Measuring Magnetic Fields
AbstractHave you ever noticed how magnets appear to have no effect on each other when they are far apart? Then, when you slowly move them closer together, you will start to feel a gentle pull until they suddenly snap together? How exactly does the strength of a magnet change with distance, and how would you measure it if you wanted to find out? In this project you will build a circuit that can measure the strength of a magnetic field and see how the field strength changes with distance.
By Akram Salman , Andrew Olson, PhD, and Ben Finio, PhD, Science Buddies
Recommended Project Supplies
Build a circuit that can measure magnetic field strength and measure how field strength changes with distance.
Magnets and magnetic fields are used in everyday electrical equipment such as motors and refrigerators. You will also find them in electronic equipment like cell phones and radios. A magnetic field can be produced by a permanent magnet, or by electrical current flowing through a wire. You can make an electromagnet by wrapping a coil of wire around a magnetic material (such as iron, magnesium, or cobalt). When current flows through the coil, a magnetic field is produced. Magnetic fields are also important in communication systems. The waves used to transfer information for television and radio broadcasts or cell phone calls are electromagnetic waves. Light, x-rays, and radio waves are all examples of electromagnetic waves.
A magnetic field can be visualized as magnetic field lines, as shown in Figure 1. The strength of a magnetic field is defined as the density of magnetic field lines and is strongest close to the magnet. The strength of the magnetic field diminishes (lessens) with increasing distance from the magnet.
Figure 1. Left: magnetic field lines are represented by arrows that originate at the north pole of a magnet and curve around toward the south pole. The lines are spaced closer together near the magnet, and farther apart away from the magnet (image credit Wikimedia Commons user Geek3, 2010). Right: Field lines can be visualized by sprinkling iron filings on a piece of paper over a bar magnet.
In general, a device that measures the strength of a magnetic field is called a magnetometer. The official SI unit for magnetic field strength is the tesla (T). Magnetic field strength is also measured in units of gauss (G) (1 G = 10-4 T). A device that measures magnetic field strength in gauss, specifically, is called a gaussmeter. The gaussmeter that you will build for this project is based on the Hall effect, discovered by Dr. Edwin Hall in 1879. Hall discovered that when a current is passing through a thin sheet and a magnetic field is applied perpendicular to the sheet, a voltage (called the Hall voltage) is generated across the third dimension, perpendicular to the direction of the original current. The magnitude of the Hall voltage is proportional to magnetic field strength. The Hall effect is used in different applications, including making an electric motor.
Your gaussmeter will be based on an integrated circuit called a Hall sensor that allows you to measure the Hall voltage generated by a magnetic field. You will measure the voltage using a multimeter. Once you have constructed the gaussmeter, you can use it to measure how the strength of the magnetic field varies with distance from the Hall sensor. How do you expect field strength to vary with distance? Will the relationship be linear or nonlinear?
Terms and Concepts
To do this project, you should do research that enables you to understand the following terms and concepts:
- Magnetic field
- Electrical current
- Electromagnetic waves
- Hall effect
- What is a magnetic field?
- What is the Hall effect and how does a Hall effect sensor work?
- How do you expect the strength of a magnetic field to change with distance? Will the relationship be linear or something else?
To learn about magnetism and magnetic fields, see this Science Buddies tutorial:
- Science Buddies staff. (n.d.). Electricity, Magnetism, & Electromagnetism Tutorial. Retrieved May 7, 2015.
To learn about the Hall effect, this website is a good start:
- Nave, C.R. (2006). Hall Effect. HyperPhysics, Department of Physics and Astronomy, Georgia State University. Retrieved May 10, 2006.
To learn about the Hall effect sensor used in this project, see the product's datasheet:
- Allegro Microsystems. (2010). Continuous-Time Ratiometric Linear Hall Effect Sensor ICs. Retrieved May 7, 2015.
This Science Buddies project has information on making your own electromagnets: The Strength of an Electromagnet
For more information about how to use a breadboard, see this tutorial:
- Science Buddies. (n.d.). How to Use a Breadboard. Retrieved September 25, 2015.
For more information about how to use a multimeter, see this tutorial:
- Science Buddies. (n.d.). How to Use a Multimeter. Retrieved May 11, 2016.
Recommended Project Supplies
- Electronic Sensors Kit, available from our partner
Home Science Tools.
You will need these items from the kit. (See Table 1 in the Procedure if you do not know what these parts look like.)
