Ultrasonic welding

Ultrasonic welding (USW) is a solid-state welding process that produces a weld by local application of high-frequency vibratory energy while the work pieces are held together under pressure; is one of the most popular methods for joining plastics and it is becoming an important method for welding polymeric composites. Ultrasonic welding uses a piezo-electric element to generate […]

Measuring lag

The delay in the response of a measuring instrument to a change in the measured quantity is known as measuring lag. Thus it is the retardation delay in the response of a measurement system to changes in the measured quantity. This lag is usually quite small, but this small lag becomes highly important when high-speed measurements […]


In Metrology the term measurement is closely associated with all the activities about scientific, industrial, commercial, and human aspects. It is defined as the assignment of a number to a characteristic of an object or event, which can be compared with other objects or events. The knowledge of the reality that surrounds us is based on the […]


An accelerometer is a measuring instrument able of detecting and/or measuring acceleration (or the gravitational force), calculating the force measured with respect to the mass of the object (force per unit of mass). Therefore the operating principle of an accelerometer is based on the detection of the inertia of a mass when it is subjected to an […]


In Metrology, tolerance means the limit or acceptable limits of the variations of a physical dimension, a physical property of a manufactured object, of a system, or other measured values such as temperature, humidity, or time. In other words, it is the maximum allowable error in the measurement is specified in terms of some value. The tolerance in mechanical devices is […]

Biofuel (biomass fuel)

Biomass fuels are exclusively combustion fuels and when they are burnt they generate carbon dioxide that enters the atmosphere in exactly the same way as fossil fuel would generate carbon dioxide. Biomass fuel is biomass converted directly to energy or converted to liquid or gaseous fuels such as ethanol, methanol, methane, and hydrogen.


A lubricant is an organic or synthetic substance (it can occur in any physical state: liquid, solid, gaseous and even semi-solid or viscous) which has the property of reducing the friction between surfaces in contact under any operating condition, dissipating the heat generated during the relative movement between the surfaces, maintaining its chemical stability, protecting the mechanical parts from corrosive attacks, cushioning possible shocks, […]

Response time

Response time is the time required by a measuring instrument or system to settle to its final steady position after the application of the input. For a step input function, the response time may be defined as the time taken by the instrument to settle to a specified percentage of the quantity being measured, after […]

Ampere (unit of electric current)

Electric units, called “international units,” for current and resistance, were introduced by the International Electrical Congress held in Chicago in 1893, and definitions of the “international ampere” and “international ohm” were confirmed by the International Conference in London in 1908. Although it was already obvious on the occasion of the 8th CGPM (1933) that there […]

Direct current (DC)

Direct current (DC) is electricity flowing in a constant direction. So, electrons always flow constantly in the same direction within the electrical circuit, and/or possessing a voltage with constant polarity, therefore they will always circulate in the same direction. Direct current was produced in 1800 by Italian physicist Alessandro Volta’s battery, by his “Voltaic pile.” Direct […]

Alternating current (AC)

Alternating current describes the flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current. In other words, it is a type of electric current characterized by the fact of inverting the electric polarity continuously over time. Basically, unlike the direct current, in which the polarity is fixed […]


In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. There are two types of overcurrents: overload currents; short-circuit currents. Possible causes for overcurrent include short circuits, excessive load, […]

Wave impedance

The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields (the transverse components being those at right angles to the direction of propagation). If the electric field strength is expressed in volts per meter and the magnetic field strength is expressed in ampere-turns per meter, the wave […]

Mechanical impedance

Mechanical impedance is a measure of how much a structure resists motion when subjected to a harmonic force. It relates forces with velocities acting on a mechanical system. The mechanical impedance of a point on a structure is the ratio of the force applied at a point to the resulting velocity at that point.

