The name is derived from the name for the SI unit for electric current, amperes A. This is necessary because objects in series experience the same current. They must not be connected to a voltage source — ammeters are designed to work under a minimal burden, which refers to the voltage drop across the ammeter, typically a small fraction of a volt. Ammeter in Series : An ammeter A is placed in series to measure current. All of the current in this circuit flows through the meter.
The ammeter would have the same reading if located between points d and e or between points f and a, as it does in the position shown. Note that the script capital E stands for EMF, and r stands for the internal resistance of the source of potential difference. Analog meters have needles that swivel to point at numbers on a scale, as opposed to digital meters, which have numerical readouts. The heart of most analog meters is a device called a galvanometer, denoted by G. Current flow through a galvanometer, I G , produces a proportional movement, or deflection, of the needle.
The two crucial characteristics of any galvanometer are its resistance and its current sensitivity. By connecting resistors to this galvanometer in different ways, you can use it as either a voltmeter or ammeter to measure a broad range of voltages or currents.
A galvanometer can function as a voltmeter when it is connected in series with a large resistance R. The value of R is determined by the maximum voltage that will be measured. The total resistance must be:. R is so large that the galvanometer resistance, r, is nearly negligible. This voltmeter would not be useful for voltages less than about half a volt, because the meter deflection would be too small to read accurately.
For other voltage ranges, other resistances are placed in series with the galvanometer. Many meters allow a choice of scales, which involves switching an appropriate resistance into series with the galvanometer. The same galvanometer can also function as an ammeter when it is placed in parallel with a small resistance R , often called the shunt resistance. Since the shunt resistance is small, most of the current passes through it, allowing an ammeter to measure currents much greater than those that would produce a full-scale deflection of the galvanometer.
Suppose, for example, we need an ammeter that gives a full-scale deflection for 1. Since R and r are in parallel, the voltage across them is the same. Null measurements balance voltages so there is no current flowing through the measuring devices that would interfere with the measurement. Standard measurements of voltage and current alter circuits, introducing numerical uncertainties.
Voltmeters draw some extra current, whereas ammeters reduce current flow. Null measurements balance voltages, so there is no current flowing through the measuring device and the circuit is unaltered. Null measurements are generally more accurate but more complex than standard voltmeters and ammeters.
Their precision is still limited. When measuring the EMF of a battery and connecting the battery directly to a standard voltmeter, as shown in, the actual quantity measured is the terminal voltage V. Voltmeter Connected to Battery : An analog voltmeter attached to a battery draws a small but nonzero current and measures a terminal voltage that differs from the EMF of the battery.
Note that the script capital E symbolizes electromotive force, or EMF. Since the internal resistance of the battery is not known precisely, it is not possible to calculate the EMF precisely.
The EMF could be accurately calculated if r were known, which is rare. However, standard voltmeters need a current to operate. One solution to the problem of voltmeters and ammeters interfering with the circuits being measured is to use galvanometers with greater sensitivity. This allows construction of voltmeters with greater resistance and ammeters with smaller resistance than when less sensitive galvanometers are used.
There are practical limits to galvanometer sensitivity, but it is possible to get analog meters that make measurements accurate to a few percent.
Note that the inaccuracy comes from altering the circuit, not from a fault in the meter. Making a measurement alters the system being measured in a manner that produces uncertainty in the measurement. For macroscopic systems, such as the circuits discussed in this module, the alteration can usually be made negligibly small, but it cannot be eliminated entirely.
For submicroscopic systems, such as atoms, nuclei, and smaller particles, measurement alters the system in a manner that cannot be made arbitrarily small. This actually limits knowledge of the system—even limiting what nature can know about itself. We shall see profound implications of this when the Heisenberg uncertainty principle is discussed in the modules on quantum mechanics. There is another measurement technique based on drawing no current at all and, hence, not altering the circuit at all.
These are called null measurements and are the topic of Null Measurements. Digital meters that employ solid-state electronics and null measurements can attain accuracies of one part in 10 6. Why should you not connect an ammeter directly across a voltage source as shown in Figure 9? Note that script E in the figure stands for emf. Suppose you are using a multimeter one designed to measure a range of voltages, currents, and resistances to measure current in a circuit and you inadvertently leave it in a voltmeter mode.
