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Ohms are a measurement of electrical resistance. The less ohms you have the less resistant the subject is to the electrical current passing though it. In the terms a slot car armature the lower the ohm rating then the quicker the arm because it allow electricity to flow through it more easily.
Once you have removed your armature from the car you will see at the top three brass coloured plates.
This part is known as the commutator or comm for short (they maybe a bit black and dirty from previous use). First thing to do is remove the two washers that sit on top of the comm by sliding them off the end of the shaft. Keep these washers to one side as you'll need them later.
Your comm might be dirty so using some lighter fluid or Simichrome with a cloth you should clean it and get it looking as shiny as possible. Use a needle or pin to remove any dirt between the comm plates to ensure there is a clear gap between each one. Now you have a cleaned and prepared your armature we'll check its ohm value.
For this part you'll want to keep the arm stable and upright, I use a small piece of AFX track as the shaft of the arm seems to fit into the small fixture holes quite well.
Now turn you multimeter on and select the ohms setting that will give you 100ths or 200ths of an ohm.
Now get your multimeter probes (red/black) and push the tips together. A reading will appear on the screen. This is the resistance value of the probes. In this case my probes are 0.08, keep a note of your value.
Now you need to start testing. Carefully place the tip of each probe against separate plates and wait for the multimeter to settle, you'll need to keep your hands very steady for this and the reading can take some time to settle. Remember this reading and write it down. You'll also need to remember which plates you have tested. In total you should have three readings from the three different combinations of plates on the comm.
Now that you have three sets of readings you will need to deduct the value, 0.08 in this example, from each one. This deduction is for the resistance of the probes.
Finally you get all three readings and add them together. Divide the result by three and that will be the true ohm rating of your armature.
Please check WHO Racing rules to see what armature ohm rating is allowed in each class.