Electrical impedance

Electrical impedance is a physical quantity that represents the opposition force of a circuit to the passage of alternating electric current, or, more generally, of a variable current. In other words, is the amount of opposition that an electrical element offers to current flow in a circuit when a voltage is applied at a specific frequency. […]

Acoustic impedance

Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal second per cubic metre (Pa·s/m3). There is a close analogy with electrical impedance, which measures the opposition that a system presents […]

Hypoid gear

Hypoid gears are similar to bevel gears, but the axes of the two connecting shafts do not intersect. They carry curved teeth, are stronger than the common types of bevel gears, and are quiet-running. These gears are mainly used in automobile rear axle drives.

Bevel gear

Bevel gears are used to connect shafts at any desired angle to each other. The shafts may lie in the same plane or in different planes.

Worm gear

A worm gear is similar to a screw having single or multiple start threads, which form the teeth of the worm. The worm drives the worm gear or worm wheel to enable transmission of motion. The axes of worm and worm gear are at right angles to each other. Worms and worm wheels are not limited to […]

Herringbone gear

Herringbone gears have two sets of helical teeth, one right-hand and the other left-hand, machined side by side.

Gear rack

The gear rack have the same sized and shaped teeth cut at equal distances along a flat surface or a straight rod. A gear rack is a cylindrical gear with the radius of the pitch cylinder being infinite. By meshing with a cylindrical gear pinion, it converts rotational motion into linear motion. Gear racks can be broadly divided […]

Helical gear

Helical or “dry fixed” gear, is a type of gear that offers a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Each tooth has a helical or spiral form. Since the gear is curved, this angling makes the tooth shape a […]

Spur gear

Spur gears, or straight-cut gears, are the simplest and the most widely used gears that can achieve high accuracy with relatively easy production processes. The gear teeth are cut on the periphery and are parallel to the axis of the gear. Though the teeth are not straight-sided (but usually of special form to achieve a constant […]


Plasticity is the capacity to resist plastic deformation (dislocation movement of a solid material undergoing non-reversible changes of shape in response to applied forces), while toughness measures the ability of a material to resist crack propagation. The rocks, subjected to external forces, are deformed continuously and permanently, without however being subjected to rupture phenomena and without […]


The gravimeter is a particular type of accelerometer specifically designed to measure the acceleration of gravity. According to the equivalence principle of general relativity, the effects of gravity and acceleration are the same; therefore, an accelerometer cannot distinguish between the two cases. As gravimeters, it is possible to use improved versions of accelerometers for static measurements, in […]

Meter (or metre) unit of length

The unit of length is the meter (or metre), defined by the distance, at 0 °C under normal atmospheric pressure, between the axes of the two central lines marked on the bar of platinum-iridium (90% platinum and 10% iridium) kept at the Bureau International des Poids et Mesures and declared Prototype of the meter by the 1st […]

Wimshurst machine

The Wimshurst influence machine is is a historic electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst (1832-1903). The appearance is particular and is characterized by two vertically mounted discs that rotate in the opposite direction and two metal balls for discharging. It produces electrical discharges of a […]


An airfoil (in American English) or aerofoil (in British English) is a structure shaped to produce an aerodynamic reaction – lift – at right-angles to its direction of motion. Familiar examples include the wings of an airplane, a blade (of a propeller, rotor, or turbine), or sail (as seen in cross-section).

Unit operation

In chemical engineering and related fields, a unit operation is a basic step in a process; it consists of a single physical transformation that can take place inside a chemical apparatus of a chemical plant. Unit operations involve a physical change or chemical transformation such as separation, crystallization, evaporation, filtration, polymerization, isomerization, and other reactions.


The belt is a mechanical transmission element used to connect two shafts in a slightly elastic, but integral way, through the use of pulleys, whose center distance is rather high. The most common belts are distinguished, according to their shape, in flat, trapezoid (or trapezoidal) and toothed. The most widely used material is the rubberized fabric, synthetic […]


In metallurgy, the fracture is a sudden break of metal materials due to stresses, caused by stress, applied at a point far from that in which the fracture begins. There are two major types of fractures: fragile and ductile (or tenacious).

