Accuracy of Current and Voltage Measurements

[mantillab]

In general, are the measurements of current or voltage more accurate?

From our lab experiments, it seems that voltage is more accurate. I'm curious as to why this is the case.

My guess is that because current itself is the measurement of the flow of the electric charges it would be more likely to fluctuate due to external forces or changes in the environment than the voltage (the difference of the electric potential between two points of a circuit) would?

 

[berkeman]

In general, all you can measure is voltage. To measure current, we generally convert it to a voltage by running it through a known resistance. This is an invasive measurement, because you need to add series resistance to the circuit to measure the current. This often can change the current, so you will generally use as small a resistance as possible for the measurement. But this makes the measurement less accurate, so in making a current measurement, there is often a tradeoff between the invasiveness of the measurement, and its accuracy.

Does that make sense? Can you think of ways that you could measure the current in a less invasive way? Do you think you can make less invasive measurements better on AC or DC currents?

 

[mantillab]

Thanks! I appreciate the response. Is an ammeter more accurate than a multimeter, or are they the same? Would measurement of DC currents be more accurate because they do not alternate like AC currents do?

 

[berkeman]

Thanks! I appreciate the response. Is an ammeter more accurate than a multimeter, or are they the same? Would measurement of DC currents be more accurate because they do not alternate like AC currents do?

By ammeter, I think you're referring to a moving coil galvanometer, correct?

http://en.wikipedia.org/wiki/Galvanometer

I honestly don't know how accurate they are, in comparison to inserting a series resistance and measuring the votage drop. Sounds like an interesting question that you could spend a little time reading about on the web -- a refined google search will probably get you some good websites to check out.

BTW, here's a good experiment to try the next time you're in the lab. Use 2 DVMs, and use one to measure the series resistance that the other DVM inserts in-circuit for various current measurement ranges. How much resistance is inserted for mA range current measurements? How much is inserted for A range current measurements?