Experimenting with Resistance – How to Measure
Electrical Conductance through Liquids
How to Make the Conductance Meter
This describes how to make a conductance meter that measures the conductivity of liquids. It uses
two pieces of copper (e.g. two 2p coins) that act as electrodes to complete the circuit once
submerged into a liquid held within a glass beaker. This allows the resistance and therefore
conductance of the liquid in question to be measured.
The theory behind conductance, in particular that of liquids, can be found on page 8.
Conductance Meter in Action. To obtain the most consistent results it is important to ensure the
electrodes (copper coins) are the same distance apart, and the same depth, for every liquid that is
analysed. To facilitate this the crocodile clips, to which the coins are connected, are attached to a
ruler. It is also fundamental to not overfill the beaker/container that the liquid is held in because if
the metal of the crocodile clips is in contact with the liquid a ‘false’ reading will result. Below you
can see the sensor in action.
Safety: Please note that you use these resources at your own risk. Correct use of some components requires care.
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources
Required Components:









1 ExperiSense board + Arduino (UNO) + cable
3 crocodile clips (double-ended)
2 x 1p/2p coins (1p coins were used here)
1 ruler
1 plastic/glass utensil (a pen was used here)
Blue-tack (or tape)
Container for the liquid(s) (a glass beaker was used here)
Ingredients, i.e. table salt, vinegar, sugar and tap water (distilled/deionised water is more ideal)
Extension task – a variety of different liquids (e.g. lemon juice, milk, cooking oil)
Step 1: Attach the copper coins to two of the crocodile clips. It is best to try to get two relatively
new/clean coins because any dirt present will affect the ion transfer and thus the resistance that is
measured – see image below.
Step 2: Fix the crocodile clips to a ruler as shown above using either tape or blue-tack (we used
blue-tack here). The distance between the clips depends on the size (width) of the container, 4 cm
was used in this case since the beaker employed was about 5 cm wide.
Safety: Please note that you use these resources at your own risk. Correct use of some components requires care.
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources
Step 3: Carefully secure the ExperiSense board onto the Arduino UNO; the prongs of the
ExperiSense board should fit perfectly into the holes on the Arduino (see images below).
Step 4: Attach one of the free crocodile clip ends to one of the ports on the ExperiSense board (we
chose port A) and the end of the other to ground (GND). You now have your two electrodes setup.
Step 5: The circuit needs to be grounded for it to be complete. To do this, simply connect a third
crocodile clip between ground (GND) and a different port on the board (we chose port D).
Step 6: Finally, connect the Arduino to the computer via (type A to B) USB cable.
Safety: Please note that you use these resources at your own risk. Correct use of some components requires care.
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources
Experimenting with Resistance – How to Measure
Electrical Conductance through Liquids
How to Use the Conductance Meter
Scratch sensors give readings from 0 to 100. This is ok for giving a qualitative measurement of
resistance/conductance, however for a more quantitative and realistic approach we need an
accurate reading of resistance. To this end we have constructed our own board (ExperiSense) to
be used in conjunction with ScratchX.
ScratchX, which can only be accessed online, grants you the ability to play with
experimental extensions to Scratch, allowing the creation of Scratch projects that connect with
external hardware (such as electronic devices and robotics) and online resources (including web
data and web services).
The conductance meter works by applying a potential difference across the copper coins
(electrodes). When the electrodes are apart and not in solution, the circuit is ‘incomplete’ and no
current flows. However, when the electrodes are touching the circuit becomes ‘complete’ and
current flows, which then gives a value of resistance back to ScratchX.
Step 1: Before you can start using your conductance meter you’ll first need to get the Arduino
UNO, and hence the ExperiSense board, communicating with ScratchX – see separate worksheet
for the ExperiSense board.
Step 2: We have chosen to represent the results in a graph of Conductance against Time, for
which we have drawn a set of x- and y-axes for the stage (see below), because we feel this is the
best way to illustrate the data. [N.B. You can represent the conductance in any way you see fit,
however this worksheet will only give instructions for recording the conductance graphically].
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources
Step 3: Create 2 variables called “Resistance / Ωk” and “Conductance / mS”. To create a variable
go to the Data tab, click on ”Make a Variable”, type in a name and make sure ”For all sprites” is
selected, then click OK (as demonstrated in pictures blow).
Step 4: Make sure the “Resistance” and “Conductance” variables have a tick next to their name as
in the picture on the right below.
Step 5: Create the following sequence of script for the Stage. You will find the blocks in “Events”,
“Control”, “Data”, “Operators” and “More Blocks” tabs. The block that reads “(normal) resistance
on (A) (kΩ)” is a custom block that has been built by us that allows the resistance across a circuit
to be measured. Some things to note:


The first drop-down list gives the choice between ‘normal’ and ‘sensitive’, which determines
the sensitivity of the measurement; in cases where smaller changes in resistance are required
then ‘sensitive’ is better (‘normal’ is sufficient for our use).
The second drop-down list is the port of the ExperiSense board that is to be measured, i.e.
which port is in use. This will depend on how you connected up your crocodile clips on page 3
(we connected our circuit up to port A, therefore we want to measure the resistance across
this port).
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources
Step 6: Click the green flag and you should see the “Resistance” and “Conductance” variables
change values in the Stage.
Step 7: Create a sprite and draw a small dot, a good size is shown below. Call this sprite “Data
Point”.
Step 8: Create the following sequence of script for the “Data Point” sprite. You will find the
required blocks under “Events”, “Control”, “Motion”, “Pen”, “Operators” and “Data”. Things to be
aware of:
 The x- and y-coordinates that are set within the script are those of the origin – this will be
dependent on the size and position of your axis (for us it is at x: -172, y: -132).
