# Voltage measurement circuit

I have a material and I want to measure the voltage and current through it over time. Unfortunately I only have an automatic voltmeter and the resistance of the material changes over time. Because I can't measure the current and I don't know the resistance I created a circuit to measure this. See image below.

R1 is the material with variable resistance (disregard the 2kO). I choose a known resistor (R2) and measure the voltage through that. I also know the applied voltage. This way I can figure out R1 as well as the current through the circuit.

The current is I = V2 / R2

The voltage is V1 = Vtot - V2

The resistance is R1 = (Vtot/V2 - 1)*R2 or R1 = V1 / I

My question: will this work? I did the math and it should, however, I have done some tests and it is not working as expected. Should R2 be a value close to R1? Will the voltmeter affect the current path and create a parallel circuit? I need to know the stuff that someone who has some experience with circuits would know.

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simulate this circuit – Schematic created using CircuitLab

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• The voltmeter does add a parallel path across R2. However, so long as R1 and R2 are small relative to the internal resistance of the voltmeter (on the order of $M\Omega$s) you can use this circuit. If the variation in R1 is extremely small however, you will need to setup a precision measurement circuit using op-amps. – Captainj2001 Jul 11 '16 at 20:49
• Thanks. That actually answers another question I had. By the way, what is op-amp? – Gethal Jul 12 '16 at 18:39
• An Op-Amp is a type of amplifier circuit used in place of discrete transistors as amplifiers. They have several nice properties and can be used in many configurations to perform a multitude of functions, including precision measurements. – Captainj2001 Jul 12 '16 at 19:17