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I was performing a basic verification of Ohm's law in a DC circuit experiment the other day, during my exams and I mistakenly used an ammeter with range 0-10A instead of the 0-1A ammeter that the practical manual expects us to use.

The invigilator/examiner noticed this error and told me that this is a really bad error on my part and this will not be forgiven. She cut half my marks just because of this small error(?). I retorted by saying that it shouldn't really matter, because all I am doing is just reading the current value in the circuit and the range of the ammeter used won't affect that value. She said that isn't the case and the range of the ammeter WILL affect the current reading. I didn't know much of the specifics about the working of an ammeter, so I just ignored her. I don't even care about the marks too.

All I want to know is whether the range of the ammeter affects the current reading shown by it? If possible, I also want to know how the current value is affected just by swapping it out with an ammeter of a different range.

I looked up the working of an ammeter and I found that an ammeter is just a galvanometer with a resistance in parallel (I don't understand what they mean by parallel). Maybe the resistance increases the overall resistance of the circuit and hence leads to decrease in current(?). Please let me know if my speculation is correct or not. Thanks in advance for tolerating my zero knowledge of the subject :)

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    \$\begingroup\$ The range might affect the accuracy of the measurement. Also the wider range setting is usually unfused, so if you have a high current (out of this range) you will fry the device, while on the fused range it will be only the fuse. \$\endgroup\$ – Eugene Sh. Dec 4 '19 at 16:47
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    \$\begingroup\$ Analog or digital meter? The answer is similar for either case, but the answer will be more useful to you if we know. \$\endgroup\$ – Mattman944 Dec 4 '19 at 16:51
  • \$\begingroup\$ Mattman944, it is a digital meter and the accuracy is the same for both of them (2 numbers after decimal point) \$\endgroup\$ – newbietoelec Dec 4 '19 at 16:53
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    \$\begingroup\$ For the definition of a parallel circuit, see here. \$\endgroup\$ – The Photon Dec 4 '19 at 16:55
  • \$\begingroup\$ @Eugene Sh. But will it affect the value of current if both the apparatus have same accuracy (2 digits after decimal point) \$\endgroup\$ – newbietoelec Dec 4 '19 at 16:55
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Yes it does matter a little. Say you have a speedometer that is 0-1000 mph scale You also have a scale of 0 to 10 mph

You are walking 1mph, which scale would be most accurate, of course the 0 to 10.

All meters scales are like this, the 0 to 1000 would still measure but not as good as the lower scale.

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  • \$\begingroup\$ What if the accuracy of measurement if both ammeters are same (2 digits after decimal point) \$\endgroup\$ – newbietoelec Dec 4 '19 at 17:01
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    \$\begingroup\$ This answer is mixing up precision and accuracy. The lower range will be more precise. But, as I explain in my answer, the higher range might be more accurate. \$\endgroup\$ – The Photon Dec 4 '19 at 17:05
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I looked up the working of an ammeter and i found that an ammeter is just a galvanometer with a resistance in parallel

We call this the shunt resistance of the ammeter.

The difference between the high current range and the low current range is that the value of this resistance is lower for the high current range.

If the circuit you're measuring has a relatively low impedance (less than maybe 20x the shunt resistance) as measured from the points you connected the ammeter, then the choice of range will affect the measured current.

But using a higher range will actually give you a more accurate result, because its lower shunt resistance will affect the measured circuit less.

(On the other hand, the result will be less precise because each step of the ADC in the ammeter will correspond to a larger increment of current)

In comments you added,

it is a digital meter

In this case, there's no actual galvonometer in your meter. But just the same, the principle is to measure the voltage across a low-valued shunt resistor. And a larger shunt resistor is needed for the lower ammeter range, leading to the same result: The higher range affects the measured circuit less than the lower range.

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  • \$\begingroup\$ Yeah! I encoutered shunt resistamce when i googled the working of an ammeter so i had to google what shunt meant too. Thanks for clarifying :D \$\endgroup\$ – newbietoelec Dec 4 '19 at 17:04
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    \$\begingroup\$ (this answer should be accepted because it addresses your question directly but it's up to you decide which one really answers your question... I don't see how Roger's answer provides that.) \$\endgroup\$ – KingDuken Dec 4 '19 at 17:48
  • \$\begingroup\$ @KingDuken this answer has a lot of jargon that went totally over my head, whereas Roger's question was way more simpler and explained it better (to me, atleast ) \$\endgroup\$ – newbietoelec Dec 4 '19 at 19:19
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    \$\begingroup\$ @newbietoelec That's your decision :) \$\endgroup\$ – KingDuken Dec 4 '19 at 19:31
  • \$\begingroup\$ @newbietoelec If you are seriously learning or studying about this stuff, you should learn the needed jargon. \$\endgroup\$ – glglgl Dec 6 '19 at 8:26
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This was a "practical" exam?
Very many current meters dedicate a front-panel jack to the 10A scale (jack #1, below). If your range selector switch is on lower-current scales, you use a different front-panel jack (jack #2,below):
enter image description here
Your marker would be justified in a significant down-grade if the improper jack was used.

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  • \$\begingroup\$ This is the ammeter we used for the exams. drive.google.com/file/d/14OF23XLYHWSI4sBu6RiKs8ZlTqC98iss/… \$\endgroup\$ – newbietoelec Dec 4 '19 at 19:10
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    \$\begingroup\$ Google blocks those of us with no account. It would help everyone if you could add the ammeter photo to your Original post above. \$\endgroup\$ – glen_geek Dec 5 '19 at 3:13
  • \$\begingroup\$ @glengeek im really sorry i made you go through all the trouble, ive opened the link now :) \$\endgroup\$ – newbietoelec Dec 5 '19 at 15:15
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    \$\begingroup\$ @newbietoelec Thankyou. Your photo shows a very old analog ammeter with 2 terminals. Yet in comments elsewhere, you say it was a digital meter with plus/minus 0.01A resolution. \$\endgroup\$ – glen_geek Dec 5 '19 at 15:57
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    \$\begingroup\$ Regarding the old-analog-ammeter, yeah.. that's pretty strange in context of OP's words. I had some of them in our school lab, but we used them exactly once, just for show that they work. Noone would trust them to the students for use, just because it's quite easy to burn them accidentaly if connected with wrong 'range'. So I'd expect a different kind of scolding from the teacher and completly different explanation that the one was quoted. \$\endgroup\$ – quetzalcoatl Dec 6 '19 at 8:03

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