PASCO EM-8812 Resistance Apparatus - Manual (Page 4)

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R e s i s ta n c e A p p a r a t u s W i r e s
4
Wires
The set of wires included with the apparatus contains two of each sample. Place one
of each sample in the storage trough on the apparatus for immediate use and set the
others aside as spares to replace lost or damaged wires. Order part EM-8813 for a new
set of replacements with two of each wire.
The set includes wires of five different materials with the same diameter, and four dif-
ferent diameters of the same material (brass). Refer to the table to identify the wires.
Voltage Measurement
Measure the voltage drop along the wire with a model PS-2160 Galvanometer Sensor
or a multimeter with a resolution of 0.1 mV or better. The maximum voltage mea-
sured will be less than 1 V.
Current Supply and Measurement
Current is established in the wire by an external power supply. Select a power supply
capable of at least 1 A at 1 V. The apparatus contains a 2 A fuse to protect against
excessive current (see page 6 for fuse replacement instructions). If you are using a
current-regulated power supply capable of more than 2 A (such as model SE-9720A),
set the current regulation to 2 A before connecting it to the apparatus. The apparatus
contains a series resistance of 0.5
Ω, which makes it easier to tune the current through
the wire by changing the applied voltage.
To measure current you can use a model PS-2115 V/I Sensor or a multimeter. If you
are using a power supply with an accurate built-in current meter (such as model
PI-9877), a separate meter is not necessary.
Four-wire Measurement
In the apparatus, the resistance of a length of wire is determined by applying a known
current and measuring the voltage. This technique is known as a four-wire measure-
ment. (The “four wires” are the two leads through which current is applied and the
two leads of the voltmeter.) The voltage is measured only across the part of the wire
under test (excluding the power supply leads), and virtually no current flows through
the voltmeter leads (so there is no voltage drop in them). This technique allows a very
Material Color
Attracted
to magnet?
Approximate
Resistivity 1
(µΩ·cm)
Diameter(s)
(inches)
Maximum 2 Constant
Current
(A)
Copper Red No 1.8 ± 0.1 0.040 2
Aluminum Light gray No 4.9 ± 0.1 0.040 2
Brass Yellow No 7.0 ± 0.5 0.020, 0.032, 0.040, and 0.050 2
Nichrome Dark gray No 105 ± 5 0.040 0.5
Stainless Steel Dark gray Yes 79 ± 1 0.040 1
1 All samples are alloys. The actual resistivity of a sample depends on its composition.
2 Excess constant current will cause wires to heat up, changing their resistivities. Current up to 2 A can be applied briefly to
all wires.