Magnetic Quantities and Units of Measurement

Magnetomotive force is the quantity of magnetic field force, or "push." Analogous to electric voltage (electromotive force).

Field flux is the quantity of total field effect, or "substance" of the field. It is analogous to electric current.

Field intensity is the amount of field force distributed over the length of the electromagnet. Sometimes it is referred to as magnetizing force.

Flux density is the amount of magnetic field flux concentrated in a given area.

Reluctance is the opposition to magnetic field flux through a given volume of space or material. It is analogous to electrical resistance.

Permeability is the specific measure of a material's acceptance of magnetic flux, analogous to the specific resistance of a conductive materia, except inverse (greater permeability means easier passage of magnetic flux, whereas greater specific resistance means more difficult passage of electric current).

We have several different systems of measurement for each of these quantities: one is called the cgs, which stands for Centimeter-Gram-Second, denoting the root measures upon which the whole system is based; SI, or International System of Units, and English system (Table 1) (from ‘Physics for Scientists and Engineers’).

Table 1. Magnetic Quantities and Units of measurement.

Quantity	Symbol	Units of Measurement and Abbreviation
		CGS	SI	English
Field force	mmf	Gilbert (Gb)	Amp-turn	Amp-turn
Field flux	Φ	Maxwell (Mx)	Weber (Wb)	Line
Field intensity	H	Oersted (Oe)	Amp-turns per meter	Amp-turns per inch
Flux density	B	Gauss (G)	Tesla (T)	Lines per square inch
Reluctance	R	Gilberts per Maxwell	Henry per meter, Amp-turns per Weber	Amp-turns per line
Permeability	µ	Gauss per Oersted	Tesla-meters per Amp-turn	Lines per inch-Amp-turn

Exercise 64. Match the terms in the list with the appropriate definition.

Pole Magnetomotive force To repel Compass

Flux density To deflect Magnet To attract

Permeability Electromagnet Reluctance Polarity

Term	Definition
	the property of a magnetic circuit of opposing the passage of magnetic flux lines, equal to the ratio of the magnetomotive force to the magnetic flux.
	a quantity measuring the influence of a substance on the magnetic flux in the region it occupies.
	a magnet that consists of a piece of iron or steel surrounded by a coil.
	the state of an object or system in which some measure has opposite value or sign at two different points.
	each of the two points or regions of a magnet to and from which the lines of magnetic force are directed.
	a quantity representing the sum of the magnetizing forces along a circuit; it is considered analogous to the electromotive force in an electric circuit.
	the amount of magnetic, electric, or other flux passing through a unit area.
	an instrument containing a magnetized pointer which shows the direction of magnetic north and bearings from it.
	a piece of iron or other material which has its component atoms so ordered that the material exhibits properties of magnetism, such as attracting other iron-containing objects or aligning itself in an external magnetic field.
	cause (something) to change direction; turn aside from a straight course.
	exert a force on (an object) which is directed towards the source of the force.
	to drive or force back or away.

Exercise 65. Answer the following questions.

1. What did Pierre de Maricourt find in 1269?

2. What can you tell about magnet poles?

3. What did William Gilbert suggest about the earth?

4. How did Hans Christian Oersted discover the relationship between magnetism and electricity?

5. When is magnetic field produced around the conductor?

6. What does the left-hand rule state?

7. What magnetic field force is stronger: generated by a straight-line wire or a coiled wire? Why?