FCC Exam Question: 3-75L4

On an analog wattmeter, what part of the scale is most accurate and how much does that accuracy extend to the rest of the reading scale?

A. The accuracy is only at full scale, and that absolute number reading is carried through to the rest of the
B. The accuracy is constant throughout the entire range of the meter.
C. The accuracy is only there at the upper 5% of the meter, and is not carried through at any other reading.
D. The accuracy cannot be determined at any reading.
Correct Answer: A

Explanation: Analog wattmeters, like many analog measuring devices, typically specify their accuracy as a percentage *of full scale*. This means the absolute error value (e.g., +/- 5 watts) is determined by the maximum reading of the meter and remains constant across the entire scale. Therefore, the most accurate *relative* reading will occur at or near full scale. For instance, a 100-watt meter with 5% full-scale accuracy has an absolute error of +/- 5 watts. When reading 100 watts, the error is 5% (5/100). However, if you read 20 watts, the same +/- 5-watt absolute error now represents a much larger relative error of 25% (5/20). * **A is correct** because the absolute accuracy number derived from the full-scale specification is indeed applied across the entire range. This makes readings closer to full scale more *proportionally* accurate. * **B is incorrect** because while the *absolute* error value might be constant, the *relative* accuracy (percentage error) is not constant; it degrades significantly at lower readings. * **C is incorrect** because the accuracy specification isn't limited to just the top 5%; the full-scale error value applies to all readings. * **D is incorrect** because meter accuracy specifications are a fundamental part of their design and are always determinable.

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Includes Elements 1, 3, 6, 7R, 8, and 9.