Video Transcript
Which of the following circuit diagrams most correctly shows a voltmeter being used to measure the voltage of a direct current source?
We’ve been given three circuit diagrams (A), (B), and (C) to choose from.
Now, a direct current source can be any kind of device that produces a direct current. Usually we’d use this phrase to refer to a battery or a cell. We can see that all three of these circuit diagrams contain a cell. So we want to pick the circuit diagram that shows a voltmeter being used to measure the voltage of a cell.
We might recall that the circuit symbol for a voltmeter is a circle containing a capital V for volts. And as we can see, none of the three circuit diagrams provided contain this symbol. So, at first glance, it doesn’t look like any of these circuit diagrams show a voltmeter at all.
However, what we can see is that each of these circuit diagrams shows a component known as a galvanometer. We can recall that a galvanometer is an electromechanical device with a needle on the front that deflects whenever a current passes through the galvanometer. We can also recall that a galvanometer is one of the components that’s used to make a voltmeter. This is because if we connect a galvanometer to a component with a potential difference across it, for example, a cell, then the resulting current causes the needle to deflect. The greater the potential difference, the greater the current and the greater the deflection of the needle.
So a galvanometer on its own does function as a rudimentary voltmeter. However, a problem with using just a galvanometer on its own as a voltmeter is that galvanometers are incredibly sensitive. This means that even a very small current, for example, 100 microamps, is enough to cause the needle to deflect all the way to the end of the scale.
When we design a voltmeter, we need to solve this problem by reducing the size of the current that passes through the galvanometer. And we can do this simply by connecting a fixed resistor in series with the galvanometer. This resistor reduces the size of the current and moves the needle back into a usable range, where it doesn’t deflect all the way to the end of the scale.
So, if we disconnect our cell from the circuit, we can see that we can build a voltmeter just by connecting together a resistor and a galvanometer in series. Drawn as a standard circuit diagram, that looks like this. This voltmeter will measure the potential difference of any component that is connected in this gap. So, for example, that could be a resistor in a circuit. Note that in this case, we would say that the voltmeter is connected in parallel with this component. And this is generally the case whenever we use a voltmeter to measure the potential difference across a component that’s in a circuit.
However, if we wanted to use our voltmeter to measure the potential difference across just a single component on its own, for example, a cell, we would just connect it like this. So, in this specific case, the voltmeter is connected in series with the cell.
So we’ve shown that we can construct a voltmeter by connecting together a galvanometer and a resistor in series. And if we wanted to use this voltmeter to measure the potential difference or voltage of a cell, we would just need to connect it in series with the cell like this.
Looking at the answer options we’ve been provided with, we can see that only option (A) matches the circuit which we’ve described. Looking at option (B), we can see that while this circuit does contain all the right components, the galvanometer and the resistor are connected in parallel rather than in series. And if we look at option (C), we can see that the galvanometer and this resistor are connected in series with each other. However, the presence of this extra parallel branch with a resistor in it prevents this circuit from behaving like a voltmeter.
So the diagram that most correctly shows a voltmeter being used to measure the voltage of a direct current source is option (A).
