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Why does the internal resistance of an analog ammeter decrease as the range used increases? For example say I am using the 100 mA range of a ammeter, if I measure the internal resistance, it is a larger value than when I use for instance the 300 mA range of the ammeter.

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  • \$\begingroup\$ Decrease when the range is doing what? \$\endgroup\$
    – Eugene Sh.
    Commented Oct 18, 2019 at 20:26
  • \$\begingroup\$ More current means more heat for the same current sense resistance used. More heat than you want and more voltage drop (or needle deflection) than you need . BTW this is not a coherent question since you say the range is used but there is always a range in operation when the meter is used. You need to actually say whether the range increases or decreases. I just happen to know what you are trying to ask. \$\endgroup\$
    – DKNguyen
    Commented Oct 18, 2019 at 20:46
  • \$\begingroup\$ a bigger drop would equal more power to lift the needle \$\endgroup\$
    – dandavis
    Commented Oct 18, 2019 at 21:36
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    \$\begingroup\$ @TimWescott, no, it does not. It swings proportional to the current passing through the movement. The rest of your comment is almost right, in that the current dividers producing the same result from different input have differing overall impedance. \$\endgroup\$ Commented Oct 19, 2019 at 2:06
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    \$\begingroup\$ @TimWescott the point is that it's the current that actual produces the deflection \$\endgroup\$ Commented Oct 19, 2019 at 3:56

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A DC analog ammeter is essentially a voltmeter in parallel with a shunt resistor.

It's a bit more complex than that, because a typical (d'Arsonval moving coil with taut band suspension) meter movement actually measures current, and has a resistance that is temperature sensitive since it's typically wound with copper wire rather than an alloy like Constantan or Manganin. In a real meter movement, a temperature-sensitive network such as an NTC thermistor in parallel with a resistor may be put in series with the copper winding to counteract the positive temperature coefficient of the coil, yielding a voltage sensitive movement with perhaps 50uA full scale and a full scale voltage of around 100-200mV.

Then to get different ranges you would switch a resistor in parallel with the (say) 150mV movement that yields 150mV at the full-scale current (taking into account the meter movement resistance and the external shunt).

So in the case of a 150mV burden and 3K meter + network resistance, the shunt resistors would be:

50uA - no shunt

100uA 6.00K

...

50mA - 3.03 ohms

etc.

In this example, the ammeter will always drop 150mV at full scale deflection regardless of what the scale is- so the "internal resistance" of the meter as a whole instrument is inversely proportional to the full-scale (full deflection of the pointer) current range.

Here is a photo of the inside of the venerable Simpson 260 multimeter, courtesy of this site:

enter image description here

The disk-shaped object is a thermistor. The 260 used a jeweled movement (tiny bearings rather than a taut band). There's also usually some protection components such as diodes and fuses to help keep the meter and shunt components from being damaged by overloads.

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