisms that were invented by William Shortt and first used in the Shortt clock.

Why It's Wrong: The passage mentions atomic clocks only as a comparison standard for testing the Shortt clock in 1984. There's no indication that atomic clocks use any mechanisms invented by Shortt. The passage suggests atomic clocks represent a different technology entirely, not an evolution of Shortt's work.

No type of clock that keeps time more stably and accurately than a Shortt clock relies fundamentally on the operation of a pendulum.

Why It's Right: The passage establishes the Shortt clock as 'the ultimate pendulum clock' with extraordinary accuracy. All evidence points to the Shortt clock being the peak achievement in pendulum-based timekeeping. The comparison with atomic clocks suggests that more accurate timekeeping requires non-pendulum technology.

Subtle changes in the earth's rotation slightly reduce the accuracy of all clocks used in observatories after 1921.

Why It's Wrong: The passage shows that Shortt clocks could detect earth's rotation changes, but this was a capability, not a problem. The passage doesn't suggest that earth's rotation changes reduce accuracy of all observatory clocks. The tidal effects mentioned were measurable by the Shortt clock because of its extreme sensitivity, not because they impaired all clocks.

At least some mechanical clocks that do not have pendulums are almost identical to Shortt clocks in their mode of operation.

Why It's Wrong: The passage calls the Shortt clock 'the ultimate mechanical clock of any kind,' which would include non-pendulum mechanical clocks. No evidence suggests that non-pendulum mechanical clocks operate similarly to the Shortt clock. The Shortt clock's key innovation was its dual-pendulum system, which wouldn't apply to non-pendulum clocks.

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