# Earth's Magnetic Field Lab question

1. Jan 28, 2016

### lloyd21

1. The problem statement, all variables and given/known data
Using the Logger Pro Program plot a graph with the angle on the horizontal axis and the measured field on the vertical axis. The curve should have its maximum value at (theta) = S . where
cos( theta - S) = cos(0) = 1. Use your graph to determine S, the declination at your location.

My graphs are uploaded, how do i solve this or did I already get the answer of 1.44 deg?

2. Relevant equations

3. The attempt at a solution
My graphs are uploaded, how do i solve this or did I already get the answer of 1.44 deg?

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2. Jan 28, 2016

### haruspex

I do not understand where you are getting 1.44 degrees from. At what x coordinate does you graph reach a maximum?

You will not get a very accurate answer just picking out the highest reading. The samples are too widely spaced. Your best bet is to fit a sine curve to all the points and see where that maximises.

3. Jan 28, 2016

### lloyd21

the 1.44 is from my 20 degrees at 0.0144.....for the maximum....if i find the declination on the web for the west side of germany it should be around 1.54.....so I thought I may be close....do those numbers look alright from the magnetic sensor ?

4. Jan 28, 2016

### haruspex

The 0.0144 is a field strength, not an angle. By what logic do you turn that into an angle of 1.44 degrees? The angle at which that field was measured is 20 degrees.

I think there is some confusion here between declination and inclination. Declination is the difference between magnetic north and true north. Yes, that might be about 1.5 degrees in Germany. But how are you going to measure it in a laboratory? You have no easy way to find true north.

The graph looks more like you are measuring inclination, i.e. the angle at which the field dips below horizontal. It should be roughly the same as your latitude. If you disagree, please describe the experiment in detail.

20 degrees is much too low for inclination in Germany, and looking at the graph it is clearly not where the true maximum lies. Draw any smooth curve through those points and the maximum will be closer to the 40 degree mark, which is more reasonable. As I posted, best bet is to fit the best sine curve to those points and read the maximum off that. Do you need help with that?

Last edited: Jan 28, 2016
5. Jan 28, 2016

### lloyd21

Yeah please I'd appreciate that big time. I don't really know what I'm doing here, and I'm still unsure if I measured the magnetic field with the sensor properly.

6. Jan 28, 2016

### haruspex

Ok. You want to fit a curve of the form $y=c_1+c_2\sin(c_3x+c_4)$ to the data. In general that's hard, but here we know two things:
- the average field should be zero, so c1=0.
- the cycle should repeat after 360 degrees, so if we measure x in radians then c3=1.
So let's rewrite it more conveniently as $y=Asin(x+c)$.
We can expand the sine term as $\sin(x)\cos(c)+\cos(x)\sin(c)$. If we then write F=Asin(c) and G=Acos(c) it becomes
$y=G\sin(x)+F\cos(x)$.
This is now easy. It's just a linear regression of y as a function of two variables, sin(x) and cos(x).
See e.g. https://www.easycalculation.com/statistics/multiple-regression.php.
What tools do you have... matlab, excel...?

7. Jan 29, 2016

### haruspex

The route I'm suggesting might be too advanced for you, so I've though up an easier way that should be better than trying to guess where those curves should hit max.
Instead, look at where the field is zero. Sketch a straight line through the points where the curve crosses the y axis and read off the x coordinates at the two crossing points. You should have two angles approximately 180 degrees different. You can safely assume that halfway between these two is either a maximum or a minimum. What angle do you get for that?

8. Jan 30, 2016

### haruspex

@lloyd21, please don't simply stop responding. How are you going with this?
A question: in your table of results, what direction is "0" degrees? Is that straight up, straight down, North or South? Which way did you rotate it from there?

Out of interest, I tried both methods that I proposed. The curve fitting gives me 28.6 degrees. If that is measured from vertically down, rotating to the north, it's reasonably accurate. The other method, drawing a straight line through the points near where they cross the y axis, gives me more like 26 degrees.

9. Jan 31, 2016

### lloyd21

Sorry about the late reply, I'm 9 hours ahead of you! I'm just rotating it clockwise, the 0 degrees is pointed towards true north.

10. Jan 31, 2016

### lloyd21

Also, on the graph, is there a way to find a curve fit in options? Not very familiar with this program

11. Jan 31, 2016

### lloyd21

I'm not sure how you got 26 degrees with the second method? I don't understand how you did that haha!

12. Jan 31, 2016

### haruspex

Then I'm still confused as to whether we are discussing inclination or declination.
How do you know where true north is in the lab?
Are you rotating the sensor in a horizontal plane or in a vertical plane?

My second method is this:
- Look at the two places where the curves are steepest and cross the y axis.
- Put straight lines through the points there, say two or three points each side of the crossing.
- Read off the x coordinates where these straight lines cross the y axis.
- These show the angles at which the field has minimum magnitude. The maximum magnitude should be half way between them, so take the average of these two x values.
- You can see this will give you the angle where the field is most negative. To get the angle where it will be most positive just subtract 180.
That procedure gave me roughly 26 degrees. If you get something very different, please post what you get for each step so that I can see where we diverge.

(If you are in Germany I'm 10 hours ahead of you.)

13. Feb 1, 2016

### lloyd21

looking halfway between the points, I see an angle of 200 degrees?

14. Feb 1, 2016

### lloyd21

true north is figured out by finding out the street im located at and direction of true north

15. Feb 1, 2016

### lloyd21

This is first part instructions.

16. Feb 1, 2016

### lloyd21

So my first post is the data in the graph and table.

17. Feb 1, 2016

### haruspex

The two intercepts of the x axis are about 100 and 310, so the midway is more like 205. As I posted, that gives an estimate of the location of the minimum, and looking at the graph around 200 that seems reasonable. You want the maximum, so subtract 180 degrees, giving 25. Again, eyeballing the graph, that does seem reasonable for the maximum.

From all the extra detail you have provided, I see that indeed you are trying to find the declination, but 25 degrees is far too much. In Western Germany it should be no more than 2 degrees. I suspect you have true North set wrongly. What map did you depend on to figure out the orientation of the street?

18. Feb 1, 2016

### lloyd21

19. Feb 1, 2016

### haruspex

Yes, that should be ok. Perhaps you made an error mapping it from street orientation to a direction in the lab? Or maybe there is another constant source of field within the lab that deflected it?

20. Feb 1, 2016

### lloyd21

thats a possibility but I kept it away from anything magnetic that might have altered the sensor readings?