E-book Correction Understanding the PMMC figure in Direct Currents 8.1

Thread Starter

chogg

Joined Dec 8, 2019
7
This isn't necessarily a "correction"; more of a request for clarification.

In the Direct Currents textbook PDF, page 237 in section 8.1, the PMMC meter movement figure shows a coil attached to a needle between two permanent magnets. It looks like the axis of the coil is perpendicular to the needle. Based on the symmetry of the situation, why would the needle ever move past the 50% mark? It seems like the fields would be fully aligned then.

Am I misunderstanding something? Or am I just taking the figure too literally?
 

Reloadron

Joined Jan 15, 2015
5,424
Unless someone has the same text book you are looking at they haven't a clue what the image looks like.

The needle won't move past the halfway point and has no reason to. Below are a few old analog meters using PMMC meter movements.
270 Ohm Volt1.png

Meter 269 270.png

Neither of these meter movements comes close to 180 degree rotation. This is another read on the subject but considering the use and application of a PMMC meter movement I am not sure what you are asking?

Ron
 

Thread Starter

chogg

Joined Dec 8, 2019
7
Thanks for responding! I can see I communicated badly at first.

The textbook version I was reading is here: https://www.allaboutcircuits.com/assets/pdf/direct-current.pdf

By "moving past the 50% mark", I meant half the distance from the beginning to the end of the scale. I didn't mean exceeding a 180 degree rotation, but I see now how I could have given that impression.

Anyway: googling for "D'Arsonval movement" was very illuminating. I found other figures (eg: https://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_movement.htm) where the needle would clearly move past the halfway point on the scale.

Overall, I think I was probably just thinking too hard about the figure. :) I think it's "wrong", but probably not a big deal.
 

Reloadron

Joined Jan 15, 2015
5,424
Kind of funny that the meter movement I grew up using is now seldom seen as digital meters have replaced them for the most part. I can remember balancing D'Arsonval meter movements using tweezers to move the tiny counterweights. Looking back the mechanics were just plain interesting. I wouldn't overthink these things, it will make your head hurt. :)

Ron
 
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