Potentiometer..when you hear the word ..what comes in your mind ..yes .. potential and meter ..means the meter that measures potential ..yes it is used to measure the potential as well as used for comparing the e.m.f. of different cells and for calibration and standarizing voltmeters, Ammeters.
Potentiometer are of two types ..
- D.C. Potentiometer
- A.C. Potentiometer
Now first we discuss about Slide wire DC Potentiometer.
Construction of D.C. Slide Wire Potentiometer
Look at the circuit diagram above, what you see ..
The following components you see
- A switch S1
- A battery B
- A rheostat Rh
- A one meter long wire made up of maganin or German silver
- Two cells E1 and E2
- A doouble pole double throw Switch S2 for connecting E1 and E2 to potentiometer
- A Galvanometer
Two cells E1 and E2 are taken ..if we know the e.m.f. of one cell , the emf of other cell can be find out by potentiometer or if we donot know emf of both the cells, than we can compare emf of both the cells …how ??
Below is the equation of potentiometer.
- E1 and E2 you already know , the emf of cell E1 and E2 respectively in volts.
- L1 and L2 is the reading of potentiometer ..L means length of the magnin wire(discussed later)
Now suppose L1 is 300 mm , L2 is 600 mm.
Than E1/E2= 300/600= 1/2
Means E1= E2/2 means E1 cell emf is half of the emf of cell E2.
Now suppose we know E1 emf ..9 volts ..than E2 cell emf will ve 18 volts ..
So you have seen above , with the help of potentiometer , you can find comaparsion of emfs of two cell and also find out exactly the emf of a cell if one cell emf is known.
But you will ask, how to get these L1 and L2 readings of magnin wire ..this is discused below in working ..
Working of Slide wire DC potentiometer:
The magnin wire is of 1 meter or 1000 mm.. and potentiometer reading L1 and L2 means we will read or take certain length of this wire during reading ..read below now
Under normal conditions, when the switch S2 remains open. So what is in the circuit , we can draw the circuit as below ..
When Switch S1 is closed than current flows driven by battery voltage and causes a uniform voltage drop..and this voltage drop is maximum at point A and falls as we approach or comes near to point C.
Now Cell E1 is connected with Potentiometer wire through Switch S2 and galvanometer as shown below figure. The Galvanometer contact with Potentiometer wire is movable and we can move through the length of the wire.
When we move the Galvanometer contact through the wire, we find a position correponds to point D where Galvanometer gives no deflection and current through Galvanometer is zero or we can say cell E1 voltage is equal to the voltage drop along AD.
The AD is L1 reading of potentiometer ..
Now we have to find out L2 ..same ..now connect cell E2 to potentiometer wire with switch S2 as shown below and move position of the Galvanometer across the potentiometer wire until we find zero deflection in Galvanometer..now we find such a position E.
Now AE is the reading L2.
These points D and E when no current flows through Galvanometer is known as null or balance point.