# Magnetic strength cannot be measured

1. Jan 14, 2012

### bobsmith76

I don't get the following passage from my textbook. Why can't the magnetic force be measured? I don't see why there is no unit for p.

If one pole of a long, thin bar magnet is placed in the vicinity of one pole of another long, thin bar magnet, Coulomb’s magnetic force law states: The magnetic force F between one pole of magnetic strength p1 and another pole of magnetic strength p2 is inversely proportional to the square of the distance r between
them, or F ∝ p1p2/2 . It is not possible, however, to find a proportionality constant, because it is not possible to define a unit for p, a magnetic monopole

2. Jan 14, 2012

### Drakkith

Staff Emeritus
I *think* it's just saying that because there are no magnetic monopoles, which would be an object that is only 1 pole of a magnet, aka a "magnetic charge", we cannot define a constant to it's strength like we can with the electric force, which does have single charges of both positive and negative. Not sure though.

3. Jan 15, 2012

### technician

I would agree with Drakkith. In electricity and gravitation it is possible to define a unit charge and a unit mass but I have never met a definition of a unit magnetic pole.

4. Jan 15, 2012

### clem

The magnetic pole strength at the end of a bar magnet equals MA, where M is the magnetization and A the area lof the end in Gaussian units. The unit of pole strength is
gauss-cm^2. The end acts like a point monopole when its area is small compare to r^2.