3
012-04367E Basic Electricity
VOM:
The Volt-Ohm-Meter or VOM is a multiple scale, multiple
function meter (such as the PASCO SB-9623 Analog
Multimeter), typically measuring voltage and resistance,
and often current, too. These usually have a meter move-
ment, and may select different functions and scales by
means of a rotating switch on the front of the unit.
Advantages: VOM’s may exist in your laboratory and
thus be readily accessible. A single meter may be used to
make a variety of measurements rather than needing
several meters.
Disadvantages: VOM’s may be difficult for beginning
students to learn to read, having multiple scales corre-
sponding to different settings. VOM’s are powered by
batteries for their resistance function, and thus must be
checked to insure the batteries are working well. Typi-
cally, VOM’s may have input resistances of 30,000 Ωon
the lowest voltage range, the range that is most often used
in these experiments. For resistances in excess of
1,000 Ω, this low meter resistance affects circuit opera-
tion during the taking of readings, and thus is not usable
for the capacitor, diode and transistor labs.
DMM:
The Digital Multimeter or DMM is a multiple scale,
multiple function meter (such as the PASCO SB-9624
Basic Digital Multimeter or the SE-9589 General Purpose
DMM), typically measuring voltage and resistance, and
often current, too. These have a digital readout, often
with an LCD (Liquid Crystal Display). Different func-
tions and scales are selected with either a rotating switch
or with a series of push-button switches.
Advantages: DMM’s are easily read, and with their
typically high input impedances (>106Ω) give good results
for circuits having high resistance. Students learn to read
DMM’s quickly and make fewer errors reading values.
Reasonable quality DMM’s can be purchased for $60 or
less. PASCO strongly recommends the use of DMM’s.
Disadvantages: DMM’s also require the use of a battery,
although the lifetime of an alkaline battery in a DMM is
quite long. The battery is used on all scales and func-
tions. Most DMM’s give the maximum reading on the
selector (i.e., under voltage, “2” means 2-volt maximum,
actually 1.99 volt maximum). This may be confusing to
some students.
VTVM:
The Vacuum Tube Voltmeter or VTVM is a multiple
scale, multiple function meter, typically measuring
voltage and resistance. They do not usually measure
current. The meter is an analog one, with a variety of
scales, selected with a rotating switch on the front of the
meter.
Advantages: VTVM’s have high input resistances, on
the order of 106Ωor greater. By measuring the voltage
across a known resistance, current can be measured with
a VTVM.
Disadvantages: VTVM’s have multiple scales. Students
need practice to avoid the mistake of reading the incorrect
one. An internal battery provides the current for measur-
ing resistance, and needs to be replaced from time to time.
Grounding problems can occur when using more than one
VTVM to make multiple measurements in the same
circuit.
Panelmeters:
Individual meters, frequently obtained from scientific
supply houses, are available in the form of voltmeters,
ammeters, and galvanometers (such as PASCO’s
SE-9748 Voltmeter 5 V, 15 V , SE-9746 Ammeter 1 A,
5 A and SE-9749 Galvanometer ± 35 mV). In some
models, multiple scales are also available.
Advantages: Meters can be used which have the specific
range required in a specific experiment. This helps to
overcome student errors in reading.
Disadvantages: Using individual meters leads to errors
in choosing the correct one. With limited ranges, students
may find themselves needing to use another range and not
have a meter of that range available. Many of the
individual meters have low input impedances
(voltmeters) and large internal resistances (ammeters).
Ohmmeters are almost nonexistent in individual form.
Light Bulbs
The #14 bulbs are nominally rated at 2.5 V and 0.3 A.
However, due to relatively large variations allowed by
the manufacturer, the wattage of the bulbs may vary by
15 to 30%. Therefore, supposedly “identical” bulbs may
not shine with equal brightness in simple circuits.
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