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Commonly Used Instruments

Commonly-Used Instruments


Abstract


This appendix describes some of the common instruments used in the laboratory.

BNC/BANANA ADAPTOR


A BNC/Banana adaptor allows one to plug a BNC connector into something with
Banana plugs (such as a DMM). Note that one side of the adaptor has a little "ear"
marked GND (the bump on the left side of the adapter in Figure 1). This side is
ground and connects to the outer conductor of the BNC connector (and thus the
outer conductor of the coaxial cable).


COAXIAL CABLE
(with BNC connector)

The coaxial cable actually is two
wires: one inner conductor which
runs down
the center of the cable and an outer
wire (also called the Shield or
Ground). Consider it to be like a
flexible metal pipe with a wire
running down the center.

All the coaxial cables used in this lab
have a latching connector at the end
which is called a BNC connector.


DIGITAL MULTIMETER (DMM)


A Digital MultiMeter is the swiss-army knife of
electrical measurement tools: it can serve as a voltmeter (to
measure voltage, or electrical potential), an
ammeter (to measure current), or an ohmmeter
(to measure resistance).

First decide whether you want to measure
voltage, current, or resistance. Turn the dial into
that area. Note that if you are measuring voltage
or current, there are separate positions for DC
and AC. If you are unsure about which you are
using, ask a lab instructor. (Batteries and the
little numbered wall plugs are DC, the 110 volt
wall outlets are AC.)
DCV: DC Volts
ACV: AC Volts
DCA: DC Amps (current)
ACA: AC Amps (current)
:
Ohms (resistance)



Side View
Front View
inner conductor
outer conductor
(shield/ground)
Figure 2
: BNC Connector attached to a Coaxial Cable
200
�

2m

20m

200m

2000m

DC10A

200

2K

200K

20K

2000K

200m

2

20

200

1000

OFF

750

200

V mA

COM

10 A DC

DCA

ACV
DCV
47
47
47
47 kkkk
Figure 3
: Digital MultiMeter (DMM)
Figure 1
:
BNC/Banana
Adapter
200
�

2m

20m

200m

2000m

DC10A

200

2K

200K

20K

2000K

200m

2

20

200

1000

OFF

750

200

V mA

COM

10 A DC

DCA
ACV
DCV
47 k

Within that area, you are to select the range. In general, one tries to use the lowest possible
range. For example, if you have a circuit which uses a 1.5V battery, you know that at no point in
the circuit will there be more than 1.5V. Therefore, you would choose the lowest range which is
still greater than 1.5V, which is 2V.


VOLTAGE
CURRENT
RESISTANCE

1000 V
2000 mA (2A)
20 M

200 V
200 mA (0.2 A)
200k

20 V
20 mA or 10 A (see below)
20 k

2 V
2 mA
2 k

200 mV (0.2 V)
not used
200

If you do not have any idea what your range should be, you should start at the largest range. For
example, you might have to measure a current and not really know how large it will be. You
would then start with the 10A range (which is discussed below). If the current measured below
2A (2000 mA), you could then switch to the 2000mA range or lower, as appropriate.

Selecting which jacks to plug your wires into can be a bit confusing at first. If you are using the
DMM as a voltmeter, an ohmmeter, or as an ammeter with a current less than 2A, connect the
ground (also called "common") wire into the
center jack marked COM and the positive
wire into the jack labeled V- -mA. A
negative voltage or current reading indicates
that the positive and negative wires are
reversed.

*
If you are measuring a current between 2A
and 10A, plug the ground (negative) wire into
the jack marked COM and the positive wire
into the left jack marked 10A DC.

UNCERTAINTY:
The uncertainty of the
DMM can normally be considered to � � the
smallest shown place value, if the number is
not varying. For example, if the DMM is
consistently reading 8.97 volts, then the
smallest place value is 0.01 volts. Therefore,
the uncertainty is 0.005 volts, yielding a
measurement of (8.97 � 0.005) volts. If, on
the other hand, the DMM reading is
fluctuating between 8.97 and 9.02 volts, the
measurement would be (9.00 � 0.03) volts.

