# How can I detect magnetic fields which are weaker than the Earth's?

• FHDCUTjg
In summary, someone is trying to simulate an electrical line by inducing a magnetic field. They are limited by their equipment and need to find a way to reduce the effect of the Earth's magnetic field.f

#### FHDCUTjg

TL;DR Summary
I don't have access to high power systems, or ultra precise detectors.
Can I detect small variations (frequencies ?) of magnetic field, while ignoring the noise caused by the Earth's magnetic field ?
I don't have access to high power systems, or ultra precise detectors.(around 10V/ 1A)
Can I detect small variations (frequencies ?) of magnetic field, while ignoring the noise caused by the Earth's magnetic field ?
My goal is to measure the magnetic field through a line in order to trace it. The problem is to overpass Earth's magnetic field, a quick approximation with the Biot-Savart law gives me a current of 23A to produce a field equal to the intensity of the Earth magnetic field, so I need to go beyond this value, which obviously requires a higher intensity, something that I can't do/access.

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Welcome to PF.
If you can induce a small current with a frequency of say 1 kHz into the line, you can then sense the magnetic field with a small pickup coil and detector tuned to 1 kHz. You then do not have to compete with the Earth's field because the Earth produces almost no 1kHz component, while your detector is AC coupled.

Twigg, sophiecentaur and DaveE
It is not problematic, it is done in certain magnetic tests to aerospace standards.

Line up a non-magnetic track/indicator north to south (magnetic) so that any magnetic fields in that direction are neutralised. Then align your measurement device with it and it should therefore read a maximum field strength, which you can also generally calibrate with some local government map which usually is more accurate a figure on it than you can measure unless you have magnetic rocks around.

You will need to do this all out in the open space, some several meters away from any structures, using a wooden table or wooden plank of some description and perhaps mark the table with a piece of plastic tape running N-S.

Then, with the indicator N-S and measurement device indicating maximum, introduce your magnetic field and then re-orient the device to read the maximum and determine that angle. A good quality orienteering compass will be quite adequate for this and much easier.

You then have some simple trigonometry to play with; you have a deflection angle and the Earth's field, now you can work out the field strength of the deflecting field relative to the local Earth's field.

If you want to create a field equal to the Earth's field, then simply ramp your current until it is a 45 degree deflection.

hutchphd
Summary:: I don't have access to high power systems, or ultra precise detectors.
Can I detect small variations (frequencies ?) of magnetic field, while ignoring the noise caused by the Earth's magnetic field ?

I don't have access to high power systems, or ultra precise detectors.(around 10V/ 1A)
Can I detect small variations (frequencies ?) of magnetic field, while ignoring the noise caused by the Earth's magnetic field ?
My goal is to measure the magnetic field through a line in order to trace it. The problem is to overpass Earth's magnetic field, a quick approximation with the Biot-Savart law gives me a current of 23A to produce a field equal to the intensity of the Earth magnetic field, so I need to go beyond this value, which obviously requires a higher intensity, something that I can't do/access.
I'd do what @Baluncore said. If that's not good enough, look into synchronous detection (or lock-in amplifiers). The sensitivity in rejecting unwanted signals this way can be astounding.

This doesn't have to be that expensive. Used lock-in amplifiers can be bought relatively cheaply.

ignoring the noise caused by the Earth's magnetic field
What sort of magnetic signal level do you want to measure? The numbers are important here. It's pretty easy to orientate to reduce the effect of the Earth's field by a factor of say 1/100. Would that be enough for your purpose? Otherwise, using a low frequency signal for your measurements as @Baluncore suggests can give you even more noise advantage.

hello and thank you for your responses

My friend and I are trying to simulate an electrical line (with the PI model of a line) in order to conduct experiment on it, and especially trying to locate it like if it was burried. We've done some researchs and we found that people use the magnetic field they induce through the line to trace it ( we mainly use this document as a guiding line: https://www.radiodetection.com/sites/default/files/Theory-Buried-pipe-manual-V10.pdf)
You might find it funny but we haven't done our lesson on electromagnetism yet... But since the magnetic field is proportionnal to the current and we are limited by our equipment (1A max ), the standard of the magnetic field will be low

These early doubtless used some sort of AC detection. My guess is that they used that coil in a "tank" (L-C oscillator) circuit and detected the change in the inductance of the coil caused by pipe by a change in resonant frequency. This is how most metal detectors work.
If memory serves the British used big coils to make their ships invisible to magnetically t5riggered mine during WW2. Might be worth looking at.

But since the magnetic field is proportional to the current and we are limited by our equipment (1A max ), the standard of the magnetic field will be low
I'm still not clear what you actually want to do here. What is your "equipment"? If you are using AC then the Earths magnetic field is irrelevant. A 1A AC (low frequency RF) current flowing round a loop would be easy to detect using the sort of resonant circuit you get in a modified standard AM receiver.

It all depends on your ability to build some special equipment, though. DC would be a dead loss so you would need some skill (or help) to build something. Also there are regulations concerning permitted interference levels. What's the size of the kit?

