Vanguard Labs

Dept. H

196-23 Jamaica Ave., Hollis, NY 11423

Getting Your

Higher Class License

Part VII — Measurements in the preceding six installments of this study course for the Advanced Class license exams, we've leaned heavily on theory—because the point of the whole series is to provide the additional theory you'll need to pass the exam!

But all this theory must be tempered somewhat with its practical applications. Any practical application of any part of the theory is going to involve some type of measurement — even though we may not realize that we're making it.

For example, simply tuning a receiver is ma; mg a measurement of the receiver local oscillator's frequency; when a station is tuned in on the nose/' the receiver oscillator is operating at a "measured" frequency, Just what tlie measurement may mean is a different question.

Most measurements we make are more deliberate than that; we may measure time in order to make our log entries, frequency 111 order to stay within the band limits, current in order to tune our transmitters, and voltage (via the S-meter) to check received signal strength.

The Commission recognizes the importance of measureinei ts in practical applications of theory, and several questions dealing directly with measurement are included on the FCC study guide, Among them are (numbers are from the FCC list):

10. How close to the edges of a certain amateur band can you safely operate a VFO-equipped CW transmitter if you are using a frequency meter which has a maximum possible error of 0,01 percent?

16. What do oscilloscope patterns showing 25% modulated signals (with no distortion) look like? 50% 75$?

22, What are Lissajous figures in oscilloscope operation? What patterns would be produced on the oscilloscope if the signal applied to the horizontal input has a frequency equal to 2 times the frequency of the signal applied to the vertical input? 3 times? 4 times?

37. Should a voltmeter have high, or low, internal circuit resistance? Explain.

These four questions, with their many parts, actually span almost the entire art of making electrical measurements. As usual, well re-phrase them to make the basic points a little more obvious, and move the emphasis from that of specific answers to specific problems over to that the general reasoning behind typical problems.

The first ciuestion, in this phase, must be "What is measurement?", for most of us probably think of it as something more than it actually is.

Directly following from die first is the second, How are measurements made? \ The third, "How accurate can measurements be?", naturally accompanies the second. One of the remaining key factors is the question, "Can a measurement affect itself?", and of vital practical point is the final question of our paraphrased group: "What are measurement "standards'?"

What h **Measurement"? The word 'measurement" means many things to many persons, but most of us tend to think of it as a process including more than it actually does.

A measurement cannot be anything more than a comparison of two like quantities, to determine whether they are equal, and if not, which is greater, he mythical scales of Justice are a typical example of measurement at its most rudimentary level.

One of the two quantities—the one which we are "'measuring"—is unknown. The other, hopefully, is known, and we call it "standard" against which we are measuring the unknown.

When our comparison device is able only

Fig, 1—Comparison of the unknown quantity (such as the weight of the letter) and a known standard (the I-ounce weight \ farms the basis of ail measurements, Any measuring instrument which does not have the comparison standard within itself is merely acting as an indirect comparison device. In this ease, the initial comparison to a standard is known as "calibration" of the instrument.

Fig, 1—Comparison of the unknown quantity (such as the weight of the letter) and a known standard (the I-ounce weight \ farms the basis of ail measurements, Any measuring instrument which does not have the comparison standard within itself is merely acting as an indirect comparison device. In this ease, the initial comparison to a standard is known as "calibration" of the instrument.

to indicate exact correspondence and relative unbalance, we must have not just one but many "standards", each of which bears some relation to the others. Staying with weight and scales for our example, we would need not only 1-pound standards but also 1-ounce, 2-ounce, 4-ounce, 8-ounce, 2-pound, 4-pound, and 8-pound standards in order to be able to "measure" unknown weights from 1 ounce to 16 pounds within 1-ounce accuracy,

Some comparison devices are able to indicate the amount of unbalance more precisely; we'll get to them a bit later. The point we're looking at now is the fact that all measurements are tonus of comparison against standards. Sometimes the measurement includes a counting action (as in measurement of time by a clock) but the comparison to a defined standard is always involved..

