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) 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 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, …
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 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 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 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 …
The accelerometers realized with MEMS technology (Micro-Electro-Mechanic System) are nothing more than miniaturized accelerometers based on a mobile micromechanical structure, realized by engraving a silicon substrate with standard photolithographic methods. The main advantages of these devices are the low production cost (in particular for two or three measurement axes) and the presence inside them of …
The laser accelerometer is a particular type of accelerometer family, used when it is necessary to carry out extremely precise measurements that cannot be obtained with other types of instruments. The operating principle is based on the physical principle that acceleration is a derivative of speed over time. In this device, a laser interferometer measures instant by …