Physics Forums

Physics Forums (http://www.physicsforums.com/index.php)
-   Electrical Engineering (http://www.physicsforums.com/forumdisplay.php?f=102)
-   -   Why must ammeter be connected in series? (http://www.physicsforums.com/showthread.php?t=580747)

kahwawashay1 Feb23-12 05:40 PM

why must ammeter be connected in series?
 
lets say you have a circuit consisting only of a resistor and battery. Is it absolutely necessary to connect an ammeter in series with the resistor to measure current through it, rather than in parallel?

Current doesn't want to go through a resistor, so if you connect a much lower resistance (ammeter) parallel to the resistor, won't the current prefer to follow the path of least resistance? So you would still get a reading on the ammeter...

What made me question this whole notion of always connecting ammeter in series is the fact that we did a lab in school and at first we accidentally connected the ammeter in parallel to a resistor and recorded that current. Then when we connected it in series, we got basically the same current reading for each of the three resistors we were measuring..

berkeman Feb23-12 06:28 PM

Re: why must ammeter be connected in series?
 
Quote:

Quote by kahwawashay1 (Post 3780791)
lets say you have a circuit consisting only of a resistor and battery. Is it absolutely necessary to connect an ammeter in series with the resistor to measure current through it, rather than in parallel?

Current doesn't want to go through a resistor, so if you connect a much lower resistance (ammeter) parallel to the resistor, won't the current prefer to follow the path of least resistance? So you would still get a reading on the ammeter...

What made me question this whole notion of always connecting ammeter in series is the fact that we did a lab in school and at first we accidentally connected the ammeter in parallel to a resistor and recorded that current. Then when we connected it in series, we got basically the same current reading for each of the three resistors we were measuring..

You need to connect the current meter in series with the current you want to measure. You could alternately connect a voltage meter in parallel with the resistor, and use the voltage and resistance value to infer the current through the resistor.

Current meters have a finite resistance (for the shunt element). That resistance is usually low when you are measuring on the 10A scale or similar, but when you are using a mA scale, the resistance is significant. That's probably why you didn't see much difference in your lab measurement that you mention in your last paragraph. If the resistors you were measuring were small compared to the current meter's internal resistance, then you would get simlar answers whether the meter were connected in parallel or series with the resistors.

kahwawashay1 Feb23-12 06:47 PM

Re: why must ammeter be connected in series?
 
Quote:

Quote by berkeman (Post 3780870)
If the resistors you were measuring were small compared to the current meter's internal resistance, then you would get simlar answers whether the meter were connected in parallel or series with the resistors.

I think you meant: "If the resistors you were measuring were (large) compared to the current meter's internal resistance"...no?

vk6kro Feb23-12 06:57 PM

Re: why must ammeter be connected in series?
 
lets say you have a circuit consisting only of a resistor and battery. Is it absolutely necessary to connect an ammeter in series with the resistor to measure current through it, rather than in parallel?

Can you see that, if you did this, the current could just flow out of the battery through the meter and back to the battery? Since ammeters have very low resistance, this current could get very high, depending on the type of battery in use.
Apart from not measuring the current through the resistor, this is very likely to destroy the shunt resistor in the meter or damage the meter movement if it was an analog meter.


In your case, the other resistors limited the current and avoided damage.

Doesn't your teacher check the circuits before you apply power to them?

Averagesupernova Feb23-12 07:38 PM

Re: why must ammeter be connected in series?
 
Look up the definition of an ampere. It is a rate. X number of electrons passing a point in a given period of time. So, how can you measure electrons passing through a resistor with an ammeter if you are not connected in series? You can't. A water meter although not 100% analogous does something similar. It measures the amount of water flowing through the pipe that enters your house.

jim hardy Feb23-12 09:46 PM

Re: why must ammeter be connected in series?
 
It's this simple.
My high school electronics teacher hollered this with emphatic arm-waving "AN AMMETER IS DEFENSELESS - IT CAN'T OPPOSE CURRENT ! "

If you place an ammeter in parallel, it will accept as much current as the circuit is capable of delivering.
Your lab experiment was intentionally set up so you woldn't fry the meter by your mistake.

Many analog meter needles have got wrapped around their high end peg .
Many digital meters have got literally blown up when somebody "put in a bigger fuse".

Here's some good information. See pyrotechnic fireball on second page and read paragraph "when does a tester become a grenade"

http://support.fluke.com/find-sales/...01_ENG_A_W.PDF
Quote:

This effectively places
a short across the voltage source.
Years ago, when analog
meters were the only instrument
for making these measurements,
this mistake pretty well destroyed
the meter movement (the needle
wrapped around the top peg), not
to mention the internal circuitry.
You REALLY need to understand this concept.

sophiecentaur Feb24-12 07:02 AM

Re: why must ammeter be connected in series?
 
Quote:

Quote by kahwawashay1 (Post 3780901)
I think you meant: "If the resistors you were measuring were (large) compared to the current meter's internal resistance"...no?

I think both statements would be true for appropriate values of internal meter resistance. With a high enough internal resistor, the meter would become a voltmeter!. But I'm splitting hairs.

If you just do what you are advised and put the ammeter in series than it will measure the current that is going through the resistor. If you put it anywhere else, it won't. Sounds like a good reason to put it in series.

psparky Feb24-12 07:04 AM

Re: why must ammeter be connected in series?
 
I certainly agree with all the replies.

Keep in mind that today's more expensive voltmeter/amp meters are quite advanced.

Especially the amp meter that you simply wrap around the wire loosely (no cutting) and it gives you the amps! How sweet is that!

sophiecentaur Feb24-12 07:15 AM

Re: why must ammeter be connected in series?
 
