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to a low voltage coefficient they are also beneficial due to long term stability 12 metal oxide film metal oxide film resistors are made of metal oxides which results in a higher operating temperature and greater stability and reliability than metal film they are used in applications with high endurance demands wire wound high power wire wound resistors used for dynamic braking on an electric railway car such resistors may dissipate many kilowatts for an extended length of time types of windings in wire resistors common bifilar common on a thin former ayrton perry wirewound resistors are commonly made by winding a metal wire usually nichrome around a ceramic plastic or fiberglass core the ends of the wire are soldered or welded to two caps or rings attached to the ends of the core the assembly is protected with a layer of paint molded plastic or an enamel coating baked at high temperature these resistors are designed to withstand unusually high temperatures of up to 450 c 12 wire leads in low power wirewound resistors are usually between 0 6 and 0 8 mm in diameter and tinned for ease of soldering for higher power wirewound resistors either a ceramic outer case or an aluminum outer case on top of an insulating layer is used if the outer case is ceramic such resistors are sometimes described as cement resistors though they do not actually contain any traditional cement the aluminum cased types are designed to be attached to a heat sink to dissipate the heat the rated power is dependent on being used with a suitable heat sink e g a 50 w power rated resistor overheats at a fraction of the power dissipation if not used with a heat sink large wirewound resistors may be rated for 1 000 watts or more because wirewound resistors are coils they have more undesirable inductance than other types of resistor however winding the wire in sections with alternately reversed direction can minimize inductance other techniques employ bifilar winding or a flat thin former to reduce cross section area of the coil for the most demanding circuits resistors with ayrton perry winding are used applications of wirewound resistors are similar to those of composition resistors with the exception of high frequency applications the high frequency response of wirewound resistors is substantially worse than that of a composition resistor 12 metal foil resistor metal foil resistor in 1960 felix zandman and sidney j stein 19 presented a development of resistor film of very high stability the primary resistance element of a foil resistor is a chromium nickel alloy foil several micrometers thick chromium nickel alloys are characterized by having a large electrical resistance about 58 times that of copper a small temperature coefficient and high resistance to oxidation examples are chromel a and nichrome v whose typical composition is 80 ni and 20 cr with a melting point of 1420 c when iron is added the chromium nickel alloy becomes more ductile the nichrome and chromel c are examples of an alloy containing iron the composition typical of nichrome is 60 ni 12 cr 26 fe 2 mn and chromel c 64 ni 11 cr fe 25 the melting temperature of these alloys are 1350 c and 1390 c respectively 20 full citation needed since their introduction in the 1960s foil resistors have had the best precision and stability of any resistor available one of the important parameters of stability is the temperature coefficient of resistance tcr the tcr of foil resistors is extremely low and has been further improved over the years one range of ultra precision foil resistors offers a tcr of 0 14 ppm c tolerance 0 005 long term stability 1 year 25 ppm 3 years 50 ppm further improved 5 fold by hermetic sealing stability under load 2000 hours 0 03 thermal emf 0 1 μv c noise 42 db voltage coefficient 0 1 ppm v inductance 0 08 μh capacitance 0 5 pf 21 the thermal stability of this type of resistor also has to do with the opposing effects of the metal s electrical resistance increasing with temperature and being reduced by thermal expansion leading to an increase in thickness of the foil whose other dimensions are constrained by a ceramic substrate 22 ammeter shunts an ammeter shunt is a special type of current sensing resistor having four terminals and a value in milliohms or even micro ohms current measuring instruments by themselves can usually accept only limited currents to measure high currents the current passes through the shunt across which the voltage drop is measured and interpreted as current a typical shunt consists of two solid metal blocks sometimes brass mounted on an insulating base between the blocks and soldered or brazed to them are one or more strips of low temperature coefficient of resistance tcr manganin alloy large bolts threaded into the blocks make the current connections while much smaller screws provide volt meter connections shunts are rated by full scale current and often have a voltage drop of 50 mv at rated current such meters are adapted to the shunt full current rating by using an appropriately marked dial face no change need to be made to the other parts of the meter grid resistor in heavy duty industrial high current applications a grid resistor is a large convection cooled lattice of stamped metal alloy strips connected in rows between two electrodes such industrial grade resistors can be as large as a refrigerator some designs can handle over 500 amperes of current with a range of resistances extending lower than 0 04 ohms they are used in applications such as dynamic braking and load banking for locomotives and trams neutral grounding for industrial ac distribution control loads for cranes and heavy equipment load testing of generators and harmonic filtering for electric substations 23 the term grid resistor is sometimes used to describe a resistor of any type connected to the control grid of a vacuum tube this is not a resistor technology it is an electronic circuit topology special varieties cermet phenolic tantalum water resistor variable resistors main article list of resistors adjustable resistors a resistor may have one or more fixed tapping points so that the resistance can be changed by moving the connecting wires to different terminals some wirewound power resistors have a tapping point that can slide along the resistance element allowing a larger or smaller part of the resistance to be used where continuous adjustment of the resistance value during operation of equipment is required the sliding resistance tap can be connected to a knob accessible to an operator such a device is called a rheostat and has two terminals potentiometers potentiometer with case cut away showing parts a shaft b stationary carbon composition resistance element c phosphor bronze wiper d shaft attached to wiper e g terminals connected to ends of resistance element f terminal connected to wiper a potentiometer colloquially pot is a three terminal resistor with a continuously adjustable tapping point controlled by rotation of a shaft or knob or by a linear slider 24 the name potentiometer comes from its function as an adjustable voltage divider to provide a variable potential at the terminal connected to the tapping point volume control in an audio device is a common application of a potentiometer a typical low power potentiometer see drawing is constructed of a flat resistance element b of carbon composition metal film or conductive plastic with a springy phosphor bronze wiper contact c which moves along the