More than a feeling?
Using magnets to find studs is nothing new.
In fact, for a very long time, it was the ONLY way to find studs since the technology to locate studs using electronics wasn’t invented yet.
The first stud finder that I ever used was a little magnetic thing 
that worked…sort of. The magnet was so tiny that it didn’t move much when it came across a metal fastener. And it would also react to all the tiny bumps that are on wall surfaces. Nice idea in theory but not in real life. But since this was the only method available (other than the knuckle test), people made the best of it.
It’s electric!
Sometime in the 80’s, someone figured out that if electronic ultrasonic waves were aimed at the wall, the pattern of reflected or bounced waves would vary depending on the density of the wall at any particular location. That’s how radar works.
You can think of this technique as being kind of an electronic knuckle. 
Just as your finger tapping sound goes from “hollow” (low density) to “solid” (high density), the transmitted ultrasonic waves will produce a similar response and a receiver circuit decodes this pattern and lights up where it thinks the wall density changes are.
Rule of thumb is that the electronic stud finders work about as good as your knuckles…
…meaning if the wall is hollow and covered with simple sheetrock, then the stud finder should be very accurate because there’s a big difference in sound (density) across the wall.
But when the construction becomes more dense, this makes for less of a distinction between “wall without a stud” and “wall with a stud”. This means that the electronic stud finder has to make a tougher decision and is more error prone.
Which brings us to plasterboard walls, with their layer of hard plaster over a form of sheetrock. These make stud finding more difficult because the reflected “sound” doesn’t change as much (as sheetrock) when you move around the wall.
But it gets worse…
No discussion of the frustrations of stud finding would be complete without mentioning the dreaded “lath and plaster“. This is how most walls were constructed throughout the 1800’s and as recent as 1940-1950. Walls made this way are inherently hard and solid—no matter where you scan or tap. Do a Google search “how to find studs in lath and plaster” and you’ll see that most people have yet to find anything that can find a %*^! stud in their old homes.
Earlier in this post I mentioned that magnets are very effective at finding studs because they are looking for actual metal fasteners that are screwed or nailed into actual studs.
So when you find the screw/nail, you’ve probably found a stud.
As I said earlier, sheetrock is the easiest type of construction in which to locate studs, using any of the various methods: magnets, ultrasound/electronic and of course, “knuckles”.
For magnetic stud finders, sheetrock has the metal fasteners just below the surface–a little more than the thickness of paint. So just about any magnet will be pulled hard enough for you to “feel” the tug thus letting you know exactly where the screw/nail is.
Plasterboard is not as easy. I’ve already explained why electronic devices are challenged by this type of dense construction. But what about magnets? Are they any better than the battery operated units?
Well it all depends.
In these older types of construction, plaster is troweled onto the wall, over the metal fasteners. This increases the distance between the magnet and the fastener and the magnet’s strength or “pull” drops off very, very quickly. This makes it difficult for you to feel a definitive tug. So just as the electronic devices have a harder time making a clean decision in thicker walls, so it is with your typical magnetic stud finder.
A typical magnetic stud finder is nothing more than a magnet wrapped in plastic or rubber.
While it is true that magnets aren’t fooled by variations in wall density like the electronic stud finders,
it’s also true that a magnet’s effectiveness is only as good as its ability to “communicate” to you where the metal fasteners are. In other words, if you can’t feel the weak tug of little nail that’s under a 1/2″ of horsehair plaster, then a magnetic stud finder is not very useful because the tiny tugs that happen over the distant nails are so weak that they feel more like wall friction than a well defined “tug”.
Unless your magnetic stud finder has some other way to precisely indicate the location of the fastener, then the accuracy of a “stationary” magnet on anything other than sheetrock is not much better than the electronic testers.
It’s safe to say that, on a hollow sheetrock wall, a magnet is about as effective as a battery operated unit.
In summary, since most walls are not simple sheetrock, there’s got to be a way to easily find studs in any kind of wall: thick or thin, dense or hollow. Or is that too much to ask?
Not anymore.
