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| A '''galvanostat''', (also known as '''amperostat''') is a [[control system|control]] and [[measuring]] device capable of keeping the [[Electric current|current]] through an [[electrolytic cell]] in [[coulometric]] [[titrations]] constant, disregarding changes in the load itself.
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| Its main feature is its ''nearly'' "''infinite''" (i.e. extremely high respect to common loads) internal [[electrical resistance|resistance]].
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| The designation "''galvanostat''" is mainly used in [[electrochemistry]]: this device differs from common [[current source|constant current sources]] by its ability to supply ''and [[measurements|measure]]'' a wide range of [[Electric current|current]]s (from [[picoampere]]s to [[amperes]]) of [[Polarity (physics)|both polarities]]. | |
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| The galvanostat responds to changes in the [[Electrical resistance|resistance]] of the cell by varying its output potential: as [[Ohm's law]] shows,
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| :<math>{R} = {U \over I}</math>
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| the variable system resistance and the controlled [[voltage]] are [[Directly proportional#Definition|directly proportional]], i.e.
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| :<math> U_c ={R_v \times I_o}</math>
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| where
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| *''<math>I_o</math>'' is the electric current that is kept constant
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| *''<math>U_c</math>'' is the output control voltage of the amperostat
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| *''<math>R_v</math>'' is the electrical resistance that varies;
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| thus, an increase of the load resistance implies an increase of the voltage the amperostat applies to the load. This device differs from common constant-current sources by its ability to supply and [[Measurement|measure]] a wide range of currents (from [[picoampere]]s to [[ampere]]s) of both polarities
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| == Technical realization ==
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| The simpler galvanostat consists of a high-voltage source producing a constant voltage ''<math>U</math>'' with a resistor ''<math>R_{x}</math>'' connected in series: in order to force an almost constant current through a load, this resistor shall be much higher than the load resistor <math>R_{load}</math>. As a matter of fact, the current ''<math>I</math>'' through the load is given by
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| :<math> I = \frac{U}{R_x + R_{load}} </math>
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| and if <math>R_x</math>>><math>R_{load}</math>, the current ''<math>I</math>'' is approximately determined by ''<math>R_x</math>'' as follows
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| :<math>I \cong \frac{U}{R_x}</math>
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| This simple realization requires rather high voltages (~100 V) to keep the load [[Electric current|current]] constant with sufficient approximation for all practical purposes. Therefore, more complex versions of galvanostats, using electronic amplifiers with [[Electronic feedback loops|feedback]] and lower voltages, have been developed and produced. These instruments are capable to feed constant currents in the ranges from few [[picoampere]]s (pA) to several [[ampere]]s (A); typical construction for use in the lower range of feed currents uses [[operational amplifier]]s.
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| == See also ==
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| *[[Current source]]
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| *[[Potentiostat]]
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| == External links ==
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| *Pierre R. Roberge (Webmaster) "''[http://www.corrosion-doctors.org/Instrumentation/galvanostat.htm Galvanostat]''", corrosion-doctors.org [http://www.corrosion-doctors.org/Principles/Glossary.htm Electrochemistry Dictionary].
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| [[Category:Electroanalytical chemistry devices]]
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