A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit.
Analog voltmeters move a pointer across a scale in proportion to the voltage measured and can be built from a galvanometer and series resistor. Meters using amplifiers can measure tiny voltages of microvolts or less. Digital voltmeters give a numerical display of voltage by use of an analog-to-digital converter.
Resistance Of Galvanometer By Ha
Voltmeters are made in a wide range of styles, some separately powered (e.g. by battery), and others powered by the measured voltage source itself. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Portable instruments, usually equipped to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.
A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. The galvanometer has a coil of fine wire suspended in a strong magnetic field. When an electric current is applied, the interaction of the magnetic field of the coil and of the stationary magnet creates a torque, tending to make the coil rotate. The torque is proportional to the current through the coil. The coil rotates, compressing a spring that opposes the rotation. The deflection of the coil is thus proportional to the current, which in turn is proportional to the applied voltage, which is indicated by a pointer on a scale.
One of the design objectives of the instrument is to disturb the circuit as little as possible and so the instrument should draw a minimum of current to operate. This is achieved by using a sensitive galvanometer in series with a high resistance, and then the entire instrument is connected in parallel with the circuit examined.
The sensitivity of such a meter can be expressed as "ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale measured value. For example, a meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would be 200000 ohms and at full scale, the meter would draw 1 milliampere from the circuit under test. For multi-range instruments, the input resistance varies as the instrument is switched to different ranges.
The sensitivity and input resistance of a voltmeter can be increased if the current required to deflect the meter pointer is supplied by an amplifier and power supply instead of by the circuit under test. The electronic amplifier between input and meter gives two benefits; a rugged moving coil instrument can be used, since its sensitivity need not be high, and the input resistance can be made high, reducing the current drawn from the circuit under test. Amplified voltmeters often have an input resistance of 1, 10, or 20 megohms which is independent of the range selected. A once-popular form of this instrument used a vacuum tube in the amplifier circuit and so was called the vacuum tube voltmeter (VTVM). These were almost always powered by the local AC line current and so were not particularly portable. Today these circuits use a solid-state amplifier using field-effect transistors, hence FET-VM, and appear in handheld digital multimeters as well as in bench and laboratory instruments. These largely replaced non-amplified multimeters except in the least expensive price ranges.
Most VTVMs and FET-VMs handle DC voltage, AC voltage, and resistance measurements; modern FET-VMs add current measurements and often other functions as well. A specialized form of the VTVM or FET-VM is the AC voltmeter. These instruments are optimized for measuring AC voltage. They have much wider bandwidth and better sensitivity than a typical multifunction device.
DVM measurement accuracy is affected by many factors, including temperature, input impedance, and DVM power supply voltage variations. Less expensive DVMs often have input resistance on the order of 10 MΩ. Precision DVMs can have input resistances of 1 GΩ or higher for the lower voltage ranges (e.g. less than 20 V). To ensure that a DVM's accuracy is within the manufacturer's specified tolerances, it must be periodically calibrated against a voltage standard such as the Weston cell.
Although today digital multimeters provide the simplest way to measure a resistance. The Wheatstone Bridge canbe used to compare an unknown resistance to that of a known resistance to determine its value allowing very low values of resistances down in the milli-Ohms (mΩ) range to be measured.
The Wheatstone bridge (or resistance bridge) circuit can be used in a number of applications and today, with modern operational amplifiers we can use the Wheatstone Bridge Circuit to interface various transducers and sensors to these amplifier circuits.
The Wheatstone Bridge circuit is nothing more than two simple series-parallel arrangements of resistances connected between a voltage supply terminal and ground producing zero voltage difference between the two parallel branches when balanced. A Wheatstone bridge circuit has two input terminals and two output terminals consisting of four resistors configured in a familiar diamond-like arrangement as shown. This is typical of how the Wheatstone bridge is drawn.
Then we can see that the source voltage VS is divided among the two series resistors in direct proportion to their resistances as VR1 = 4V and VR2 = 8V. This is the principle of voltage division, producing what is commonly called a potential divider circuit or voltage divider network.
Then we can see that the resistance ratio of these two parallel arms, ACB and ADB, results in a voltage difference between 0 volts (balanced) and the maximum supply voltage (unbalanced), and this is the basic principal of the Wheatstone Bridge Circuit.