Parallam – Parallel strand lumber (PSL)

Parallam© is a composite material obtained by the union of long and thin strips of adequately selected and dried wood, impregnated with water repellent resin, oriented and cured under high pressures, and with the help of microwave energy. The most commonly used wooden strips come from trees such as fir, pine, hemlock, and yellow poplar. […]


A transducer is a device that takes a useful signal in one form, say electrical, and converts it to another useful form, perhaps optical. Note that “sensors” and “actuators” are, in a more general sense “transducers,” but common usage restricts the meaning of the term.


An actuator is an element of a control system that will move another element, by providing a force, pressure or moment (a force acting through a lever arm) in response to a command signal.


A transponder is a component which, on receiving an ElectroMagnetic (EM) signal, often coded, will respond by sending a similar signal, usually after a known, controlled delay time.


Bioinformatics is the use of information technology for the study, collection, and storage of genomic and other biological data.


A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power.

Compression strength

Compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce the size, as opposed to tensile strength, which withstands loads tending to elongate. In other words, compressive strength resists compression (being pushed together), whereas tensile strength resists tension (being pulled apart). In the study of the strength of materials, […]


Spectrometers are devices for the precise measurement of energies or masses of the particles in a beam. These devices are used, for example, in the secondary beams to sort out the products of high-energy or nuclear reactions. Charged particles produced in these reactions are guided through a spectrometer to analyzing stations. Although no acceleration is performed […]


A semiconductor is a substance with an electrical conductivity between that of a conductor and an insulator — the conductivity of a semiconductor increases as temperature increases. Adding appropriate impurities also increases conductivity. Amorphous semiconductor An amorphous semiconductor also called “thin film“ is a non-crystalline semiconductor material that has no long-range order. Although easier and cheaper to make than […]

Ultimate tensile strength (UTS)

Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or Ftu within equations, is the capacity of a material or structure to withstand loads tending to elongate, as opposed to compressive strength, which withstands loads tending to reduce size. In other words, tensile strength resists tension (being pulled apart), whereas compressive strength resists compression (being pushed […]

Rigid body

A rigid body (also known as rigid object) is defined as an ideal (theoretical) material body such that the distances between assigned points of the same body remain always constant during motion. That is, each particle in a rigid body maintains defined the mutual distance with the other particles of the body itself, as well as being identified […]


Ductility is a technological property of matter that indicates the ability of a body or material to deform plastically under load before breaking, which may be expressed as percent elongation or percent area reduction from a tensile test — for example, the ability to withstand plastic deformations. A body is much more ductile, the higher the […]

Ground (or earth)

In an electrical circuit, there are always points with different electrical potentials. For convenience when studying an electrical system is considered a point in the system as a reference, for the same reason for which to measure the altitude of the earth’s crust is considered the sea level as the reference altitude. Below you can […]

Grashof number

The Grashof number is a dimensionless parameter which, unlike the Rayleigh number, contains information of a mechanical nature only. It contrasts the exclusive effects of viscosity with the floating force: \[Gr=\dfrac{F_g}{(F_v)^2}\] once the motion is triggered (Ra > 1708) to understand if the motion is laminar or turbulent (in natural convection) the Grashof number is evaluated; if […]

Rayleigh number

The Rayleigh number (Ra) is a dimensionless parameter that expresses the relationship between all the actions that positively contribute to motion in a fluid (floating force) and everything that is negatively opposed (viscosity and thermal diffusivity): \[Ra=\dfrac{F_g}{\mu\alpha}\] It is, therefore, possible to understand whether there is motion, or not, in conditions of natural convection; or if, due […]


The electric capacitance is a scalar physical quantity that quantifies the aptitude of a conductive material to accumulate electric charge when it has an electric potential with respect to the environment or is subject to an electric potential difference with respect to other conducting bodies. Capacitance is the ratio of the change in an electric charge […]


