Determining Factors that Affect Magnetic Field Strength
Consider a magnet’s characteristics. Magnetic properties are described using these characteristics:
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.
Note the material a permanent magnet is made from. Permanent magnets are typically made from one of the following materials:
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.
Count the number of turns in an electromagnet’s coil. The more coil turns per length of the core, the greater the magnetic field strength. Commercial electromagnets have sizable cores of one of the magnetic materials described above and large coils around them. However, a simple electromagnet can be made by wrapping a coil of wire around a nail and attaching its ends to a 1.5-volt battery.
Check the amount of current flowing through the electromagnetic coil. Use a multimeter to do this. The stronger the current, the stronger the magnetic field generated.
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.