So we can see that a Wheatstone bridge circuit can be used to compare an unknown resistance RX with others of a known value, for example, R1 and R2, have fixed values, and R3 could be variable. If we connected a voltmeter, ammeter or classically a galvanometer between points C and D, and then varied resistor, R3 until the meters read zero, would result in the two arms being balanced and the value of RX, (substituting R4) known as shown.
An LDR, also known as a cadmium-sulphide (Cds) photocell, is a passive resistive sensor which converts changes in visible light levels into a change in resistance and hence a voltage. Light dependent resistors can be used for monitoring and measuring the level of light intensity, or whether a light source is ON or OFF.
A typical Cadmium Sulphide (CdS) cell such as the ORP12 light dependent resistor typically has a resistance of about one Megaohm (MΩ) in dark or dim light, about 900Ω at a light intensity of 100 Lux (typical of a well lit room), down to about 30Ω in bright sunlight. Then as the light intensity increases the resistance reduces. By connecting a light dependant resistor to the Wheatstone bridge circuit above, we can monitor and measure any changes in the light levels as shown.
The Wheatstone Bridge has many uses in electronic circuits other than comparing an unknown resistance with a known resistance. When used with Operational Amplifiers, the Wheatstone bridge circuit can be used to measure and amplify small changes in resistance, RX due, for example, to changes in light intensity as we have seen above.
But the bridge circuit is also suitable for measuring the resistance change of other changing quantities, so by replacing the above photo-resistive LDR light sensor for a thermistor, pressure sensor, strain gauge, and other such transducers, as well as swapping the positions of the LDR and VR1, we can use them in a variety of other Wheatstone bridge applications.
It depends on the size and weight limit of the weighbridge. There could be one load cell in each corner and a mean average value taken, or one single pivoted load cell. Internally, one or more strain gauges whose electrical resistance varies with applied force are arranged to form the resistive arms of a Wheatstone Bridge Circuit. An excitation voltage (in Volts) is applied to the load cell with the low output voltage (in milli-volts) indicating the weight.
Tetracycline efflux proteins have amino acid and protein structure similarities with other efflux proteins involved in multiple-drug resistance, quaternary ammonium resistance, and chloramphenicol and quinolone resistance, including methylenomycin A (MetA) from Streptomyces coelicolor, aminotriazole transport (Atr1) from Saccharomyces, and arabinose transport (AraB) from Escherichia coli (147, 269). Homology between the Tet and other efflux proteins has also been found with a new protein (EfpA) cloned from Mycobacterium tuberculosis (65).
The gram-negative efflux genes are widely distributed and normally associated with large plasmids, most of which are conjugative. They come from a number of different plasmid incompatibility groups (115, 176). These plasmids often carry other antibiotic resistance genes, heavy metal resistance genes, and/or pathogenic factors such as toxins (70). Thus, selection for any of these factors selects for the plasmid. This phenomenon of cross-selection has contributed to the dramatic increase in the number multiple-drug-resistant bacteria over the last 40 years (146, 226).
Group 2 includes Tet(K) and Tet(L), with 58 to 59% amino acid identity; these proteins are found primarily in gram-positive species. Group 2 has 14 predicted transmembrane α-helices. These genes code for proteins which confer resistance to tetracycline and chlortetracycline. Their presence is indicated when gram-positive bacteria are resistant to tetracycline but not to minocycline or glycylcyclines (287, 295). The tet(K) and tet(L) genes are generally found on small transmissible plasmids, which on occasion become integrated into the chromosome of staphylococci (84) or the chromosome of Bacillus subtilis (255) or into larger staphylococcal plasmids (184, 266). Large staphylococcal plasmids carrying the tet(K) genes are relatively uncommon, whereas small plasmids carrying the tet(K) genes are common. The small plasmids represent a family of closely related plasmids, which range in size from 4.4 to 4.7 kb (213). Plasmid pT181 is the prototype of the family and has been completely sequenced (124). The pT181 family of plasmids can carry antibiotic resistance genes other than tet(K) (124). 2ff7e9595c
Σχόλια