Quote:

Quote by psparky (Post 3781720)

Especially the amp meter that you simply wrap around the wire loosely (no cutting) and it gives you the amps! How sweet is that!

It's great that you are excited by these things. I am too. (That type of meter is measuring the magnetic field around the wire - Hall Effect). Shame that they are not very sensitive, though.

psparky Feb24-12 07:23 AM

Re: why must ammeter be connected in series?
 
Quote:

Quote by sophiecentaur (Post 3781730)
It's great that you are excited by these things. I am too. (That type of meter is measuring the magnetic field around the wire - Hall Effect). Shame that they are not very sensitive, though.

I kinda guessed it was measuring the magnetic field....simply because it couldn't possibly be "feeling" anything else. I'm sure they will get more "sensitive" over time.

But wow....how convenient especially for measuring high current devices such as three phase air conditioners, air handlers, motors and so forth. Turn the meter on, open up the clip, toss it on....read the meter....have a nice day.

One other big bonus is that these fancy amp meters don't have to withstand start up current like the old ones did....big plus!

sophiecentaur Feb24-12 07:42 AM

Re: why must ammeter be connected in series?
 
They are just as much subject to the high magnetic field at startup but the 'signal' they are working on is so far down that they are less likely to overload damage. However, if one were to make an even more sensitive magnetic field measuring device, it might require some sort of overload protection to prevent it from (perhaps short-term) damage. Like when we go temporarily blind after a bright flash.

As with all measuring systems, they must be affecting the system they measure. By putting a ring of magnetic material around the wire, a tiny change in inductance is introduced which will delay any change in current.

psparky Feb24-12 07:47 AM

Re: why must ammeter be connected in series?
 
Quote:

Quote by sophiecentaur (Post 3781759)
They are just as much subject to the high magnetic field at startup but the 'signal' they are working on is so far down that they are less likely to overload damage. However, if one were to make an even more sensitive magnetic field measuring device, it might require some sort of overload protection to prevent it from (perhaps short-term) damage. Like when we go temporarily blind after a bright flash.

As with all measuring systems, they must be affecting the system they measure. By putting a ring of magnetic material around the wire, a tiny change in inductance is introduced which will delay any change in current.

I certainly agree. With the new fancy meter, the electrician has the option of only measuring steady state if that's all that interests him. With the old series meters, this was not an option.

sophiecentaur Feb24-12 07:59 AM

Re: why must ammeter be connected in series?
 
Quote:

Quote by psparky (Post 3781766)
I certainly agree. With the new fancy meter, the electrician has the option of only measuring steady state if that's all that interests him. With the old series meters, this was not an option.

You mean the 'clamp' meters?

psparky Feb24-12 08:04 AM

Re: why must ammeter be connected in series?
 
Quote:

Quote by sophiecentaur (Post 3781773)
You mean the 'clamp' meters?

Probably?

"Clamp" meter = New fancy amp meter. Pro: Convenient Con: Not as accurate

"series" meter = Older type ampmeter. Pro: accuracy Con: Not convenient

psparky Feb24-12 08:10 AM

Re: why must ammeter be connected in series?
 
To add to the original question....in older type cars.....hobbyist like to add guages to their car. Water temperature, tachometer.....and you guessed it.....amp meter or volt meter.

Op for the volt meter!!! Simple parallel connection.

To add a amp meter to the car....you would have to cut the fusible link and have the main amperage cable of your car running through your meter! What a pain...not to mention reliability down the road.

That being said....you probably can buy a fancy amp meter in your car these days that clamps on and just measures the magnetic field and converts it to amps.

NascentOxygen Feb26-12 02:34 AM

Re: why must ammeter be connected in series?
 
Quote:

Quote by psparky (Post 3781789)
To add a amp meter to the car....you would have to cut the fusible link and have the main amperage cable of your car running through your meter! What a pain...not to mention reliability down the road.

psparky, you have awakened a long-suppressed memory from my teenage years. :smile:

Developing a keen interest in electricity and electronics, I would on weekends raid a nearby car dump and, doing my best to avoid snakes and spiders, excitedly salvage coils, windscreen motors, lights, switches & gauges from old rusting car wrecks, to take home and investigate closely. One ammeter that I freed by unbolting and cutting its wires initially appeared to be of no use--it had no terminals to connect to! All it had were two loops at the back through which the thick copper wire (insulated) was tightly threaded--their purpose being to hold that wire against the back of the meter. While freeing the gauge from the car's panel I had cut through two wires thinking they were connected to the meter's terminals, but they proved to be the ends of the same piece of a short length of copper wire. It was some time before I concluded that it relied on the magnetic field around that conductor for coupling to the meter. Neato!
Quote:

That being said....you probably can buy a fancy amp meter in your car these days that clamps on and just measures the magnetic field and converts it to amps.
I bear witness that that was available on at least one model over 50 years ago. :smile:

sophiecentaur Feb26-12 04:42 AM

Re: why must ammeter be connected in series?
 
NascentO
That type of meter was called a Moving Iron meter, when I was at School. The scale on it was hugely non linear but it could not 'overload'.

jim hardy Feb26-12 09:39 PM

Re: why must ammeter be connected in series?
 
Quote:

I bear witness that that was available on at least one model over 50 years ago.
I have two such meters . One is an antique automotive current meter, other a much more recent "Gauss" meter made by these folks:

http://rbannis.com/products/magnets.html click "Model 25"

They're the same instrument with different scales. Ammeter has a ridge on back to align it with the wire you wish to measure.


All times are GMT -5. The time now is 02:52 AM.

Powered by vBulletin Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
© 2014 Physics Forums