# how do we actually count the number of magnetic field lines and what does the gap between those lines describe?

on the internet,(http://nuclearpowertraining.tpub.com/h1011v1/css/h1011v1_53.htm), there is a statment which says that 1 Wb is $1\times 10^{8}$ magnetic field lines. My question is, How do we count those lines exactly or what type of experiments are conducted to claim a number? can we extend this concept to electric fields(can we say $n$ electric field lines?). in this picture, I can see two lines at the top, but I don't think it is actually two lines, its just a representation(may be proportionally drawn like magnitude of a vector.) The part I don't understand is the gap between them.Does this show that there is no force between the gaps, meaning if there are n magnetic field lines, there is a gap between the $n_{i}^{th}$ and $n_{i+1}^{th}$ magnetic field line because they can't cross each other. So, does this mean that there is no force experienced by a small ferromagnetic material in that gap?

## 2 Answers

First of all, there is no real or observable lines. Even the magnetic and electric fields are nice and abstract fields which describe observable forces. The term "line" you read is an old unit of magnetic flux. One line is the flux of a uniform magnetic field of one gauss across a surface of one square centimeter perpendicular to the field, $$1\ line = 1 G\cdot cm^2 = 10^{-8} Wb.$$

I suppose the origin of this name is related to the fact that we normally associate magnetic flux with the difference between lines crossing out and in a surface. Just like the flow of a fluid. However it is still an analogy, nothing else but a way to visualize the flux.

The "gap" between them is also a "representation". As Faraday told us, the tangent along the lines give us the direction of the field while the density of the lines give us the intensity of the field. There definitely is field in between the drawn lines.

Field lines are a good concept for imagining things, but it does not reach too far.
Imagine for example the field of two distinct sources -- the field lines would cross if you just draw them both. But this does not represent the sum field.
Field lines are drawn by convention so, that their density is approximately proportional to the field strength. This is can obviuosly be only approximate.

So the statement about the number is just wrong/doesn't make sense.

I didn't call write anything about magnetic on purpose, it's the same for electric/gravitation