# Magnetic field and logarithmics

• GeertTimmerman
In summary, Dale provides an example of a logarithmic correlation between two quantities. The professor wants the students to build a RL circuit and measure the field near the inductor.f

#### GeertTimmerman

Hello everyone!

I'm in my first year of my bachelor in physics and astrophysics at Leiden University, The Netherlands. We have to perform a small experiment were we use a transducer to measure a logarithmic correlation between two certain quantities. As an example our prof. said you could measure light intensity as a function of time with a photodetector. Every group has a different transducer. Me and my partner have to work with a magnetic field detector (GMR), but the problem is that we cannot think of a quantity that has a logarithmic relation to the magnetic field. I did some research, but I couldn't find anything that matches our needs (we only have four hours to do the experiment itself, we have more time for the analysis of the data)

Does anyone of you have any suggestions?

thanks alot!

PS I am new to this forum but I already love it!

We have to perform a small experiment were we use a transducer to measure a logarithmic correlation between two certain quantities.
Do both quantities have to change logarithmically or is it ok if one changes linearly and the other changes logarithmically?

@Dale It is ok if one changes linearly and the other logarithmically. The most important thing is that it is easy and fast to perform (in less than 4 hours). The main point for this experiment is that we learn how to analyse data sets (plotting, fitting and such) and how to document our experiences during the experiment, not the experiment itself (i.e. it does not have to be groundbreaking

So I would take a strong permanent bar magnet and measure the field at different distances. Try one set along the axis and one set perpendicular to the axis. The field should fall off and the slope on a log-linear scale will tell you the 1/r power.

So I would take a strong permanent bar magnet and measure the field at different distances. Try one set along the axis and one set perpendicular to the axis. The field should fall off and the slope on a log-linear scale will tell you the 1/r power.
Thanks! maybe this will help us out!

Thanks! maybe this will help us out!
We just dicusses this idea with our prof and he says it has to be exponential, like a decay process... Anyone who can help is. We can really not think of a magnetic exponential growth or magnetic exponential decay.

Sounds like your professor wants you to build a RL circuit and measure the field near the inductor.

berkeman
We just dicusses this idea with our prof and he says it has to be exponential, like a decay process... Anyone who can help is. We can really not think of a magnetic exponential growth or magnetic exponential decay.
The RL circuit example given by @Dale should work -- can you say how you might set that up?

I can also think of an interesting mechanically damped exponential decay system (at least I think it can be designed to give a good exponential decay detected by your GMR sensor). It involves a lightly damped pendulum setup with a permanent magnet affixed to the bottom. If you put your GMR sensor directly adjacent to the path of the swinging magnet at the bottom of the pendulum's travel, it will not show a decay in the magnetic field that it picks up (In fact it will show a steady strong magnetic field when the pendulum comes to a halt at the bottom). But, I can think of one thing that you could put in the place of the GMR at the bottom of the pendulum's swing, and a way to transfer the received signal to the GMR a little ways away from the pendulum. I believe that this will result in GMR pulses as the pendulum passes the bottom part of its travel that will decay exponentially over time as the pendulum slows down.

Can you guess of the setup I'm thinking of? Can you mention a couple of ways that you could control the damping of the pendulum?

kuruman
An alternative arrangement to what @berkeman suggests might be to use the permanent magnet + mass not as a pendulum bob but in a vertical spring-mass arrangement. Put your sensor directly below the mass bobbing up and down and record the maximum signal as a function of time. Be sure to adjust the starting amplitude and the equilibrium position so that you get a good signal drop from one to the other. What's a measure of "good"? And to echo @berkeman, can you mention ways to control the damping of the motion?

berkeman
use the permanent magnet + mass not as a pendulum bob but in a vertical spring-mass arrangement.
Ooo, that's better than my idea. Much simpler!

kuruman