Lewin's NCF Experiment and Lecture

[tinman]

Brad, look at your schematic, and consider the direction of conventional current as you have indicated with the arrows. Look at the locations of the probe Tips and Grounds with respect to the current flow, hence the voltage drop across the resistors.   Also consider that the probe ground references are connected together at the scope chassis.

I think it might be interesting to make these two measurements separately. That is, do a shot with only CH1 connected, save the waveform, then do a shot with only CH2 connected. Then compare the waveforms. Also you might try the CH2 measurement (separately) with probe tip connected to the "current in" side of the resistor and ground connected to the "current out" side.The CH1 probe is already connected this way, with tip on "current in" and ground on "current out".

That is not my schematic,it is Poynt's.
I have the scope hooked as per the volt meters on the schematic.

Why would the common ground of the scope matter ?,as the wire between the two resistors is common anyway.

I stated in the other thread that the polarity should be the same ,due to the common wire on each side of each resistor,and looking at all the scope shot's i have taken,that is exactly how it is.
I will try each channel 1 at a time,but i dont see that making any difference.

Below is Poynts schematic of the test setup,where i have a 1k ohm resistor instead of the 900R.
I still see no reverse voltage--both traces show same polarity.


Brad

[tinman]

Well i have no idea why,but we get the desired result by using 1 channel of the scope at a time.
I know my scope has a common ground,but the wire between the two resistors is also common--so why the difference?.

The first scope shot below shows the trace across the 1k ohm resistor only,and the second shows the trace across the 100 ohm resistor only. Here we now see that we do have the reversed polarity we were looking for,but in order for me to get that trace across the 100ohm resistor,the loop formed by the scope probe and ground wire,must be a vertical loop as apposed to the horizontal loop of the two resistors. If the scope probe and ground wire also form a horizontal loop,then only a very small voltage is seen across the 100 ohm resistor.


Brad

[tinman]

So when we just started to think we had the answer's.

Decided to scope across the wire from one resistor to the other-->and what do you know ,The very same voltage potential and polarity as the previous scope trace across the 100R in my last post.

And no,the channel is not inverted

See scope probe position in schematic.


Brad

[poynt99]

Well i have no idea why,but we get the desired result by using 1 channel of the scope at a time.
I know my scope has a common ground,but the wire between the two resistors is also common--so why the difference?.

This is highly germane to the entire problem at hand.

Consider this; taking into account that we are dealing with Faraday induction here, is it correct to presume the wire between the resistors is simply wire, and it is doing nothing more than connecting one resistor to another?

The answer is no. The wire is where the emf is being induced, so if you have shorted it out with your scope's gnd leads, it isn't going to work properly.

The first scope shot below shows the trace across the 1k ohm resistor only,and the second shows the trace across the 100 ohm resistor only. Here we now see that we do have the reversed polarity we were looking for,but in order for me to get that trace across the 100ohm resistor,the loop formed by the scope probe and ground wire,must be a vertical loop as apposed to the horizontal loop of the two resistors. If the scope probe and ground wire also form a horizontal loop,then only a very small voltage is seen across the 100 ohm resistor.


Brad

You may want to double check that it is 100 Ohms, and not 10 Ohms or something. I am not sure why you had to rotate the loop 90 degrees in order to measure -100mV, but you shouldn't have to. I didn't.

[poynt99]

So when we just started to think we had the answer's.

Decided to scope across the wire from one resistor to the other-->and what do you know ,The very same voltage potential and polarity as the previous scope trace across the 100R in my last post.

And no,the channel is not inverted

See scope probe position in schematic.


Brad

If all is going as expected, you should measure essentially 0V between the wires when the measuring probes are in the horizontal plane. What should you measure between the wires when the measuring probes are in the vertical plane, i.e. decoupled from the experiment?