What effect will the meter have on the circuit? What would happen if you were measuring voltage but accidentally put the meter in the ammeter mode? Specify the points to which you could connect a voltmeter to measure the following potential differences in Figure a the potential difference of the voltage source; b the potential difference across R 1 ; c across R 2 ; d across R 3 ; e across R 2 and R 3.
Note that there may be more than one answer to each part. To measure currents in Figure 10, you would replace a wire between two points with an ammeter. Specify the points between which you would place an ammeter to measure the following: a the total current; b the current flowing through R 1 ; c through R 2 ; d through R 3. What is the sensitivity of the galvanometer that is, what current gives a full-scale deflection inside a voltmeter that has a 1.
What is the sensitivity of the galvanometer that is, what current gives a full-scale deflection inside a voltmeter that has a Find the resistance that must be placed in series with a Include a circuit diagram with your solution.
Find the resistance that must be placed in parallel with a Suppose you measure the terminal voltage of a 1. See Figure Suppose you measure the terminal voltage of a 3. A certain ammeter has a resistance of 5. What is the sensitivity of the galvanometer? Unreasonable Results Suppose you have a Unreasonable Results a What resistance would you put in parallel with a You cannot achieve a full-scale deflection using a current less than the sensitivity of the galvanometer.
Skip to main content. Circuits and DC Instruments. Search for:. DC Voltmeters and Ammeters Learning Objectives By the end of this section, you will be able to: Explain why a voltmeter must be connected in parallel with the circuit. Draw a diagram showing an ammeter correctly connected in a circuit. Describe how a galvanometer can be used as either a voltmeter or an ammeter.
Basic electrical quantities: current, voltage, power. Example: Analyzing a more complex resistor circuit.
Analyzing a resistor circuit with two batteries. Resistivity and conductivity. Kirchhoff's current law. Kirchhoff's voltage law. Voltmeters and Ammeters. Electrolytic conductivity. Next lesson. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript - [Voiceover] Let's say you have a circuit here and you had a battery with a voltage v and there were resistors one, resistor two, and resistor three up here, and there was current flowing through here.
What if you wanted to experimentally measure the voltage across some of these elements? You'd have to use a voltmeter. Voltmeter looks like this. So a circle with a v in it is the symbol we use for a voltmeter.
How do you use it? You take that voltmeter, you bring it over to here. I can't plug it in the circuit like that. What I do is I take the leads of the voltmeter and I just connect them to either side of the circuit element that I want to determine the voltage across. So if I do this and I connect those leads right here, this voltmeter will tell me the voltage across R three. Or take the voltmeter, put it over here, and if I connect the leads across R one in parallel, notice I'm hooking up the voltmeter in parallel.
Voltmeters you always hook up in parallel. This now will tell me the voltage across R one and if I wanted to make sure my battery was functioning correctly, I could take my voltmeter and I can hook up the leads across the positive and negative terminals of the battery and see if the voltage across the battery is what I think it is. That's how you use a voltmeter: always hooked up in parallel. But if I wanted to measure the current, I don't use a voltmeter, I use an ammeter.
And for an ammeter you do not hook up an ammeter in parallel with the element you're trying to measure. You will probably blow out the ammeter. I've done it a few times. It's embarrassing.
Don't hook up the ammeter in parallel, tell you why in a minute. But what you have to do is hook it up in series. So if I wanted to know the current going through R three, I could just stick the ammeter right in here. One lead would plug into one side of the ammeter, the other lead would plug into the other side. This current would have to flow straight through the ammeter and this is telling me how much current goes through R three.
It doesn't matter what side I put it on, the current going into R three will equal the current going out. So you can put it over here too, but it's gotta be hooked up in series. So you have to disconnect, it's kind of a pain to hook up an ammeter sometimes. You have to disconnect something here, then connect that connection to the one side of the ammeter, connect to the other side of the ammeter.
For a voltmeter, you didn't have to do that.
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