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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