Science Buddies: "Ask an Expert"

Hello, I have successfully built a wire recorder, but under the experiments part of this project, I must test to see the relationship between input current and output voltage. I get how to use "AC" mode on my multimeter, I just don't know how (or where) to connect the multimeter to get the output of the amplifier or the resistance of the coil. Also, I am using the amplifier recommended by the materials list and I have a Klein Tools MM1000 Multimeter with red and black banana jack cords. I am hoping to get my measurements in before the break ends, so any help would be great - Thank you!

Hi SophieS-

I haven't done this myself and I agree that the instructions could be clearer. I'm assuming you're talking about the first steps of the experiments.

> Use the digital multimeter to measure the output of the amplifier for a constant-intensity input (e.g., you could hum into
> the microphone, or you could connect a portable music player to the amplifier and use a suitable portion of a song). Use
> AC mode, and take an "average" reading. If you need help using a multimeter, check out the Science Buddies Multimeter
> Tutorial.

I would suggest you download and play a test tone from your MP3 player (https://www.mtx.com/f/mtx/testtones/1kHz.mp3 is a 1kHz tone that you can get from https://www.mtx.com/testtones. Record that onto the wore and then play it back. To see the approximate power output of the amplifier you need to look at the speaker in the amplifier itself. How you do this will depend on what sort of amplifier you have. If you have the Radio Shack device, the speaker is built into the cabinet but I believe it has a jack on the side to connect the output to another speaker. Whether you use the one in the case or not, connect the multi mater across the two wires of the speaker and read that as an AC voltage. To get the current, disconnect one of the wires going between the amplifier and the actual speaker itself, and use the meter in AC current mode to connect the speaker back to the amp. That will give you approximate voltage and current coming from the amplifier to the speaker. Power in Watts = voltage * current. This is a crude measurement at best but it's the best you'll get with just that meter. Using a nice clean tone will give you better results than humming as the project suggests.

> Use the digital multimeter to measure the resistance of the coil on the read/write head.

Disconnect the head from the amplifier and put your meter in resistance or Ohms. One lead goes to one side of the head coil and the other lead goes to the other side. It doesn't matter which is which.

> Use Ohm's law to calculate the recording current (voltage/resistance).

Ohm's law says that the voltage across a resistance = the current through it * the resistance, to V = IR. Flip that around and you get I = V / R. You know V from your AC voltage measurement and R from your resistance measurement.

It's a very cool project. I'm going to have to do it myself some day. Please write back if you have any more questions.

Howard

Dear Mr.Howard,
Thank you for the wonderful advice! Keeping in mind that I have the Radio Shack Amp, my only confusion is when you said to get the voltage, "connect the multi mater across the two wires of the speaker." I see the exit speaker plug-in on the amp's side, but I don't think my multimeter can go in there. Do you mean that I should open up the back of the amp and connect the multimeter's alligator clips to two inside wire pieces while in play back? If so, where could I find those wires? There are two- one red and one black- connecting to the battery- would it be these?
Thank you!

I went back and read through the project again. What it seems like the author is asking you to do is to measure the current going through the head during recording. If you measure the resistance of the coil when it's disconnected and then measure the AC voltage across the coil when it's recording, you can calculate current by using I = V / R, or current = voltage divided by resistance. What I told you was one way you could measure this directly. Simply measuring the voltage and resistance is an easier way.

You would use the output from the amplifier's phone jack, connect that to the coil and then measure. I misunderstood the instructions the first time. Sorry about the confusion.

Howard

Dear Mr. Howard,
I have recorded the sound you gave me, but no readings for resistance or voltage are coming up. I have the multimeter alligator clips attached to the wire strips at the coil- is this right?
-Happy New year!

Yes, putting the multimeter leads across the two wires of the coild is right. Disconnect the amplifier and set the meter to Ohms to read resistance, and leave the amplifier connected and the tone playing to read voltage.

If need be, play with the range settings on the meter. You might be set too high or too low.

Howard

Dear Mr.Howard,
Success! I can get a resistance of 4 ohms - can it be the same number every time? Also, I am getting the voltage- but only as a 0.1 that flashes up every once and a while- I've played around with the multimeter- not really sure why it's only giving me that.
Thank you!

The resistance *should* be the same every time you measure it. The voltage won't be. A multimeter is not the right tool for the job. That's all you have, so you just have to do what you can.

Howard

Dear Mr. Howard,
Thank you for the help- I am finished with all my data!