- 9 V battery
- 9 V battery snap connector
- LM7805 voltage regulator
- A1302 Hall effect sensor
- Jumper wires (assorted)
- Alligator clip leads (2)
- You will also need to gather these items, not included in the kit:
- Common household magnet (like a refrigerator magnet); do not use neodymium or "rare earth" magnets, as they are too strong for the Hall sensor's range.
- Plastic or wooden ruler
- Optional: Small book or stack of sticky notes
- Lab notebook
If you want to build and measure the strength of simple electromagnets, instead of permanent magnets, you can use the Strength of an Electromagnet Kit, available from our partner Home Science Tools.
Disclaimer: Science Buddies participates in affiliate programs with Home Science Tools, Amazon.com, Carolina Biological, and Jameco Electronics. Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity, and keep our resources free for everyone. Our top priority is student learning. If you have any comments (positive or negative) related to purchases you've made for science projects from recommendations on our site, please let us know. Write to us at email@example.com.
Assembling Your Gaussmeter Circuit
Important: your Sensor Kit contains two parts that look very similar: a transistor and a Hall effect sensor. They are both small black plastic parts with three metal legs. This project requires the Hall effect sensor. When viewed from the top, it is smaller the the transistor and angled on one face, not rounded, as shown in Figure 2. Make sure you use the Hall effect sensor, or your circuit will not work. There is some writing on one side of the Hall effect sensor (the smaller side). The direction this writing faces is important, but it can be hard to see. Look carefully and try tilting the sensor under a bright light to see which side has the writing.
Figure 2. Hall effect sensor (left) and transistor (right) viewed from the top.
Assemble your gaussmeter circuit on a breadboard, as shown in the slideshow and described in Table 1. If this is your first time using a breadboard, refer to the Science Buddies reference How to Use a Breadboard for Electronics and Circuits. For a circuit schematic, see the Help section.
Slideshow with step-by-step instructions viewable online.
|E2, E3, E4
Writing facing to the left
|Hall effect sensor
|B9, B10, B11
Writing (smaller side) facing to the left. Look carefully, the writing is hard to see!
|Jumper wires (6)
|C2 to (+) bus
B3 to (+) bus
C4 to C9
A10 to (-) bus
(-) bus to multimeter
A11 to multimeter
|9 V battery and snap connector
|Red lead to (+) bus
Black lead to (-) bus
|Set to measure 20 volts DC.
Black probe in COM. Connect to ground bus with alligator clip and jumper wire.
Red probe in VΩmA. Connect to A11 with alligator clip and jumper wire.
Measuring Magnetic Fields
- Once you have assembled your circuit, your multimeter should display about 2.50 V when no magnets are nearby. If you bring a magnet near the Hall sensor, the voltage should fluctuate (whether the voltage goes up or down depends on which pole of the magnet faces the front of the sensor). If the voltage goes all the way down to 0 or all the way up to 5, then your magnet is causing the sensor to saturate, or reach the limits of its range, and you should use a weaker magnet. Experiment with your circuit briefly to see if it is working. If it does not behave as described here, see the Help section.
- Prepare a data table in your lab notebook to record distance between the magnet and Hall sensor, voltage, and magnetic field strength. You may want to pre-determine the distances you will test (for example, every 5 mm).
- Set up your experiment so you can measure the distance between your magnet and the front of the Hall sensor (the side with the writing on it) using a ruler. Depending on the size and shape of your magnet, you may want to prop it up on something (like a small book) so it is level with the front of the sensor. It is important for the magnet to remain still while you take your readings; your readings may fluctuate too much if you try to hold the magnet in front of the sensor. Figure 3 shows an example experimental setup.
Figure 3. Experimental setup to measure the effect of distance on magnetic field strength. A stack of sticky notes is used to hold the magnet level with the front of the Hall sensor.
- Make sure the sensor is not near any magnets. Record the voltage displayed on the multimeter in your lab notebook and label it as "V0". Refer to the Science Buddies reference How to Use a Multimeter if you need help using a multimeter.
- Now, starting with the magnet touching the face of the sensor (a distance of zero), record the voltage displayed on the multimeter.
- Slide the magnet directly away from the sensor (make sure you move it straight backwards, not to the side). Record the new distance and voltage in your data table.
- Repeat step 6 until the voltage stops changing.
- Repeat steps 5–7 at least two more times, for a total of at least three trials.
- Calculate an average voltage for each distance.
- Now, convert voltage to magnetic field strength. You can do this using information from the sensor's datasheet, which says that the sensor has a sensitivity of 1.3 mV/G (note that the sensitivity is given in millivolts (mV) and you took your readings in volts (V), so you will need to convert from V to mV). You can convert voltage to field strength using the following equation:
- B is the magnetic field strength in gauss (G).