surface an alternate construction is resistance wire wound on a form with the wiper sliding axially along the coil 24 these have lower resolution since as the wiper moves the resistance changes in steps equal to the resistance of a single turn 24 high resolution multiturn potentiometers are used in precision applications these have wire wound resistance elements typically wound on a helical mandrel with the wiper moving on a helical track as the control is turned making continuous contact with the wire some include a conductive plastic resistance coating over the wire to improve resolution these typically offer ten turns of their shafts to cover their full range they are usually set with dials that include a simple turns counter and a graduated dial and can typically achieve three digit resolution electronic analog computers used them in quantity for setting coefficients and delayed sweep oscilloscopes of recent decades included one on their panels typical panel mount potentiometer an assortment of small through hole potentiometers designed for mounting on printed circuit boards resistance decade boxes main article decade box resistance decade box a resistance decade box or resistor substitution box is a unit containing resistors of many values with one or more mechanical switches which allow any one of various discrete resistances offered by the box to be dialed in usually the resistance is accurate to high precision ranging from laboratory calibration grade accuracy of 20 parts per million to field grade at 1 inexpensive boxes with lesser accuracy are also available all types offer a convenient way of selecting and quickly changing a resistance in laboratory experimental and development work without needing to attach resistors one by one or even stock each value the range of resistance provided the maximum resolution and the accuracy characterize the box for example one box offers resistances from 0 to 100 megohms maximum resolution 0 1 ohm accuracy 0 1 25 special devices there are various devices whose resistance changes with various quantities the resistance of ntc thermistors exhibit a strong negative temperature coefficient making them useful for measuring temperatures since their resistance can be large until they are allowed to heat up due to the passage of current they are also commonly used to prevent excessive current surges when equipment is powered on similarly the resistance of a humistor varies with humidity one sort of photodetector the photoresistor has a resistance which varies with illumination the strain gauge invented by edward e simmons and arthur c ruge in 1938 is a type of resistor that changes value with applied strain a single resistor may be used or a pair half bridge or four resistors connected in a wheatstone bridge configuration the strain resistor is bonded with adhesive to an object that is subjected to mechanical strain with the strain gauge and a filter amplifier and analog digital converter the strain on an object can be measured a related but more recent invention uses a quantum tunnelling composite to sense mechanical stress it passes a current whose magnitude can vary by a factor of 10 12 in response to changes in applied pressure measurement the value of a resistor can be measured with an ohmmeter which may be one function of a multimeter usually probes on the ends of test leads connect to the resistor a simple ohmmeter may apply a voltage from a battery across the unknown resistor with an internal resistor of a known value in series producing a current which drives a meter movement the current in accordance with ohm s law is inversely proportional to the sum of the internal resistance and the resistor being tested resulting in an analog meter scale which is very non linear calibrated from infinity to 0 ohms a digital multimeter using active electronics may instead pass a specified current through the test resistance the voltage generated across the test resistance in that case is linearly proportional to its resistance which is measured and displayed in either case the low resistance ranges of the meter pass much more current through the test leads than do high resistance ranges this allows for the voltages present to be at reasonable levels generally below 10 volts but still measurable measuring low value resistors such as fractional ohm resistors with acceptable accuracy requires four terminal connections one pair of terminals applies a known calibrated current to the resistor while the other pair senses the voltage drop across the resistor some laboratory quality ohmmeters milliohmmeters and even some of the better digital multimeters sense using four input terminals for this purpose which may be used with special test leads called kelvin clips each of the two clips has a pair of jaws insulated from each other one side of each clip applies the measuring current while the other connections are only to sense the voltage drop the resistance is again calculated using ohm s law as the measured voltage divided by the applied current standards production resistors resistor characteristics are quantified and reported using various national standards in the us mil std 202 26 contains the relevant test methods to which other standards refer there are various standards specifying properties of resistors for use in equipment iec 60062 iec 62 din 40825 bs 1852 is 8186 jis c 5062 etc resistor color code rkm code date code eia rs 279 din 41429 resistor color code iec 60063 iec 63 jis c 5063 standard e series values mil prf 26 mil prf 39007 fixed power established reliability mil prf 55342 surface mount thick and thin film mil prf 914 mil r 11 standard canceled mil r 39017 fixed general purpose established reliability mil prf 32159 zero ohm jumpers ul 1412 fusing and temperature limited resistors 27 there are other united states military procurement mil r standards resistance standards the primary standard for resistance the mercury ohm was initially defined in 1884 in as a column of mercury 106 3 cm long and 1 square millimeter in cross section at 0 degrees celsius difficulties in precisely measuring the physical constants to replicate this standard result in variations of as much as 30 ppm from 1900 the mercury ohm was replaced with a precision machined plate of manganin 28 since 1990 the international resistance standard has been based on the quantized hall effect discovered by klaus von klitzing for which he won the nobel prize in physics in 1985 29 resistors of extremely high precision are manufactured for calibration and laboratory use they may have four terminals using one pair to carry an operating current and the other pair to measure the voltage drop this eliminates errors caused by voltage drops across the lead resistances because no charge flows through voltage sensing leads it is important in small value resistors 100 0 0001 ohm where lead resistance is significant or even comparable with respect to resistance standard value 30 resistor marking main article electronic color code wheel based rma resistor color code guide circa 1945 1950 axial resistor cases are usually tan brown blue or green though other colors are occasionally found as well such as dark red or dark gray and display three to six colored stripes that indicate resistance and by extension tolerance and may include bands to indicate the temperature coefficient and reliability class in four striped resistors the first two stripes represent the first 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