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I. Direct Current Meters
A. Ayrton Shunt
A shunt used to increase the range of a galvanometer without changing the damping. Also known as universal shunt.
An Ayrton shunt used with an ammeter consists of several series connected resistors all connected in parallel with the PMMC instrument. Range change is effected by switch between resistor junctions
B. Ohmmeter
Is an electrical instrument that measures electrical resistance, which is known as the opposition to an electric current. The unit of measurement for resistance is ohms (Ω).
The original design of an ohmmeter is supplied by a small battery to apply a voltage to a resistance. It uses a galvanometer to measure the electric current through
the resistance. The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance is decreased, the current through the meter would increase.
A more accurate type of ohmmeter has an electronic circuit that passes a constant current (I) through the resistance, and another circuit that measures the voltage (V) across the resistance. According to the following equation, derived from Ohm's Law, the value of the resistance (R) is given by:
C. Multimeter
Also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance. Multimeters may use analog or digital circuits—analog multimeters and digital multimeters (often abbreviated DMM or DVOM.) Analog instruments are usually based on a microammeter whose pointer moves over a scale calibrated for all the different measurements that can be made; digital
instruments usually display digits, but may display a bar of length proportional to the quantity measured.
A multimeter can be a hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such aselectronic equipment, motor controls, domestic appliances, power supplies, and wiring systems.
The first moving-pointer current-detecting device was the galvanometer. These were used to measure resistance & voltage by using awheatstone bridge, and comparing the unknown quantity to a reference voltage or resistance. While usable in a lab, the technique was very slow and impractical in the field. These galvanometers were bulky and delicate.
The D'Arsonval/Weston meter movement used a fine metal spring to give proportional measurement rather than just detection, and built-in permanent field magnets made deflection independent of the position of the meter. These features enabled dispensing with Wheatstone bridges, and made measurement quick and easy. By adding a series or shunt resistor, more than one range of voltage or current could be measured with one movement.
Multimeters were invented in the early 1920s as radio receivers and other vacuum tube electronic devices became more common. The invention of the first multimeter is attributed to Post Office engineer Donald Macadie, who became dissatisfied with having to carry many separate instruments required for the maintenance of the telecommunication circuits. Macadie invented an instrument which could measure amperes, volts and ohms, so the multifunctional meter was then named Avometer.[2] The meter comprised a moving coil meter, voltage and precision resistors, and switches & sockets to select the range.
D. Other Meters1. Electrometer
An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solid state technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere. A simpler but related instrument, the electroscope, works on similar principles but only indicates the relative magnitudes of voltages or charges.
2. Hall Effect Sensor
A Hall effect sensor is a transducer that varies its output voltage in response to changes in magnetic field. Hall sensors are used for proximity switching, positioning, speed detection, and current sensing applications.
In its simplest form, the sensor operates as an analogue transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced.
Electricity carried through a conductor will produce a magnetic field that varies with current, and a Hall sensor can be used to measure the current without interrupting the circuit. Typically, the sensor is integrated with a wound core or permanent magnet that surrounds the conductor to be measured.
II. Alternating Current Meters
A. D’Arsonval Meter Movement in AC
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil. The term has expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.
By connecting a rectifier to a d'Arsonval meter movement, an alternating current measuring device is created.
When ac is converted to pulsating dc, the d'Arsonval movement will react to the average value of the pulsating dc (which is the average value of one-half of the sine wave). Another characteristic of using a rectifier concerns the fact that the d'Arsonval meter movement is capable of indicating current in only one direction. If the d'Arsonval meter movement were used to indicate alternating current without a rectifier, or direct current of the wrong polarity, the movement would be severely damaged. The pulsating dc is current in a single direction, and so the d'Arsonval meter movement can be used as long as proper polarity is observed.
B. Electrodynanometer movement
An electrodynamometer is an instrument used for measuring the electric power. The basic principle was laid out in an 1848 paper by Wilhelm Weber (1804-1891): when the same current passes through two concentric coils placed at right angles to each other, the resulting torque depends on the square of the current.
The electrical inventor and entrepreneur, Werner von Siemens (1816-1892), used this principle in his electrodynamometer, first described in 1880. In order to measure the power dissipated in an electrical load, it is necessary to measure the current through the load and the potential drop across it. In the Siemens instrument, the stationary coil is made of relatively few turns of heavy wire and is connected in series with the circuit. The rotating coil consists of many turns of fine wire, and is connected across the load with a multiplier resistance in series with it to measure the potential drop. The currents through the two coils are I and a current proportional to V, and the product of the two currents is proportional to the power dissipated in the load.
