Ammeter
An ammeter is a device used to measure electric current in a circuit. It is typically connected in series with the circuit, allowing it to measure the flow of current through the circuit. Ammeters are designed to have low resistance to minimize their impact on the circuit being measured, and they are essential tools for understanding and analyzing electrical systems.
7 Key excerpts on "Ammeter"
eBook - ePub- Adrian Waygood(Author)
- 2015(Publication Date)
- Routledge(Publisher)
...The proposed new definition of the ampere will be based on the numerical value of the elementary charge (i.e. the amount of charge on a single electron), a figure yet to be agreed! So, as far as the ampere is concerned, watch this space! Measuring current, voltage, and resistance Although we are able to observe the effects of an electric current or voltage, we cannot, of course, ‘see’ the actual current or voltage itself. So, in order to detect and to measure these quantities, together with resistance, we need instruments that will do the ‘seeing’ and ‘measuring’ for us. The three basic instruments used for this purpose are the Ammeter, voltmeter and ohmmeter (Figures 7.2, 7.3 and 7.4): Most of us are very unlikely to use these individual instruments outside a college laboratory. We are far more likely to use a multimeter (Figure 7.5), either analogue or digital, which combines the functions of these three separate instruments into one. Figure 7.5 In this chapter, though, we’ll first consider the separate functions of a multimeter. We’ll start by learning how analogue instruments work and how to use them, as they are significantly more complicated to prepare and use. If we can handle an analogue instrument, then we will have no difficulty transferring these skills to a digital instrument. Analogue instruments Measuring current Before connecting an Ammeter into a circuit, we must always start by de-energizing that circuit with its load-breaking device, such as a circuit breaker. To measure current, we must disconnect the circuit at the point where we wish to measure that current and, then, insert the Ammeter at that point. In other words, an Ammeter must be connected in series with the circuit under test, as illustrated in Figure 7.6 (where R represents the load)...
- Dale Crane, Dennis Wilt(Authors)
- 2017(Publication Date)
- Aviation Supplies & Academics, Inc.(Publisher)
...A voltmeter across an open switch reads the system voltage. A voltmeter across a closed switch reads zero. Ammeters Ammeters are connected in series with the circuit whose current is to be measured. The positive (+) terminal of the Ammeter is connected to the portion of the circuit to which the positive terminal of the power source is connected. Figure 4-3. Ammeters are connected in series with the load to measure the current flowing through the load. Ammeters installed in an aircraft cockpit can measure the current flowing from the generator without routing the high-current-carrying cables through the cockpit. This is done by placing an Ammeter shunt—a very low-resistance precision resistor—in the main generator output lead and measuring the voltage drop across it with a millivoltmeter calibrated in amps. The standard aircraft Ammeter shunt has a 50-millivolt (0.05-volt) drop across it when the rated output current from the generator is flowing through it. Small amounts of current are measured with milliAmmeters, which measure current in increments of 0.001 amp, and microAmmeters, which measure current in increments of 0.000 001 amp. Figure 4-4. Generator output current is monitored by measuring the millivoltage drop across an Ammeter shunt installed in the generator output lead. The millivoltmeter is calibrated in amperes. Ohmmeters Ohmmeters measure the resistance of a component by measuring the amount of current forced through the component by a known voltage. Ohmmeters contain a battery and a variable resistor to adjust the voltage so it will furnish the correct amount of current to cause full-scale deflection of the meter when the test leads are shorted together. Figure 4-5. Before the resistance of a component can be measured with an ohmmeter, the component must be isolated from the rest of the circuit. There are several scales marked R × 1, R × 100, and R × 10,000 to give the meter a wide range of resistance it can measure...
eBook - ePub- Adrian Waygood(Author)
- 2018(Publication Date)
- Routledge(Publisher)
...With moving-coil instruments, this is normally provided by an eccentric-screw arrangement which will slightly adjust the ‘relaxed’ position of the hairsprings, enabling the pointer to be adjusted so that it hovers exactly over its zero position. This action must always be performed before using the instrument whenever the pointer is seen to be off-zero. Because the direction of the torque depends upon the direction of the current flowing through the coil, all inoving-coil instruments are d.c. instruments. However, to enable moving-coil instruments to measure a.c. values, a rectifier (a circuit that change a.c. into d.c.) is incorporated into these meters, and their scales calibrated to indicate a.c. values. Some d.c./a.c. instruments have different scales for d.c. and a.c. values, so care must be taken to read from the correct scale. Measuring current: Ammeters To measure current, the circuit should be temporarily disconnected from its supply, and must then be broken at the point where we want that current to be measured, and the Ammeter inserted at that point. In other words, an Ammeter must be connected in series with the load under test, as shown in Figure 12.7. Figure 12.7 It’s very important that the insertion of the Ammeter into a circuit has very little effect on the circuit’s existing resistance which would alter the current normally flowing in the circuit and therefore give an inaccurate reading. So, Ammeters are manufactured with extremely low values of internal resistance. Because Ammeters have a very low internal resistance, it is vitally important that they are never inadvertently connected across, or in parallel with, any circuit component – and especially with the supply...
