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JBC3
JBC3 asks:
Q:

How is the ohmmeter supposed to be set up for this? Is a battery needed to supply a current? if so, how is that set up?

This experiment says "set up the ohmmeter" but doesn't give any directions on what that means. How is this set up so that the ohmmeter is supposed to read anything from string without a current?
Member Added on Jan 23, 2011
Thanks for the response. I thought the question was 'linked' to the experiment... sorry. I'm referring to the experiment called "I'll have the volts, please -- oh, and hold the resistance". To prepare the experiment, the instructions say to screw several sets of hooks into a piece of wood and tie string between the pair of hooks. Then it says to "set up the ohmmeter" which is part of my question above because there is no explanation as to what that means. The next step is to touch the electrodes on the ohmmeter to each string and record data in log book. No battery or source of current is mentioned but there has to be one, right?  
In Topics: School and Academics
> 60 days ago

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graham
graham writes:
I don't know what experiment you are referring to, but I'll try to answer anyway.

An ohmmeter is a device which measures electrical resistance. Typically what's called a 'multimeter' is used, which has a mode for measuring resistance. Multimeter's have two 'probes' which are touched to two points on a circuit, and it will tell you, in ohms, what the resistance is. If the device shows nothing, or infinite resistance, then your circuit is not closed. Multimeter's typically require batteries.

Sorry I cannot help more... if you tell me more about what experiment you are doing, I can answer in more detail.
> 60 days ago

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graham
graham writes:
Thanks for the update. You are right in that the details of the experiment are not very clear. I will try and clarify.

Ohmmeters detect the resistance by sending a small electric current from one electrode, and detecting how much of that current makes it to the other. The difference in voltage is caused by the resistance in the circuit.

The answer to your question is that the source of current is the  ohmmeter itself, which are typically battery powered- no extra battery or complete circuit is required.

Some clarifications on the steps:

Step 3 has you use a cookie sheet to wet the strings. This is important as if you get the wood wet, you might be measuring the resistance across the wood, not the string. Rubber or plastic coated hooks would solve this problem.

Ignore the 12 screws, you should only need the 12 hooks- you can get hooks at any home improvement store which you can screw in by hand into a piece of wood.

You need to carefully space out the hooks as resistance may vary with distance (which, in itself is a good experiment). In order to increase the sample size, each test is done 6 times: once on each of the 6 strings.

The log table at the bottom doesn't show the records for each string- which I think is an oversight. You should record each string, and its resistance, as so:

Test 1 - resistance across 4cm of dry string
String 1:
String 2:
String 3:
String 4:
String 5:  
String 6:
Average resistance:

And then repeat for the other tests (wet and frozen). This way finding the average will be more clear.


Some variations on this experiment could be:

- Testing with sports drinks instead of regular water (does resistance change with the salt content of the drink or the sugar content? which drink has the least resistance? is it 'Power Aid'?). Or a more general question: what effect does salt water have on conductivity? Does this mean you can make a device to determine salinity based on its conductivity?

- Testing conductivity (which is the inverse of resistance) with distance and your wet string. If you increased the space between the hooks by 1cm each hook: so the first hook would be 2cm, next 3cm, etc (you might want to use more than 6 strings), you could graph the results. The distance is your independent variable (plotted on the x axis) and the resistance is the dependent variable (plotted on the y axis). Is it a line or a curve? If you know enough algebra, come up with a resistance vs distance equation for your string. You would only need to do this experiment with a wet string (not dry or frozen), but repeat it a few times to increase your sample size.

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