Lewin's NCF Experiment and Lecture

[Magluvin]

You said you had to reposition the probe somehow while measuring the 100R in order to get a voltage reading if I'm not mistaken. You shouldn't have to do that. That is a sign of one potential problem. You also have not taken care to minimize the effect from the source wiring. Take a close look at mine and you will see what I did. The only horizontal wiring  is twisted to cancel the "stray" magnetic field. The top conductor of the solenoid travels down the inside so it can exit at the bottom, twisted with the other end.
I'm fairly certain I did, but it was a few years ago and I could be mistaken.
If you look back at my diagrams, I show three probe positions for the meter on the left. What are the meter voltages at each of the three positions?
Same response as above.
Yes perhaps I will. Tell me what you measure at those three positions first.

These are strange results it seems.

If there is a question of whether the meter leads are being induced, set the meter to the left of the coil, connect the test leads to the middle of the wire connecting the resistors, one lead on each side, take the measurement, then without disconnecting the test leads, move the meter to the right side, lifting the test wires over the coil assy and set  the meter down on the table. Then check the meter again. Is it the same reading?

Mags

[tinman]

These are strange results it seems.

If there is a question of whether the meter leads are being induced, set the meter to the left of the coil, connect the test leads to the middle of the wire connecting the resistors, one lead on each side, take the measurement, then without disconnecting the test leads, move the meter to the right side, lifting the test wires over the coil assy and set  the meter down on the table. Then check the meter again. Is it the same reading?

Mags

I am only discharging a cap into L1,and capturing one cycle at a time. So i cant use a DMM or meter to do any checking like that.

I have to run my top wire of L1 down through the center of L1,and twist the ends of the coils leads to eliminate stray inductance they may be causing in the scope probe lead loop.


Brad

[poynt99]

Mags raises a very good question, and actually it makes no difference whether you use meter leads (and DMM) or scope probes, the result will be the same. The problem of course is that the signal is fleeting, and it therefore must be captured on a scope.

However, that doesn't mean we are limited to using only the probes that come with the scope, i.e. the traditional scope probe. If you want, you can construct a simple unshielded "probe" from simple twin-lead (speaker wire), two alligator clips, and a BNC connector. Depending on how noisy your environment is, you may or may not have interference on your signals. My speaker wire "probes" are about 6' long, and I don't have any problems with noise. You may want to construct a probe like this for part II of the experiment, which is next when you're ready.

[lumen]

It might help to think of the wire segments as batteries placed in the direction of the arrows. It's the same thing because you are inducing a current these segments.
Once you look at the problem this way, everything makes sense and the results are correct.

[poynt99]

No quite.

Once you understand what is happening from both perspectives, then everything makes sense. Until perspective two is explored,  the results for perspective one (the one we are working with now) appear confusing when looking at the measurements and imagining the wires as sources of emf.

Hold on to those thoughts lumen and let's get through this one step at a time.