eBook - ePub- Julian Nathan(Author)
- 1998(Publication Date)
- Newnes(Publisher)
...2.2). The shunt value is a fraction of an ohm, low enough not to seriously affect the circuit under test. While voltage measurement is made across components in a circuit, current is measured in series with them in order to intercept the flow. However, current can also be calculated from voltage measured across one circuit resistance element. Like voltmeters, Ammeters measure RMS current where AC is concerned. Fig. 2.2 An Ammeter measures volts across a shunt resistor. The voltmeter method of circuit current measurement is the basis of current sensing as used in many electronic systems, for example, some amplifier overload protection and negative feedback applications. As with voltage measurement, the meter is switched to a safe Alternating or Direct Current range before the current is applied to it. It is worth repeating that the current through a single un-branched circuit is the same at any point in that circuit. Referring to Fig. 2.3, the current flowing through R2 is the same as through R3. However, if the resistances are different, then it follows that for the same current, the voltage drop across R2 and R3 will also be different. Fig. 2.3 Current measurement through R2 and R3. CURRENT IS MEASURED IN SERIES WITH A CIRCUIT OR CALCULATED FROM THE VOLTAGE DROP ACROSS A SERIES RESISTOR When making measurements for the purpose of determining watts in a known circuit resistance by calculation, as in amplifier output measurement, it would be just as accurate to measure current instead of voltage, and calculate watts from the two known quantities...
eBook - ePub- Stephen Sangwine, Stephen Sangwine(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
...Ammeter with a coil resistance of 75 Ω into a 1 V f.s.d. voltmeter. (Answer : 925 Ω.) Ideally, a voltmeter would draw zero current from the circuit being measured. As shown above, a voltmeter based on a moving-coil Ammeter must draw some current in order to work, and it is common to specify a figure of merit for this type of voltmeter. This can be done by stating the resistance of the meter divided by the full-scale deflection in volts, giving a figure in ohms per volt. A good moving coil voltmeter will typically have a figure of merit of 20,000 Ω/V. Voltmeters with a low figure of merit may draw significant current from a circuit being measured, leading to error in the measurement. Ammeter and voltmeter functions are often combined in a multipurpose instrument known as a multimeter, usually with switch-selectable ranges for both a.c. and d.c. current and voltage and ranges for measuring resistance (using an internal battery to supply a current through the resistance to be measured). A.C. measurements are obtained using rectifiers (usually full-wave bridges) to obtain a unidirectional current through the meter coil. The meter responds to the mean current, but the scale may be calibrated in r.m.s. units. This will only be correct for a sinusoidal voltage or current within the frequency range of the multimeter. Correct measurement of nonsinusoidal currents requires a true r.m.s meter such as a moving iron meter. Details may be found elsewhere in, for example, reference handbooks on electrical measurement and instruments. Figure 6.5 shows the well-known “Avometer” multimeter, which has been produced more or less in its present form since 1936, and was still in production in 2006, at the time of writing this third edition. It measures a.c. or d.c. voltage and current, and resistance. Figure 6.5 The Avometer Model 8 Mark 7 multimeter...
eBook - ePub- Lawrence Thompson(Author)
- 2006(Publication Date)
- International Society of Automation(Publisher)
...What calibration must be performed is usually in a calibration lab. Still, the principles employed to calibrate an analog Ammeter are basic to all electric calibrations. There are two methods for calibrating an analog Ammeter. 1. Comparison calibration. 2. Voltmeter-precision resistor (or indirect) calibration. COMPARISON CALIBRATION Figure 7–9 illustrates the equipment and connections used in a typical comparison calibration setup for an Ammeter. In this method, the current-limiting resistor is adjusted to allow varying amounts of current through the circuit, and the readings taken from the meter under test (MUT) are compared to the readings taken from the standard meter. Provided that the meter meets its specifications, the standard was at least as accurate as the meter under test’s specifications (although four times the accuracy is generally preferred). Where the meter does not meet the standard, a calibration curve can be developed. When it is used with the calibration curve the meter will achieve its stated accuracy. Modern-day digital manufacturing and control instruments maintain a copy of the calibration curve and linearize themselves. Most modern instruments are trimmed to specification in the factory, and there are no field-adjustable or calibration points. Figure 7–9 Comparison calibration, Ammeter. VOLTMETER-PRECISION RESISTOR Figure 7–10 illustrates the equipment and connections used in a voltmeter-precision resistor calibration setup for an Ammeter. This method utilizes Ohm’s Law to determine the true current, where the voltage drop across the precision resistor divided by the resistance of the precision resistor gives the circuit current...
eBook - ePubThe Really Useful Book of Secondary Science Experiments
101 Essential Activities to Support Teaching and Learning
- Tracy-ann Aston(Author)
- 2017(Publication Date)
- Routledge(Publisher)
...The unit for measuring resistance is ohms. An electrical current is the flow of negatively charged electrons through a conducting material. As the electrons pass through a metal such as a wire they are likely to collide with the atoms that make up the metal. If the wire is longer the resistance will be greater because there are more atoms present for the electrons to collide with. If the wire is thinner the resistance will be greater as there is less space for the current to flow through the wire. NATIONAL CURRICULUM LINKS: Current electricity • electric current, measured in amperes, in circuits, series and parallel circuits, currents add where branches meet and current as flow of charge • potential difference, measured in volts, battery and bulb ratings; resistance, measured in ohms, as the ratio of potential difference (p.d.) to current. MATERIALS NEEDED: Cells and cell holders, Ammeters, connecting wires and crocodile clips, resistance wires of different diameters. SAFETY AND TECHNICAL NOTES: • Use fine gauge wires to prevent the wires heating up and affecting the results. METHOD: To be done in advance by the teacher Ensure the different wires are labelled with their diameters. STUDENTS: 1 Build a simple series circuit using the cells, Ammeters and connecting wires. 2 Make a gap in the circuit and add the first piece of wire you will be testing. 3 Record the reading on the Ammeter in your results table. 4 Repeat the investigation with the rest of the wires you will be testing. DATA COLLECTION IDEAS: Diameter of resistance wire (mm) Current in the circuit (A) DIFFERENTIATION: • Decrease the challenge: The electrical...