A capacitor consists of two conducting plates separated by an insulating layer called a dielectric. Capacitors often differ in the size and arrangements of plates as well as the type of dielectric materials used. When a capacitor is connected in a circuit, the power source’s voltage forces electrons onto the surface of one plate and pulls electrons […]

Electrical conductor

An electrical conductor is an object or type of material that allows the flow of charge (electrical current), with the application of voltage, in one or more directions. The conductive materials are characterized by the presence of free electrons in the valence band of the crystal lattice atoms (conductors of the first species) or contain ionic species that […]


Elastomer is defined as a macromolecular material (natural or synthetic polymer) which subjected to high deformation (stretching with elongation up to 5 or 10 times the initial length) within a specific temperature range after the applied stress removal returns to the shape and the original size. Chemical characteristics The main chemical characteristics of the elastomers are: […]

Thermoplastic elastomer

Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) which, above its melt temperature, exhibits a thermoplastic character that enables it to be shaped into a fabricated article and which, within it’s designed temperature range, possesses elastomeric behaviour without […]

Filter circuit

Circuits capable of selectively filtering one frequency or range of frequencies out of a mix of different frequencies in a circuit are called filter circuits, or filters. A common need for filter circuits is in high-performance stereo systems, where certain ranges of audio frequencies need to be amplified or suppressed for best sound quality and power efficiency. […]


A gyroscope is a device used for measuring or maintaining orientation and angular velocity; the gyroscope utilizes the angular momentum of a heavy spinning disk, called a rotor, to sense angular motion of its base about one or two axes at right angles to the spin axis. The rotor is mounted and moves within gimbals within a […]


The ampere (symbol: A), often shortened to “amp“, is the base unit of electric current in the International System of Units (SI) used to measure of the rate of electron flow or current in an electrical conductor. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. One ampere of current represents […]


The ampacity of a conductor, that is, the amount of current it can carry, is related to its electrical resistance: a lower-resistance conductor can carry a larger value of current. The ampacity of a conductor depends on its ability to dissipate heat without damage to the conductor or its insulation. This is a function of the insulation […]

Port (electric circuit theory)

In electrical circuit theory, a port is a pair of terminals (of an electrical element) connecting an electrical network or circuit to an external circuit, a point of entry or exit for electrical energy. A port consists of two nodes (terminals) connected to an outside circuit, that meets the port condition; the currents flowing into the two […]

Electric resistance

The electrical resistance is the opposition to the motion of the electrons through a conductor. It can be said that conductors have low resistance and insulators have very high resistance. This opposition to electric current depends on the type of material, its cross-sectional area, and its temperature. Resistance serves to limit the amount of current through the circuit […]


The term mechanism identifies an entity that has the constructive connotations, the mobility, the ability to transmit motion, force, energy, present in the definition of machine. The complex of several mechanisms connected together constitutes a machine. Mechanisms generally consist of moving components that can include: gears; belt and chain drives; cam and followers; linkage; friction devices, such […]


Biomass energy. Biofuel (biomass fuel). Gasification (biomass gasification).


The Gilsonite (also known as “uintahite,” “asphaltum” or asphaltite), named by Samuel Gilson, is a natural resinous hydrocarbon derived from petroleum historically discovered in the 1860s in Uintah Basin in northeastern Utah and also present in Colorado, Iran, and Colombia. Gilsonite is soluble in aromatic and aliphatic solvents, as well as in petroleum asphalt. Thanks to its […]

Passive device

Components incapable of controlling current by means of another electrical signal are called passive devices. Resistors, capacitors, inductors, transformers, and even diodes are all considered passive devices.