In ancient history, the standards were considerably less precise than those wTe use today, The biblical standard of length, for instance, was the cubit—which was the length of a man's forearm. Just how many cubits long a wall happened to be depended upon whose arm was used to establish the standard. As recently as the middle ages, the englishman dard "foot" (which today is defined as 12 inches) was defined as the length of the right pedal extremity of the reigning monarch —and varied all over the kingdom when a new King took the throne.

Our electrical standards are much more precise, even if they do happen to be phrased in the language of physics. A volt is defined as the potential produced by a specified type of primary-standard cell. An ohm is defined as the resistance of a particular and highly-specified conductor. An ampere is defined as the current required to electroplate a given quantity of silver out of a solution of specified strength. Additionally, each of these standard "units of measurement" is defined in terms of each of the others by means of Ohm's law, and the whole system of physical units is geared together so that all the equations of physics hold true*

When we measure a votage in a circuit, though, we aren't performing any such direct comparison—and what we actually measure is not voltage, but length! How this can tell us the voltage in the circuit is what the next question is all about

Olden Days Measuring Instruments

Fig, 2—In olden days, standards were a bit less precise than those we use today. The standard of length in England through the Middle Ages, for instance, was the King's right foot—tvhence comes the name of the unit. This was some slight hardship to surveyors when the reign changed hut most people could live with it and those who couldn't, didn't, courtesy of the royal executioner.

Fig, 2—In olden days, standards were a bit less precise than those we use today. The standard of length in England through the Middle Ages, for instance, was the King's right foot—tvhence comes the name of the unit. This was some slight hardship to surveyors when the reign changed hut most people could live with it and those who couldn't, didn't, courtesy of the royal executioner.

How Are Measurements Made? If measurement means only a comparison of an unknown quantity against a standard, how can it be possible for us to measure the voltage in a circuit by using a voltmeter which does not contain any standard voltage source against which the unknown can be compared?

The answer, surprising though it may be, is that it's not. We speak of measuring voltage, but we don't. What we actually measure is length—the distance across the meter face travelled by the needle—and the comparison standard is the printed scale under the needle.

How can length indicate voltage? Again, it doesn't; what it does indicate is power. In the ordinary moving-coil meter movement, the electrical quantity which is measured is power. This power forces the meter needle against a spring in some cases, but in more sensitive meters the power is only capable of pushing the needle partway across its scale. The deflection of the needle is proportional to the applied power, and since the meter is of fixed construction, the distance travelled across the scale by the needle can be used to indicate the power.

We normally calibrate meters to indicate either voltage or current, rather than powerin a circuit of fixed resistance, though, either voltage or current may control the amount of power available. If the circuit resistance is low, then current is usually the controlling factor. If circuit resistance is high, voltage indications are obtained. One rather unusual result of this is the fact that a millianimeter, calibrated for current, can be used without any modifications for measurement of extremely low voltages. If the resistance of the meter movement itself is 100 ohms, for instance, and full-scale deflection is obtained with I mA of current, then the meter may also be used as a voltmeter in the range from

0 to 1/10 volt When 1/10 volt is applied to a 1.00-ohm resistance, the resulting current flow is 1 mA—and tiiat's full-scale deflection for the meter. At least one commercial VOM makes use of this capability to provide a very-low-voltage range.

Don't become confused by this approach— the power used [>y the meter is not related to the power consumed by the circuit being tested. To measure power consumption of the circuit, two meters are normally required; one measures the current flowing through the circuit and the other measures the voltage. Certain special meter movements have been built (mostly for ac uses) to combine both these measurements in a single meter, and indicate power directly on the scale, but you are not

1 i<ely to run into any of these meters in practice.

The type of measurement we've been examining thus far in this section can be called "indirect since we are not making a direct comparison and so are not, in the strictest sense of the word, measuring what we think were measuring. Indirect measurement can be summarized in the idea that we actually measure something which is related to the quantity we want to know. The process of establishing that relation so that we can trust our instruments is known as "calibration",

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Responses

  • ferdinand
    How to measure times in olden days?
    7 years ago

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