KNIFE SWITCH


The knife switch simply closes or opens a circuit
by making or breaking contact between two metal
blades. Note that there are two "double throw"
switches, one on each side, each of which is entirely
independent of the other. Each switch is called
"double throw" because there are two ways to
"throw" the switch.

MICROMETER SCALE




Use this setting for
DC current
measurements no
larger than 20mA
Use these jacks for
every measurement
other than for
currents between 2A
and 10A
Figure 4:
DMM Settings
Figure 5
: Knife
Switch, open
Figure 6
: Knife
Switch, closed

The micrometer scale consists of a rotating dial outside a fixed shaft. The fixed shaft is most
commonly graduated in twentieths of a centimeter, as shown in Figure 7. Note that the gradations
are laid out so that they can be considered to be millimeters (above the center line) and "half-
millimeters" (below the center line).

The rotating dial is most commonly
graduated in hundredths of a millimeter
(the dial turns once per half-millimeter,
and there are 50 graduations per turn,
yielding 100 gradations per millimeter.)

Before reading the micrometer, check to
see what the gradations of the shaft and
rotating dial are; if they are not as
described above, then the following
procedure will be correct, but the fractions will change appropriately.

To read the micrometer scale, first determine the last visible marking on the fixed shaft. In Figure
7, the last visible gradation is the "half-millimeter" after 11, so begin with a measurement of 11.5
mm. Add to that the number of hundredths indicated on the rotating dial by the center line. In
Figure 7, the center line points between 17 and 18 hundredths; we can estimate it to be 17.8
hundredths of a millimeter (0.178 mm). So, adding that to the first value yields 11.5 mm + 0.178
mm = 11.678 mm.

UNCERTAINTY:
Of course, no measurement is complete without an associated uncertainty!
There are two primary causes for uncertainty when reading a micrometer: the uncertainty in
estimating the last digit (the 0.008 mm in the example above) and the uncertainty in the position
of the instrument with the micrometer scale. The uncertainty in the last digit will be largely based
upon how much experience you have reading precision scales - a beginner might claim to be able
to have estimated the previous value of 8 � 3 [yielding a final measurement of (11.678 � 0.003)
mm], while someone with more experience might be able to estimate the final digit to 8 � 1
[yielding a final measurement of (11.678 � 0.001) mm].

The uncertainty in the position of the instrument is very much a function of the instrument itself
and whatever is being measured. For example, if the micrometer screw is being used in the
measurement of something somewhat flexible (say a soft plastic), one might be able to rotate the
micrometer screw a bit and not know exactly when the micrometer first touch the edge of the
plastic. This uncertainty in the position of the micrometer screw for a "correct" measurement
must be reflected in the reported measurement.

RHEOSTAT or POTENTIOMETER


A rheostat or potentiometer is just a resistor with a wiper that slides along the resistor, allowing
you to change the resistance between the ends and the wiper. Notice that as the resistance
between one end and the wiper increases, the resistance between the other end and the wiper
decreases (the resistance between the two ends is fixed). For most of the labs, only one end and
the wiper will be used (e.g., note in Figure 1 of Lab E-2, Joule's Law, the schematic drawing
shows one end not connected to anything).

Figure 7
: Micrometer Scale
0
5

10

15

20

Sometimes both ends are used to
make a voltage divider, as in Lab Q-
3, The Photoelectric Effect. A
voltage divider is just two resistors
in series, where as one resistance
increases, the other decreases. You
can easily picture how a voltage
divider works by considering the
two extremes where the wiper can be
in Figure 9. Consider the left end to
be at some voltage, V (perhaps 1.5
volts), and the right end to be at 0
volts. If the wiper is all the way to
the left, it is touching the left end,
and there is no resistance between it
and the left terminal. Therefore, the
wiper is at the same voltage as the
left terminal: V (1.5 volts, in this
example). If the wiper is all the way to the right, there is no resistance between it and the right
terminal, and is therefore at the same voltage: 0 volts. As you slide the wiper between the two
ends, its voltage will be between the two extremes: between V (1.5 volts in this case) and 0 volts.
At the halfway poin