- V0 is the voltage when there is no magnet nearby in millivolts (mV).
- V is the voltage recorded at a certain distance in millivolts (mV).
- 1.3 is the sensor's sensitivity in millivolts per gauss (mV/G).
Note that it is okay if the value you calculate is negative. See the Help section for more information.
- Make a graph of magnetic field strength versus distance.
- How does field strength change with distance?
- Are your results consistent with the behavior you observe when using magnets? In other words, can magnets push and pull on each other from across a room? How close do you need to bring them before they will snap together on their own?
For troubleshooting tips, please read our FAQ: Measuring Magnetic Fields.
Ask an Expert
- Can you use your gaussmeter circuit to measure how another variable affects magnetic field strength? For example, what about the type or size of magnet, temperature instead of distance, or the number of wire turns in an electromagnet? For the latter two ideas, see the Science Buddies projects How the Strength of a Magnet Varies with Temperature and The Strength of an Electromagnet.
- Can you use your gaussmeter circuit to create a map of the field lines around a magnet? Remember that magnetic fields have both magnitude and direction. The Hall sensor in your circuit only measures the magnitude of the field that is perpendicular to the face of the sensor (the side with writing on it). That means in order to draw field lines, like the ones shown in Figure 1, you would need two take two measurements at each point in space, with the sensor rotated 90° for X and Y measurements. Knowledge of math topics like the Pythagorean theorem, vectors, and trigonometry will be helpful for this experiment.
- The previous point might be easier if you add a second Hall sensor to your circuit, perpendicular to the first one, so you can take two readings at once. You can order individual Hall sensors from Jameco Electronics. Can you add a third sensor to measure the magnitude and three-dimensional orientation of a magnetic field at any point in space?
- What is the exact mathematical relationship between magnetic field strength and distance from the magnet? Does it follow the inverse square law or is it something else? Be careful, the answer to this question can be tricky!
Frequently Asked Questions (FAQ)
- You have your circuit set up correctly, but your multimeter set up incorrectly. Make sure the black probe is plugged into the port labeled COM, the red probe is plugged into the port labeled VΩMA, and the dial is set to measure 20 V (the white "20" in the upper-left section of the wheel).
- You have your multimeter set up correctly, but part of your circuit is incorrect. It only takes a single misplaced component lead or jumper wire to prevent the circuit from working, in which case the Hall sensor may not output a voltage and the multimeter will read zero.
- Make sure the exposed metal parts of the probes and alligator clips do not touch each other. This will cause a short circuit and make the multimeter read zero.
- You have a short circuit (power and ground are shorted directly together). This can happen, for example, if you accidentally place both leads from the battery in the same breadboard bus, or if you misplace a jumper wire. This causes a large amount of current to flow from the battery, which can cause it and the circuit to overheat. Plastic parts (like the breadboard and wire insulation) may even begin to melt.
- You connect the pins of a component incorrectly, for example by reversing power and ground, or by connecting power to an "output" pin. Some electronic parts contain built-in protection against such accidental connections, but some do not.
- You supply too much voltage to a part. For example, in this project the Hall sensor is designed to work with a supply voltage of 5 V, and it is rated for an absolute maximum supply voltage of 8 V. Connecting it directly to the 9 V battery may damage it.
However, the sensor requires a 5 V power supply to operate correctly. The battery supplied in your DIY Sensors Kit is 9 V. The LM7805 voltage regulator is used to convert the 9 V from the battery to a stable 5 V supply for the sensor. You can learn more about the voltage regulator from its datasheet.
Figure 3. Schematic for the gaussmeter circuit. Pin numbers on the physical parts are from left to right when the front (side with writing) is facing you.
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Contact UsIf you have purchased a kit for this project from Science Buddies, we are pleased to answer any question not addressed by the FAQ above.
In your email, please follow these instructions:
- What is your Science Buddies kit order number?
- Please describe how you need help as thoroughly as possible:
Good Question I'm trying to do Experimental Procedure step #5, "Scrape the insulation from the wire. . ." How do I know when I've scraped enough?
Good Question I'm at Experimental Procedure step #7, "Move the magnet back and forth . . ." and the LED is not lighting up.
Bad Question I don't understand the instructions. Help!
Good Question I am purchasing my materials. Can I substitute a 1N34 diode for the 1N25 diode called for in the material list?
Bad Question Can I use a different part?
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