Science Buddies: "Ask an Expert"

Hello,

I have already built a working model of the wire recorder, and now I am working on the experiment. In the insrtuctions on the Recording on a Wire page, https://www.sciencebuddies.org/science- ... p015.shtml I need to know so more specific instructions on how to set the digital multi-meter and where to place the probes.

[*]Step one: Using multi-meter to test output of amplifier for constant intesity input. I hummed into the mic connected to the input on the amplifier (Radio Shack Mini Audio Amplifier, part #277-1008). I put the probes from the multi-meter, one (black probe) to the external speaker and the other (red probe) to the DC 9V. (for your information, the amp has three jacks: Input, external speaker and DC 9V). I had the meter dial set to the area V~ 200...if that means anything to you. I am not really familiar with a multi meter. I got a reading of 17.8. that reading stayed constant with or without the humming sounds. Question is: Am I putting the probes in the right place and doing this step correctly?

[*]Step two: Using muti-meter to test resistance of the coil on the read/write head. What I did was to put a probe on each loose end of the coil wrapped around the ferrit bead core.I had the meter set to the Ω 20k. I think that is the right area, but not totally sure. I got a reading of .01. Does that sound right? Once again, I am unfamiliar with the workings and readings from a multi-meter. Does it sound like I am doing this positioning of probes and meter setting correctly?

[*]Step three: uses the readings from steps one and two. So once I get those two accurately measured...I can figure out the recording current using Ohm's law.

[*]Step four: Actual recording on wire. I have accomplished this. Really cool by the way!

[*]Step five: Use multi meter to measure the voltage of the coil on the read/write head during playback. On this step, I have no idea where to place the probes of the multi meter. Does anyone have any advice for how to do this?

I guess my basic problem is in using a multi meter on a device like this, and what settings to use for each of these steps.

Thanks,
Bennet

Hi Bennet and welcome to the forums!

Hopefully I can help you with your multimeter readings.

I put the probes from the multi-meter, one (black probe) to the external speaker and the other (red probe) to the DC 9V. (for your information, the amp has three jacks: Input, external speaker and DC 9V). I had the meter dial set to the area V~ 200

OK - First, the probes from the multimeter need to be across the speaker for this measurement. There should be 2 wires to the speaker. If you are using a cable with a jack on one end and the speaker connected on the other, you may need to either cut away a little insulation on each half of the wire (it should have 2 conductors inside) or open the amplifier and find the 2 connections to the output jack. Second, you want to use the AC setting on the multimeter. If the DC setting is V - (straight line) then the AC setting might be V ~ (curvy line). Also, you probably want a lower setting, such as 2V instead of 20V. If your meter shows the maximum for the range you use, just increase the range until you get a reasonable value.

Step 2: It sounds like you have the test leads in the correct place. However, you again want to use a much smaller setting. The coil will have very little resistance, so you probably want the smallest setting - my meter's is 200 ohms.

Step 5: You will measure the coil output at the same place as Step 2 - on the 2 wires of the coil. This will be a *very* small voltage, so you want the smallest setting on the multimeter.

I hope this helps - if you need more guidance, just ask us!
Keith

Step 2: I had the meter set to the Ω 20k. I think that is the right area, but not totally sure. I got a reading of .01. Does that sound right?

As Bennet mentioned, a lower resistance scale might give you a more accurate answer; however, it is extremely difficult to accurately measure DC resistances less than 0.1 ohms. Your reading of 0.01 ohms is as good as any similar value in terms of what can be expected. It is probably more of a function of your contact resistance than of the actual impedance of the coil.

Even if you had a more exotic way of measuring low DC resistance values with a "bridge" circuit, the DC resistance of a coil on a ferrite bead isn't a very good indication of its AC resistance (or impedance) so it means that this part of the experiment has some flaws in its design in terms of absolute values.

If one assumes that the impedance of the coil does not change (a good assumption as long as you use the same music so the frequency range involved is constant and the coil doesn't heat up because you are putting too much current through it), then you can compare the relative power at that point by comparing the square of an AC RMS voltage or current reading. Since you are looking for the difference between two or more operating points, the absolute value of the impedance is not important so you really don't have to measure this DC resistance in the first place. All that attempting to measure it proved is that you have continuity so it won't be an open circuit and fail to work.

My science teacher has agreed to push back the due date until Tuesday January 20
Thank You for your help on steps one and two, but I am still having trouble with step 5. I have put the probes on the ends of the wires and set it to the lowest voltage on my multi-meter (1.5V). Once that didn't work, I put the dial on each setting to try and find some sort of reading. I didn't get a single reading for anything.

I have no idea what I should do to get a correct reading and I'm freaking out because it's due soooooo soon!!
Please Help!

My science teacher has agreed to push back the due date until Tuesday January 20
I posted a question about this already, but I can only find what I need for Steps 1 and 2. I am still having trouble with step 5. I have put the probes on the ends of the wires and set the multimeter to the lowest voltage on it (1.5V). Once that didn't work, I put the dial on each setting to try and find some sort of reading. I didn't get a single reading for anything.

I have no idea what I should do to get a correct reading and I'm freaking out because it's due soooooo soon!!
Please Help