A basic question about Calibration of measuring instruments...

Dear All,
Just wanted to know the answer of basic query..

Why Calibration is necessary in case of PMMC Ammeter, while its not necessary in case of Tangent Galvanometer?
Both, PMMC Ammeter & Tangent Galvanometer measure the value of current in terms of Theta, i. e. Angle of Deflection of Pointer & some constant.

I don't know these devices well, but in general instruments used to measure a quantity often are calibrated to ensure the reported result is accurate.
Some businesses have test tools that are no longer supported by the instrument supplier (i.e. end-of-life), but the test tool still has some practical value if it is not disposed;
as a result those test tools are not used for critical measurements for customer-destined products.
Those test tools would not need calibration. In other words, the need (or absence of the need) for calibration can be a commercial decision, not a physics-related
decision.

Well, no doubt I am satisfied with the answer..
But, the thing is, I am a teacher of the subject of Instrumentation & Measurement to Diploma 2nd year students, & I want to let them understand the with the answer of above question...
So, if anyone knows about above two instruments, it will be a great thing for me!

accuracy in pointing the value becomes the priority in the tg and so they meaure current and all related stuff in theta and not in direct amperes

they also take into account the position and the location of the person in the surface of the earth as polarity of earth magnetic field is fixed relating the measured entity with it gives us higher accuracy


i dono abt the other device

Perhaps the answer is in the manner of operation of the two instruments. The Tangent Galvanometer operates by COMPARING the field generated in the coil with the earth's magnetic field, which is assumed to be a constant. The angle of the compass needle is determined only by the ratio or the earth's field to the field generated in the coil. You measure that angle and do the math and you have you r answer.

On the other hand, the PMMC Ammeter places a moving coil inside the field generated by a permanent magnet. The exact value of that permanent magnet field is not a constant in the same sense as the earth's field. It can vary from one magnet to another. It is also possible that it may change over time. So a calibration is needed.

Well, the vector of the earths magnetic field is different depending where you are. I took a compass from the USA to Australia... It pointed downward so much it rubbed on the case. A new compass for the s hemisphere was required. The surrounding proximity is important too. Anything conductive within the vicinity will probably change the magnetic field. Those instruments are for high accuracy measurements. As for the permanent magnet vs the earths field, their orientation to each other will change the result. Aren't they both vectors and add thusly?

I didn't make any claims that using the earth's field as a standard was a particularly good idea. It definitely isn't. But you must remember that technology does have a history.

So, in the year 18xx (+/-) when this was invented, perhaps it was the best option. Or at least one physicist thought so. I am very sure he/she did not have a jet plane at hand to carry either the instrument or a compass around the world. His/her experiments were very likely confined to a very small area of the earth's surface. I very seriously doubt that anybody outside of a HS or college lab class would use this instrument today and calibrate it in this manner. If it is taught in such schools today, it would be purely for instructional purposes.

The horizontal component of geomagnetic field, which is used as a field strength standard in the tangent galvalometer is varying both locally and in time. "Requiring no calibration" depends on your knowledge of the present geomagnetic field strength.

In my view, a calibration free measurement method depends on a primary physical standard. I doubt that the geomagnetic field would be considered as such.

https://en.wikipedia.org/wiki/Geomagnetic_secular_variation