# Homework Help: Voltmeter help

1. Dec 5, 2004

### joshanders_84

This is another problem I am having troubles with. Here's the prompt:

What is the reading of a voltmeter with the proper range and with resistance 5*10^4 Ohms when connected between point b and ground?

I know that for voltmeters, V = I_fs (R_c + R_s), and I tried solving for V using the I through the system as I_fs, which I solved as (400V/300000) = .001333, and then using 5*10^4 and 200000 as the resistors, but it says my answer is off by an additive constant. I don't understand what I am dong wrong...all help is appreciated. Thanks
Josh

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2. Dec 5, 2004

### ceptimus

If the voltmeter wasn't there, or was perfect, then the voltage at b would be 2/3 of that at a.

Now when the voltmeter is present, its resistance is in parallel with the 200k-ohm resistor, so you can calculate the equivalent resistance of the parallel combination, and get a new ratio for the voltages at a and b.

3. Dec 6, 2004

### joshanders_84

I don't understand how that helps...sorry. I just don't get what I am doing wrong still, or how the above would help me go further...

4. Dec 6, 2004

### joshanders_84

o i ci I got it now, thank you