I'm still not clear what you actually want to do here. What is your "equipment"?
It is not surprising you are confused.
There is a new OP, Post #6; with a quite different subject/project, added to the earlier thread.

russ_watters
It is not surprising you are confused.
There is a new OP, Post #6; with a quite different subject/project, added to the earlier thread.
I just sort of assumed that post was demonstrating that DC wouldn't work but that AC would /did.
The scale of the experiment would make a difference, I think and we still don't know the details of the proposed experiment in the OP.
The Current Balance manages to work with pretty low values of current but an AC version of it can measure much lower values of current (AKA Field).

It is not surprising you are confused.
There is a new OP, Post #6; with a quite different subject/project, added to the earlier thread.
Ok I'm sorry if you didn't understand correctly, I should have been clearer so I'm going to explain again:
The original post has been written by my friend because we thought that the Earth's magnetic field would be problematic for our measurements. Basically we are trying to reproduce an electric line with the PI model( a circuit with a resistor a capacitor and a coil up to 100kOhm, 1microF and 100mH respectively, the maximum we can reach with one component available to us) in order to test methods used by professionals to detect and trace an unknown underground cable.
According to what I've read, DC is useless because it is difficult to measure a static field against the earth’s magnetic field, so we'll need to use AC, but we are afraid that using low values of current (1A max) will make the magnetic field induce undetectable due to the Earth's one, especially because we want to experiment how the value of the current affects the distance a signal induced by it can affect the distance the given signal can be traced, so we'll probably need to start with a value lower than 1A.

You asked for the scale of the experiment, we'll use what you find in a school for experiment, the component will not be extravagant, we just need to determine the length of the circuit to detect an effective signal reduction, but I assume adding some meters to the circuit won't be a problem

Basically we are trying to reproduce an electric line with the PI model( a circuit with a resistor a capacitor and a coil up to 100kOhm, 1microF and 100mH respectively, the maximum we can reach with one component available to us) in order to test methods used by professionals to detect and trace an unknown underground cable.
Will you directly excite the PI with a low frequency signal, then use a portable receiver to locate the signal from the cable?

If the PI is a model of the underground conductor, with the inductor antenna being an point source signal, then it will not be a line source like an underground conductor.

A professional detector will accurately find the centre and alignment of a buried conductor.
That cannot be done with a point source.

This is a PI representation. The resistor reproduces the losses by Joule effect, the coilrepresents energy storage in the form of a magnetic field around and in the line (self-induction) and the capacitor the capacitive coupling between the line and the ground. If I understood correctly, you're asking if the coil is use to induce current inside the line, the answer is no, it is part of the model.
Will you directly excite the PI with a low frequency signal, then use a portable receiver to locate the signal from the cable?
yes that's the idea

(on the schema, entrée means entrance, ligne = line and sortie = exit)

A professional detector will accurately find the centre and alignment of a buried conductor.
That cannot be done with a point source.
Perhaps the exercise is just to study the signal loss and the PI circuit can get higher source signal current if there's resonance. The problem of locating the cable could be something to look at later (and, if it's a school project then it would probably not be long term).
But there are several details which we still don't know.

Perhaps the exercise is just to study the signal loss and the PI circuit can get higher source signal current if there's resonance. The problem of locating the cable could be something to look at later (and, if it's a school project then it would probably not be long term).
But there are several details which we still don't know.
Yes the goal isn't to have the most accurate values , but to experiment various parameters that can influate the propagation of the signal (like frequency or intensity) that will follow the line, allowing us to trace it by detecting the magnetic field induced.
But what are the missing details you need to know? I'll try to answer the best that I can ecen though I am not yet an expert on the subject

But what are the missing details you need to know?
You need to acknowledge that whatever you are tracking underground is just one conductive pipe, or a bundle of wires, probably inside a metallic shield.

Your low frequency model of a transmission line is then that pipe or bundle, seen against the ground it is buried in. The top wire in the model is the pipe or line, the lower wire in the model is the Earth itself.

At the near end of the line, a current signal is applied to the pipe or conductor bundle, referenced to a ground stake. At the far end of the line, the pipe or line bundle is grounded to an Earth stake.
Alternately, the signal can be inductively coupled into a long line with a split-core current-transformer.

Your detector or receiver is then free to roam, in search of the AC magnetic field radiated by the conductive underground line.

hutchphd and DaveE
But what are the missing details you need to know?
I'd like to know the sort of size of the PI circuit and what would be generating the signals. I'd like to know what you plan to receive / detect the signals with and the sort of frequencies that you plan to use.
"AC" is not just and alternating current. It covers many octaves and, apart from acknowledging its frequency would not be nominally zero, you haven't given us a clue yet. IS it a bench-top set up or is it out of doors, for instance?
I appreciate that you may need guidance from your teacher about this but that person should have some idea about those details and some plan of the work, even if it's all new to you. PF can't supply any valid advice or ideas without that knowledge.

but I assume adding some meters to the circuit won't be a problem
That could be a serious problem. If your meters are ordinary Digital Multimeters then they will be pretty insensitive. How would they be connected?
Your teacher really has to be giving you a lot more guidance on this if you want any meaningful results from the activity. I suggest you refer him to this post on this forum and ask for his comments. (He should be pleased that you are using your initiative about this.)