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. A potentiometer is a null measurement device for measuring potentials voltages. A voltage source is connected to resistor R, passing a constant current through it. There is a steady drop in potential IR drop along the wire, so a variable potential is obtained through contact along the wire.
An unknown emf x represented by script E x connected in series with a galvanometer is shown in. Note that emf x opposes the other voltage source. The location of the contact point is adjusted until the galvanometer reads zero.
Since no current flows through the galvanometer, none flows through the unknown EMF, and emf x is sensed. Potentiometer : The potentiometer is a null measurement device.
A voltage source connected to a long wire resistor passes a constant current I through it. An unknown EMF labeled script Ex is connected as shown, and the point of contact along R is adjusted until the galvanometer reads zero. The unknown EMF is thus proportional to the resistance of the wire segment. In both cases, no current passes through the galvanometer. The current I through the long wire is identical. The three quantities on the right-hand side of the equation are now known or measured, and emf x can be calculated.
There is often less uncertainty in this calculation than when using a voltmeter directly, but it is not zero. Furthermore, it is not possible to tell when the galvanometer reads exactly zero, which introduces error into both R x and R s , and may also affect the current I. Many so-called ohmmeters measure resistance.
Question: In the electric circuit diagram at right, possible locations of an ammeter and a voltmeter are indicated by circles 1, 2, 3, and 4. Where should an ammeter be located to correctly measure the total current and where should a voltmeter be located to correctly measure the total voltage? Answer: To measure the total current, the ammeter must be placed at position 1, as all the current in the circuit must pass through this wire, and ammeters are always connected in series.
To measure the total voltage in the circuit, the voltmeter could be placed at either position 3 or position 4. Voltmeters are always placed in parallel with the circuit element being analyzed, and positions 3 and 4 are equivalent because they are connected with wires and potential is always the same anywhere in an ideal wire.
Question: Which circuit diagram below correctly shows the connection of ammeter A and voltmeter V to measure the current through and potential difference across resistor R?
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. Digital meters are able to detect smaller currents than analog meters employing galvanometers. How does this explain their ability to measure voltage and current more accurately than analog meters?
Since digital meters require less current than analog meters, they alter the circuit less than analog meters. Their resistance as a voltmeter can be far greater than an analog meter, and their resistance as an ammeter can be far less than an analog meter. Consult Figure and Figure and their discussion in the text. Stimulate a neuron and monitor what happens. Pause, rewind, and move forward in time in order to observe the ions as they move across the neuron membrane.
Why should you not connect an ammeter directly across a voltage source as shown in Figure? 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 ; c across ; d across ; e across and.
Note that there may be more than one answer to each part. To measure currents in Figure , 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 ; c through ; d through.
What is the sensitivity of the galvanometer that is, what current gives a full-scale deflection inside a voltmeter that has a resistance on its What is the sensitivity of the galvanometer that is, what current gives a full-scale deflection inside a voltmeter that has a resistance on its V scale?
Find the resistance that must be placed in series with a galvanometer having a sensitivity the same as the one discussed in the text to allow it to be used as a voltmeter with a 0. Find the resistance that must be placed in series with a galvanometer having a sensitivity the same as the one discussed in the text to allow it to be used as a voltmeter with a V full-scale reading.
Include a circuit diagram with your solution. Find the resistance that must be placed in parallel with a galvanometer having a sensitivity the same as the one discussed in the text to allow it to be used as an ammeter with a Find the resistance that must be placed in parallel with a galvanometer having a sensitivity the same as the one discussed in the text to allow it to be used as an ammeter with a mA full-scale reading.
Find the resistance that must be placed in series with a galvanometer having a sensitivity to allow it to be used as a voltmeter with: a a V full-scale reading, and b a 0. Find the resistance that must be placed in parallel with a galvanometer having a sensitivity to allow it to be used as an ammeter with: a a Suppose you measure the terminal voltage of a 1.
Suppose you measure the terminal voltage of a 3. A certain ammeter has a resistance of on its 3. What is the sensitivity of the galvanometer? A voltmeter is placed in parallel with a resistor in a circuit. A ammeter is placed in series with a resistor in a circuit. Suppose you have a galvanometer with a sensitivity. You cannot achieve a full-scale deflection using a current less than the sensitivity of the galvanometer.
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