# Calculating the Internal Resistance of a Voltmeter

#### Discoveries

Joined Oct 15, 2015
4
Hello,

I'm required to use the Voltage Divider Rule to find the resistance of the Voltage Source, an ammeter, and a voltmeter in the circuits respectively below: The first problem I'm having is trying to determine what the resistance is at Ri for the first circuit. I discussed this with a friend of mine and he gave me a formula but it didn't make sense. How can I use the Voltage Divider Rule to find the resistance of Ri?

I read this part of the textbook which helped: http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/

However the link above didn't explain how I can find the resistance I need to solve the problem.

Any help is appreciated! Thanks!

#### JoeJester

Joined Apr 26, 2005
4,390
how did they describe the voltmeter and the ammeter? Did they indicate the ohms/volt for the voltmeter and a max reading for the ammeter?

#### Discoveries

Joined Oct 15, 2015
4
They did not describe them really at all. No values were given. Could we assume some default values to estimate?

#### shteii01

Joined Feb 19, 2010
4,644
You have two unknowns, you need two equations.

The third circuit has one unknown, use that circuit to find resistance of the voltmeter.

Then plug resistance of voltmeter into first circuit to find Ri.

Then use Ri in second circuit to find resistance of ammeter.

#### Discoveries

Joined Oct 15, 2015
4
You have two unknowns, you need two equations.

The third circuit has one unknown, use that circuit to find resistance of the voltmeter.

Then plug resistance of voltmeter into first circuit to find Ri.

Then use Ri in second circuit to find resistance of ammeter.
I'll give that a try and get back to you! Thanks!

#### WBahn

Joined Mar 31, 2012
26,846
Do they tell you what the voltmeter is reading in the first circuit? If not, then there is no way to find Ri because you could but any Ri in there that you want and nothing changes as far as any of the information you have supplied.

#### JoeJester

Joined Apr 26, 2005
4,390
Can the TS post the whole question from the instructor?

Thank you everyone. 