may be calibrated in volts, but it actually responds to current. In figure

2 a meter movement is shown.

RM is the series current limiting range

resistor. Rv is the resistance of the meter movement. Remembering Ohms law

I = ER, the resistor should drop the voltage to a level that the voltage

drop across Rv would not allow more than full scale on the meter movement.

Now if the voltmeter "V" is of the 20,000 ohm-per-volt type, with a movement

resistance of 2,000 ohms, and is set on the 1,000 V scale, its multiplier

resistor RM will have to be 19,998 ohms. Then, if you apply 1000 V to the

circuit you will have 50 uA or full scale on the meter.

If you were to

exchange this meter for a 1000 ohms-per-volt meter and the movement

resistance were 2000 ohms, then RM would be 998,000 ohms. The meter would

draw 1 mA from the circuit.

Therefore, you can see that the higher the

resistance of the voltmeter the less the load it presents to the circuit

under test.

(g) Impedance Matching, in AC circuits loading is often referred

to as a mismatch, due to the standing waves it creates.

An AC signal

generator will have a characteristic output impedance. In order to achieve

maximum transfer of energy, this output must be terminated into a load equal

to this impedance value.

(h) Systematic errors can always be blamed as the cause of trends,

jumps, periods, and change in precision.

Sometimes in a sequence of

measurements, the value will slowly drift in one direction. This is called

a trend.

Sometimes it will suddenly jump from one set of values, or the

amount of dispersion of the measurements may change from one value to

another, even though they do form a random sequence.

d. Random errors.

(1) Random errors are sometimes called accidental errors because they

are as likely to occur in one direction as the other. They are sometimes

called residual errors since they are what is left after all systematic

errors have been corrected. This is the type of error that is dealt with

statistically since the rules of statistics and probability are developed

around random events.

Random errors result in low precision, whereas

systematic errors reduce the accuracy of a group of measurements.

Random

errors can be reduced by taking several readings and averaging them.

Extreme values should be allowed for by discarding them before averaging the

readings. Discarded readings are written down but labeled as "disregard".

Random errors affect the standard deviation and range of a distribution.

Systematic errors affect the position of the mean of the group.

This is

illustrated in figure 3.

Four different observers each made 25 different

readings of a voltage whose nominal value was 217.645 volts.

Each used a

different technique and introduced various systematic and random errors.

One gross error was made.

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