# Imaginary plane of the fulcrum

I’m reading about a patent on a pocket-sized weighing scale and I need some help figuring out what this means …

What is the imaginary plane containing the fulcrum? Does it go vertically or horizontally through the fulcrum? If it’s vertical, is it perpendicular or parallel to the beam?

It would be easier to answer if we have a diagram to look at. Until then, I’m assuming that it means the plane in which the axis of the fulcrum lies, which would be horizontal and parallel to the ground in all balances I’ve ever seen.

QED is right, a diagram or description (or even a patent number) would help, but I interpret that sentence differently than he did.

In the context of a standard balance, it would make sense that “the imaginary plane of the fulcrum” would be the vertical plane containing the balance arm, weighing pans, and fulcrum – i.e. the plane perpendicular to the axis of the fulcrum, not a plane containing the axis. It could make good sense to assure the weight is in this plane to assure that the weight is reasonably balanced in the pan, rather than exerting an off-center (lateral) force or torsion. I can’t imagine placing weights on the plane of the axis around which the fulcrum turns: the fulcrum generally turns toward or away from the weight, and by definition, that is perpendicular to the axis.

This also has the advantage that “the plane of the fulcrum” is uniquely defined. There is only one plane perpendicular to a line, but there are an infinite number that contain a given line. Absent any further specification, I’d expect the Patent Office to request clarification before granting the patent

Of course, since you’re looking at patents, I presume that the inventor had some clever non-standard device design (and I’ve seen plenty of those) so all bets are off.

I’m starting to believe that the plane it’s talking about is a vertical plane that is perpendicular to the beam. In other words, when set to the zero position, the weight is somehow on both sides of the fulcrum – maybe there’s some kind of protrusion or something.

You can see the patent at the U.S. Patent and Trademark Office’s Web site by doing a search for Patent No. 4,744,428. There are links to images, but I couldn’t get them to come up.