C. Iron-vane Meter movement
The moving iron vane movement can be used to measure both AC current and voltage.By changing the meter scale calibration, the movement can be used to measure DC current andvoltage. The moving iron vane meter operates on the principle of magnetic repulsion betweenlike poles. The measured current flows through a field coil which produces a magnetic fieldproportional to the magnitude of current. Suspended in this field are two iron vanes attached toa pointer. The two iron vanes consist of one fixed and one moveable vane. The magnetic fieldproduced by the current flow magnetizes the two iron vanes with the same polarity regardlessof the direction of current through the coil. Since like poles repel one another, the moving ironvane pulls away from the fixed vane and moves the meter pointer. This motion exerts a forceagainst a spring. The distance the moving iron vane will travel against the spring depends onthe strength of the magnetic field. The strength of the magnetic field depends on the magnitudeof current flow.
D. Thermocouple meter
For typical metals used in thermocouples, the output voltage increases almost linearly with the temperature difference (ΔT) over a bounded range of temperatures. For precise measurements or measurements outside of the linear temperature range, non-linearity must be corrected. The nonlinear relationship between the temperature difference (ΔT) and the output voltage (mV) of a thermocouple can be approximated by a polynomial:
The coefficients an are given for n from 0 to between 5 and 13 depending upon the metals. In some cases better accuracy is obtained with additional non-polynomial terms[4]. A database of voltage as a function of temperature, and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple is available online[4].
In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table;[5] older devices use analog circuits.
Piece-wise linear approximations are an alternative to polynomial corrections.
E. AC Voltmeter
AC electromechanical meter movements come in two basic arrangements: those based on DC movement designs, and those engineered specifically for AC use. Permanent-magnet moving coil (PMMC) meter movements will not work correctly if directly connected to alternating current, because the direction of needle movement will change with each half-cycle of the AC. (Figure below) Permanent-magnet meter movements, like permanent-magnet motors, are devices whose motion depends on the polarity of the applied voltage (or, you can think of it in terms of the direction of the current).
In order to use a DC-style meter movement such as the D'Arsonval design, the alternating current must be rectified into DC. This is most easily accomplished through the use of devices called diodes. We saw diodes used in an example circuit demonstrating the creation of harmonic frequencies from a distorted (or rectified) sine wave. Without going into elaborate detail over how and why diodes work as they do, just remember that they each act like a one-way valve for electrons to flow: acting as a conductor for one polarity and an insulator for another. Oddly enough, the arrowhead in each diode symbol points against the permitted direction of electron flow rather than with it as one might expect. Arranged in a bridge, four diodes will serve to steer AC through the meter movement in a constant direction throughout all portions of the AC cycle.
Passing AC through this Rectified AC meter movement will drive it in one direction.
Another strategy for a practical AC meter movement is to redesign the movement without the inherent polarity sensitivity of the DC types. This means avoiding the use of permanent magnets. Probably the simplest design is to use a non-magnetized iron vane to move the needle against spring tension, the vane being attracted toward a stationary coil of wire energized by the AC quantity to be measured.
F. Other Meters1. Gauss Meter
This meter measures the level of ELF magnetic field radiation from power lines, computers, kitchen appliances, and more! The easy to read scale, unique built-in audio signal and auto shut-off make it simple to use and a great way to find hidden sources of ELF frequency magnetic fields.
Most experts agree that chronic exposure to more than 2.5 milli-Gauss is inadvisable. This meter will show you which areas are above or below 2.5 milli-Gauss in the 50 - 60 Hz frequency range.
Hand-held, lightweight and durable, with two easy to read scales (0-1 mG, & 0-10 mG), and dramatic audio signal. Our favorite meter for demonstrating the presence of dangerous EMFs to others and paranormal work. Requires one 9V battery (not included). Calibrated at 50-60 Hz.
2. EMF Meter
This 3-axis AC gaussmeter is very unique, especially the 60 Hz filtering switch. Flip this switch to SUM to measure a wide range of frequencies (20 Hz to 100KHz). Flip to FILTER and 60 Hz signals ((±10 Hz to 3 dB, common powerline and household electricity) are NOT included in the detection. This feature combined with the LED output and switchable sound make this an excellent choice for paranormal and routine work. Readout is easy to see and hear in low light conditions, even by a group of people. 7 LED display will show 1 LED in DOT mode, or include all lower level LEDs in BAR mode. ±10% Accuracy at 60 Hz. Uses 9V battery