Magnets are commonly found in motors, dynamos, refrigerators, credit and debit cards, and electronic equipment, such as electric guitar pickups, stereo speakers, and computer hard drives. They can be either permanent magnets, made of naturally magnetic forms of iron or alloys, or electromagnets. Electromagnets create a magnetic field when an electric current is run through a coil of wire wrapped around an iron core. There are several factors affecting the strength of magnetic fields and several ways of determining the strength of those fields, both are described in the article below.
Coercive magnetic field strength, abbreviated Hc. This represents the point at which the magnet can be demagnetized (degaussed) by another magnetic field. The higher this number, the more difficult it is to degauss the magnet.
Residual magnetic flux density, abbreviated Br. This is the maximum magnetic flux the magnet can produce.
Related to magnetic flux density is overall energy density, abbreviated Bmax. The higher this number is, the more powerful the magnet.
The temperature coefficient of the residual magnetic flux density, abbreviated Tcoef of Br and expressed as a percentage of degrees Celsius, describes how the magnetic flux decreases as the magnet's temperature rises. A Tcoef of Br of 0.1 means that if the magnet's temperature rises 100 degrees Celsius (180 degrees Fahrenheit), its magnetic flux decreases by 10 percent.
The maximum operating temperature (abbreviated Tmax) is the highest temperature the magnet can be operated at without losing any of its field strength. Once the temperature falls below Tmax, the magnet recovers its full field strength. If the magnet is heated above Tmax, it will lose some of its field strength permanently after cooling to its normal operating temperature. If, however, the magnet is heated to its Curie temperature, abbreviated Tcurie, it will become demagnetized
Neodymium iron boron. This has the highest magnetic flux density (12,800 gauss), coercive magnetic field strength (12,300 oersted), and overall energy density (40). It has the lowest maximum operating temperature and Curie temperature, at 150 degrees Celsius (302 degrees Fahrenheit) and 310 degrees Celsius (590 degrees Fahrenheit), respectively, and a temperature coefficient of -0.12.
Samarium cobalt has the next highest coercive field strength, at 9,200 oersted. But it has a magnetic flux density of 10,500 gauss and an overall energy density of 26. Its maximum operating temperature is much higher than for neodymium iron boron at 300 degrees Celsius (572 degrees Fahrenheit), as is its Curie temperature of 750 degrees Celsius (1,382 degrees Fahrenheit). Its temperature coefficient is 0.04.
Alnico is an aluminum-nickel-cobalt alloy. It has a magnetic flux density close to that of neodymium iron boron (12,500 gauss), but a much lower coercive magnetic field strength (640 oersted) and consequently an overall energy density of only 5.5. It has a higher maximum operating temperature than samarium cobalt, at 540 degrees Celsius (1,004 degrees Fahrenheit), as well as a higher Curie temperature, 860 degrees Celsius (1,580 degrees Fahrenheit), and a temperature coefficient of 0.02.
Ceramic and ferrite magnets have much lower flux densities and overall energy densities than the other materials, at 3,900 gauss and 3.5. Their magnetic flux density, however, is much better than alnico at 3,200 oersted. Their maximum operating temperature is the same as for samarium cobalt, but their Curie temperature is much lower, at 460 degrees Celsius (860 degrees Fahrenheit), and their temperature coefficient is -0.2. Therefore, they lose field strength faster in heat than do any of the other materials.
Ampere-turn per meter is another metric unit for measuring magnetic field strength. This represents how if the current, the number of coils, or both are increased, the magnetic field strength increases.
Testing Magnetic Field Range with Paperclips
Tape one of the long ends of a clothespin to the bottom of the cup.
Place the cup with the attached clothespin on the table upside down.
Insert the magnet into the clothespin.
Testing Magnetic Field Strength With a Gaussmeter
Set the maximum voltage to be read at 10 volts DC.
Read the voltage display with the meter away from a magnet. This is the baseline or original voltage, represented as V0.
Find the difference between the original and the new voltage. If the sensor is calibrated in millivolts, divide by 1,000 to convert from millivolts to volts.