Active device

An active device is any type of circuit component with the ability to electrically control electron flow (electricity controlling electricity). In order for a circuit to be properly called electronic, it must contain at least one active device. Active devices include, but are not limited to, vacuum tubes, transistors, silicon-controlled rectifiers (SCRs), and TRIACs. A case might […]

Electrical element

An electrical element is a conceptual abstraction representing idealized electrical components, such as resistors, capacitors, and inductors, used in the analysis of electrical networks. All electrical networks can be analyzed as multiple electrical elements interconnected by wires.  Every electrical element has at least two terminals: an area of an electrical component that can receive the electric current […]

Electric circuit

An electric circuit is an unbroken loop of conductive material that allows electrons to flow through continuously without beginning or end. If a circuit is broken, that means it’s conductive elements no longer form a complete path, and continuous electron flow cannot occur in it. The location of a break in a circuit is irrelevant to its inability to […]


A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. Researches Bidirectional optical signal transmission between two identical devices using perovskite diodes. https://www.nature.com/articles/s41928-020-0382-3

International System of Units (SI)

The International System of Units universally abbreviated SI (from the French Le Système International d’Unités), is the modern metric system of measurement, used prevalently in science and international commerce. There are seven base units and symbols for the seven base quantities, which are assumed to be independent. These seven base units are essential for the […]

Gear (cogwheel)

Spur gear. Helical gear. Gear rack. Herringbone gear. Worm gear. Bevel gear. Hypoid gear. Gear errors.

Indentation hardness

Indentation hardness, used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on a macroscopic or microscopic scale.

Fracture toughness

Fracture toughness is a property which describes the ability of a material to resist fracture and is one of the essential properties of any material for many design applications.

Shear strength

Shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force. When […]

Linear Variable Differential Transformer (LVDT)

A linear variable differential transformer (LVDT) is one of the most popular electromechanical devices used to convert small mechanical displacements (of the order of a few millimeters or fractions of a millimeter) into amplified electrical signals. An LVDT provides an alternating current (AC) voltage output proportional to the relative displacement of a transformer core with respect to […]

Kinematic chain

When the kinematic pairs are coupled in such a way that the last link is joined to the first link to transmit definite motion (i.e. completely or successfully constrained motion), it is called a kinematic chain. The degrees of freedom, or mobility, of a kinematic chain is the number of parameters that define the configuration of […]

Kinematic pair

A kinematic pair is a connection between two physical objects that imposes constraints on their relative movement. In other words: two links or elements of a machine, when in contact with each other, are said to form a pair; if the relative motion between them is completely or successfully constrained (i.e. in a definite direction), the pair […]

Kinematic link

A kinematic link is defined as the part of the machine which has a relative motion with respect to some other part of the same machine is called kinematic link or element. Kinematic links can be divided into three types: Rigid link: in this type of link there is no deformation while transmitting the motion. The motion […]


In Metrology, fidelity is defined as the degree to which a measurement instrument indicates changes in the measurand quantity without dynamic error. In metrology, it is said that a measuring instrument is all the more “faithful” the more it provides indications of little discordant value between them in the course of several measurements of a constant physical quantity. […]

Range of interval (or span)

In metrology, the range of the interval \([a, b]\) is the difference \((b-a)\) and is denoted by \(r[a, b]\). It also represents a static characteristic of an instrument. It defines the maximum and minimum values of the inputs or the outputs for which the instrument is recommended to use. For example, for a temperature measuring instrument the […]


In metrology, stability represents a static characteristic of a measuring instrument and the ability of an instrument to retain its performance throughout is specified operating life. Zero stability is defined as the ability of an instrument to return to the zero reading after the input signal or measurand comes back to the zero value and other variations due to temperature, […]


In metrology, drift can be defined as the variation caused in the output of an instrument, which is not caused by any change in the input. Drift in a measuring instrument is mainly caused by internal temperature variations and lack of component stability. It also represents a static characteristic of an instrument. A change in the zero output of […]

Threshold (dead space)

In Metrology, dead space or threshold is a static characteristic of an instrument defined as the range of different input values over which there is no change in output value. If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the […]

