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## Re: using current to measure a time

 Subject: Re: using current to measure a time Rich Grise Fri, 02 Dec 2005 23:37:23 GMT sci.electronics.basics
 ```On Fri, 02 Dec 2005 22:37:55 +0000, Panther wrote: >> Use a high-impedance voltmeter, like a DVM. >> >> Record the voltage while the balls are apart. It will very probably be >> very close to zero. You can assure this by momentarily shorting the >> capacitor terminals. >> >> Drop the ball. While the balls are in contact, current will flow and >> start to charge the cap through the resistor, with a time constant >> of T = RC. >> >> The voltage across the capacitor at the exact moment that the balls >> separate will tell you T by using that exponential equation that I >> can't remember now, but since you're in school you should look it up >> anyway. > > Thank you!!!!!!!!!!!!!! This looks very simple so I will give it a go. But > could you please explain what the time constant T = RC means? I've no idea. > So you're saying, for example, if I were to do the practical now and record > the voltages, I would be able to work out the time section using some > formula? IE I don't need the equipment? As that would very handy. > T = RC is the "time constant". It's the amount of time it takes for the charge to reach 63% (or something) of its final value, R is in ohms, and C is in farads. With T on the horizontal axis, the cap voltage rises at some exponential rate... Maybe one of these sites will be more helpful: http://www.google.com/search?q=%22time+constant%22 Good Luck! Rich ```
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