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Potentiometer
used to measure the potential difference between two terminal and comparing the emf between two cells .
CHARACTERISTICS
Measure the unknown voltage by comparing it with a known voltage sources
Measures using null or balance condition
BASIC PRINCIPLES
Have connected two batteries in parallel through galvanometer
Negative and positives terminals are connected to same terminal through galvanometer
If there are the differences between battery cells, there will be a circulating current and causes the galvanometer to deflect
In potentiometer, a relative high resistance wire connected in series with voltage source.
A steady current in this wire produces a uniform voltage drop along its length.
The voltage drop between the movable contact x and Y is directly proportional to length of wire.
The voltage drop across a unit length of the resistance wire depends on the value of circuit current.
Type of Potentiometer
SLIDE-WIRE
The potential dropped across a segment of wire of uniform cross-section carrying a constant current is directly proportional to its length.
Calibration method : determines the relationship between the analytical response from an instrument and the analyte concentration.
Unknown measuring method : comparing the unknown resistance with the standard resistance.
SINGLE RANGE DIAL
The variations of the slide wire potentiometer and has differences at the arrangement of wire and resistor.
The reading can be taken only in a short time.
Calibration method : setting the direction needle of the main dial and slide wire dial to the standard voltage value.
Unknown measuring method : the measured value of Ex is recorded by adding the value read from the main dial and the slide wire dial.
VOLT BOX
Used to expand the voltage measurement range of basic potentiometer circuit.
It uses voltage divider concept to reduce the large unknown voltage Vinput.
A proper range of resistance must be chosen so that the output voltage from the volt box is not more than the measurement range of potentiometer.
The resistance value must be added respectively for every range.
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