# Using ammeter to measure voltage

Suppose you have the following circuit: Where R1 = 1 kΩ, R2 = 2 kΩ, R3 = 3 kΩ, R4 = 4 kΩ, R5 = 5 kΩ, Vs = 5V. The voltage in the branch with R5 is then 5 I where I over R5 is .064A.

Suppose you put a voltmeter to measure Vab which is the voltage from node Va to node Vb. This would just be the voltage in the branch: 5 * .064, if im not mistaken.

But what if you put an ammeter in to find Vab instead. Since ammeters have low resistance, under ideal conditions, would the voltage measure 0?

• "But what if you put an ammeter in to find Vab instead." What do you mean by "put an ammeter in". In where? Be specific. – Bob D Oct 18 '19 at 21:38
• In series with R5 – Melanie Oct 18 '19 at 22:18

Suppose you put a voltmeter to measure Vab which is the voltage from node Va to node Vb. This would just be the voltage in the branch: 5 * .064, if im not mistaken.

Not sure what 5 * .064 means. I think you mean the voltmeter would measure the voltage $$V_{ab}$$. If that's the case, yes it would.

But what if you put an ammeter in to find $$V_{ab}$$ instead. Since ammeters have low resistance, under ideal conditions, would the voltage measure 0?

Based on your clarification that the ammeter is put in series with $$R_5$$, yes it could be used to determine the voltage $$V_{ab}$$, provided that the input impedance of the ammeter is much less than $$R_5$$, just like the input impedance of a voltmeter placed between a and b would have to be much greater than the equivalent impedance between node a and b. But the ammeter would give you the current in $$R_5$$, which would not be a value of 0 volts, unless $$V_{ab}=0$$. It would measure a current equal to

$$I=\frac{V_{ab}}{R_5}$$.

From which you can calculate the voltage $$V_{ab}$$ as

$$V_{ab}=IR_{5}$$

Where $$I$$ = the ammeter reading.

Hope this helps.