 Prior to introducing a liquid to the circuit, the circuit will be incomplete and thus resistance will
read ∞ (infinity) kΩ. This means conductance will technically equal 0, therefore the graph will
start plotting before you begin the experiment. Including an ‘if () then’ loop and setting a zerolimit for the conductance as shown will prevent this.
 Since the stage is a finite width, there will come a point where the time will extend off the
screen. To counteract this, we have included a ‘repeat until ()’ loop, setting conditions for the
x-position (we chose 200 because this was the maximum value of our x-axis). This makes the
graph begin plotting again from the left, making it continuous.
 The scale of the conductance axis (y-axis) needs to be configured. This is based on the
maximum possible conductance reading (ignoring infinity), which can be obtained by placing
the electrodes into some brine (salt water). Once this maximum value has been obtained,
determine the total length of the y-axis (ours is 287), and then divide this length by the
maximum conductance (we got ca. 28). The result is the calibration constant, or ‘fudge factor’,
for your graph. You now need to multiply the measured conductance by this value, then add
the minimum y-coordinate of the conductance axis (ours is -132), since the graph doesn’t start
at 0.
 Conditions for the conductance are also required in the ‘repeat until () loop’ because the graph
will continue to plot if the electrodes are suddenly removed from the liquid. Setting a zerolimit here will prevent this since the conductance will equal zero if the electrodes are removed.
 You can set the ‘wait’ duration to whatever you want, the lower the time the more frequent
the measurements and thus the more frequently data will be recorded. However, the smaller
the time the more sensitive the meter and so the larger the fluctuations will be, which results
in a less smooth curve. It might take some trial and error to find the best sampling rate for you.
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources
Step 9: Your conductance meter is now all ready to go. Pour the water (tap or distilled) into the
beaker, enough so that the copper coins – but not the crocodile clips – become submerged. Next
lower the electrodes into the liquid, holding the edge of the ruler onto the rim of the beaker (see
page 1 for the final setup). Press the green flag and the conductance of the liquid will start being
plotted on the graph – the line should be reasonably flat for now.
Step 10: Without disturbing the device, drop a very small amount of salt in the water and gently
stir the water with the plastic/glass utensil to mix the solution. Keep adding salt over time, with
stirring, and observe the change in conductance of the water/salt solution.
Step 11: Refill the original container with fresh water (tap or distilled) and repeat the above
experiment, however in place of salt use vinegar; lower the electrodes into the water then add a
few drops of vinegar and stir. Repeat this action and observe change in conductance of the
water/vinegar solution. [N.B. You may have to re-configure the scale of the conductance (y-) axis].
Step 12: Repeat step 10/11 using sugar instead of salt/vinegar and observe the conductance over
time.
Extension Task – Experimenting with Different Liquids
The conductance of electricity through other liquids besides water can also be determined. This is
achieved using the same apparatus and setup as before.
Instead of observing the effect of adding salt to the conductance of water, simply investigate the
conductance of a variety of different liquids, such as cooking oil, milk, and lemon juice, and
compare the values obtained with each other and for that of water (both with and without salt).
Are there any interesting or strange results? Which liquid(s) is the best conductor of electricity?
Are any liquids not very good conductors? What other liquids do you think will conduct electricity?
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources
Theory behind the Conductance –
Measuring Conductance of Liquids
What is Electrical Conductance?
 The ease with which an electric current passes, which is a flow of electrical charge (electricity).
 Metals: charge carried by electrons moving through the metal. Electrons are negatively charged
subatomic particles.
 Solutions: charge carried by ions moving through the solution, from one electrode to the other.
 Ions are atoms or small groups of atoms that have a positive or negative electrical charge.
 The greater the number of electrons or ions in solution, the greater the electrical conductance.
 Salts, like table salt (NaCl), form ions when they are dissolved in water – Na+ and Cl-.
 Acids, like citric acid (in lemons) and lactic acid (in milk), also form ions when dissolved in water.
 Neither salt nor citric acid conducts electricity in their own, they must be dissolved in water.
Distilled/Deionised Water
Tap Water
 Boiled and condensed water; lacking
minerals, impurities and ions – ‘pure’ H2O.
 Does not conduct electricity very well – water
molecules have no charge.
 Some H+ and OH- ions exist due to very small
number of water molecules dissociating and
recomposing spontaneously.
 Isn’t ‘pure’ – contains impurities
(dissolved minerals) like calcium,
magnesium, iron and sodium ions.
 Can conduct electricity, these ions can
help conduct electricity.
 Ion levels in tap water will vary, so
conductance will vary.
Brine (e.g. seawater)
Vinegar
 Brine is water saturated with Na+ and Cl- ions
– no more NaCl will dissociate into ions.
 Extra added salt won’t influence the
conductance of the solution.
 Has relatively high electrical conductance.
 Solution of acetic acid in water.
 In water acetic acid hydrolyses to give
hydronium (H3O+) and acetate ions.
 Conducts electricity due to the
movement of these ions.
Cooking Oil (oils)
Sugar
 Molecules in oils are non-polar
with very strong covalent bonds.
 Cannot dissociate in water,
therefore ions can’t form.
 No electrical conductance.
 Sugar is made of uncharged particles (molecules).
 Despite dissolving in water it doesn’t dissociate into
ions, it just remains as sugar molecules.
 Adding sugar to a solution won’t increase the
electrical conductivity.
Produced by Matthew Earl ([email protected])
Technology Volunteers: go.warwick.ac.uk/techvolunteers
Scratch Resources: go.warwick.ac.uk/scratchresources