Threshold of hearing

Sound level measurements in decibels are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity: \[I_0=10^{-12}\;\dfrac{\textrm{W}}{\textrm{m}^2}=10^{-16}\;\dfrac{\textrm{W}}{\textrm{cm}^2}\] or in terms of sound pressure: \[P_0=2\times 10^{-5}\;\dfrac{\textrm{N}}{\textrm{m}^2}\] This value has wide acceptance as a nominal standard threshold and corresponds to 0 decibels. It […]


In metrology, the resolution of a measuring instrument is the ability to detect the smallest change in the value of a physical property that an instrument can detect. It also represents a static characteristic of an instrument. The resolution of an instrument can also be defined as the minimum incremental value of the input signal that is required to cause […]


In metrology repeatability is a static characteristic of an instrument defined as the ability of an instrument to reproduce a group of measurements of the same measured quantity, made by the same observer, using the same instrument, under the same conditions. Specific claims of repeatability are based on multiple calibration tests (replication) performed within a given lab on the particular unit. The […]


In metrology, the term “reproducibility,” when reported in instrument specifications, refers to the closeness of agreement in results obtained from duplicate tests carried out under similar conditions of measurement. It is specified in terms of scale readings over a given period of time. It also represents a static characteristic of an instrument. As with repeatability, the uncertainty is […]


In metrology, linearity is actually a measure of the nonlinearity of the measurement instrument. When we talk about sensitivity, we assume that the input/output characteristic of the instrument to be approximately linear. But in practice, it is normally nonlinear, as shown in the figure below. The linearity is defined as the maximum deviation from the linear characteristics as a percentage of […]


In Metrology, the sensitivity of a measuring instrument is that metrological characteristic that provides information on the instrument’s ability to detect small variations in the input quantity; in other words: the increment of the output signal (or response) to the increment of the input measured signal. It can be defined also as the ratio of the incremental output and the incremental input. […]

Accuracy vs Precision

The difference between precision and accuracy needs to be understood carefully. Precision means repetition of successive readings, but it does not guarantee accuracy; successive readings may be close to each other, but far from the true value. On the other hand, an accurate instrument has to be precise also, since successive readings must be close to the true value […]


In metrology, precision indicates the repeatability or reproducibility of an instrument (but does not indicate accuracy). In other words is the degree of the repetitiveness of the measuring process of a quantity made by using the same method, under similar conditions. See also Accuracy vs Precision » The ability of the measuring instrument to repeat the same results during the act of measurements for […]


In error theory, accuracy is the degree of correspondence of the theoretical data, which can be inferred from a series of measured values (data sample), with the real or reference data, i.e. the difference between the average sample value and the true or reference. Indicates the proximity of the value found to the real one. It is a qualitative […]

Measuring instrument

A device used for the measurement of a certain physical quantity is called a measuring instrument. The instruments indicate the value of these quantities, based on which we get some understanding and also take appropriate actions and decisions. Types of measurement instruments There are two main types of measuring instruments: analog and digital. The analog instruments indicate the magnitude of […]


In metrology, a sensor is a sensitive device placed in contact with the physical quantity. It represents the compatible element and able to collect information from the physical quantity in the measurement environment. Each sensor is generally sensitive to several physical quantities, but its change of state is definitely more significant in relation to a specific quantity […]

Zero error

In metrology, zero error means the error that is made when long-term measurements are made, and it is verified that the zero of the measuring instrument undergoes a drift phenomenon, called zero drift. The zero error is evaluated in units of the quantity to be measured.

Reading error

In metrology, the reading error is that what happens when evaluating the relative position of the index of the measuring instrument with respect to the scale; this error is generally due to four causes: resolving power of the human eye: it is defined as the angle of minimum separation between two points that the eye is able […]

Rapidity error

In metrology, the error of rapidity is that metrological quality of a measuring instrument that expresses the ability to follow the (dynamic) variations in the time of the measurand; it is essential because it allows evaluating the limits within which a measuring instrument can be suitable for measuring variable quantities over time (dynamic quantities). Another practical definition […]

Mobility error

Mobility error is mainly due to the friction that develops between the mobile components of the measurement instrument and the inevitable spaces between them.