POTENTIOMETER
Potentiometer
Potentiometer is an instrument used for
measuring potential difference or
electromotive force (e.m.f) by performing
a comparison with a known voltage.
It work by varying the position of sliding
contact across a uniform resistance.
It has a long resistance wire AB of length L
( 6m to 10m long)
HOW IT WORKS
Works by moving sliding contact across the resistive element(uniform
resistance).
The position of a slider determine the output voltage so it is
compared by the known voltage.
NOTE
o The resistive element is a long wire consisting of materials like
german silver , constantan.
OUTPUT VOLTAGE
The position of the sliding contact that is the place of getting the
output voltage.
OUTPUT VOLTAGE
As the sliding contact moves to the higher voltage end the output
voltage increase and if it moves on the lower voltage end the output
voltage decrease.
CONSTRUCTION AND WORKING PRINCIPLES OF A POTENTIOMETER
COMPONENTS OF A
POTENTIOMETER
A wire material (resistive element).
* made by; -silver
-constantan
These materials have uniform cross sectional area , resistant to
temperature change.
Wooden board
The resistive element is mounted to the board.
Auxillary circuit
1. power supply
It powers the potentiometer wire connected between terminals A and
terminals B.
COMPONENTS OF A
POTENTIOMETER
Rheostat
It controls the current flowing through the potentiometer wire.
Switch 1
Activate or deactivate the circuit.
Ammeter
It measures the current in the circuit.
Secondary circuit
It contain the known voltage to calibrate the potentiometer.
COMPONENTS OF A
POTENTIOMETER
Unknown source(vx)
the unknown voltage to be measured
Galvanometer
detect if there is no current flow,indicate voltage balance
Resistor
protect the standard cell from excessive current
Switch 2
select between standard and unknown source
WORKING PRINCIPLES
Setting the sliding contact
The sliding contact is placed at the midpoint of the potentiometer wire.
Activate the auxillary circuit
You switch on the switch 1 to make the current flow in the potentiometer
wire.
Adjust the current
Use the rheostat to control the flow of current ensuring constant current
flows through the wire .
Calibration
The pontetiometer is calibrated by connecting the standard cell to the
galvanometer throught switch 2.
ABOUT CALIBRATION
Calibration- is the process of setting up the pontetiometer to ensure it
can measure unknown voltages accurately.
To determine the relationship between the length of the wire and
voltage drops across it.
In calibration you move the sliding contact ,so you look at the movement
of the galvanometer till at the point where the galvanometer shows no
deflection that is the point where the voltage across the wire is equal to
the voltage of the standard cell ,so you measure the length of the wire.
This helps us to find the unknown source voltage, to determine the
voltage per unit length.
voltage per unit length=Vs/L
Where by;
Vs-standard cell voltage (known voltage)
L-the length measured.
So to get the unknown voltage you connect the unknown source to the
standard cell and then look at the deflection of the galvanometer ,so you
move the sliding contact till the point where the galvanometer shows no
deflection and measure the length.
Unknown voltage = voltage per unit length *the length 2
CONSTRUCTION OF A LABORATORY TYPE PONTETIOMETER
