DIFFERENCE BETWEEN AMMETER AND VOLTMETER AND IT'S WORKING

AMMETER AND VOLTMETER

Ammeter is a device to measure an electric current and voltmeter is a device to measure a potential difference. In both the instrument there is a coil, suspended between the poles of a magnet. When a current is passed through the coil, it deflects. The angle of deflection is proportional to the current going through the coil. A needle is fixed to coil.  when the coil deflects, the needle moves on a graduated scale.

Ammeter

In an ammeter, a resistor having a small resistance is connected in parallel with the coil. This resistor is called the shunt. The current to be measured is passed through the ammeter by connecting it in series with the segment which carries the current. Plus and minus signs are marked near the terminals of the ammeter. The current should enter the ammeter through the terminal marked "plus". When no current passes through the ammeter, the needle stays at zero which is marked at the left extreme of the scale.

figure1.1

Suppose the coil has a resistance Rc and the small resistance connected in parallel (shunt) has a value r. When a current i is sent through the ammeter, the current gets divided in two parts. A part i1 goes through the coil and the rest, i - i1, through the shunt. As the potential difference across Rc  is the same as that across r,

i1Rc = (i - i1)r

or,

   i1 = R/Rc + r  i.

The deflection is proportional to i1 and hence to i. The scale is graduated to read the value of i directly. The equivalent resistance of an ammeter is given

Req = Rcr/ Rc + r 

When the ammeter is connected in a segment of a circuit the resistance of the segment increases by this amount Req. This reduces the main current which we wish to measure. To minimise this error, the equivalent resistant Req should be small. This is one reason why the shunt having a small resistance r is connected in parallel to the coil. This makes Req small.

Galvanometer is very similar to an ammeter in construction. When no current passes through it, the needle stays in the middle of the graduated scale. this point is marked zero. Current can be passed through the galvanometer in either direction. the needle deflects accordingly towards left or towards right.

Voltmeter

in a voltmeter, a resistor having a high resistance R is connected in series with the coil. The end points (terminals) are connected to the points A and B between which the potential difference is to be measured. plus and minus signs are marked on the terminals. The terminal marked "plus" should be connected to the point at higher potential. when no potential difference is applied between the terminals, the needle stays at zero which is marked at the left extreme of the scale. when the potential difference is applied, a current passes through the coil and the high resistance. If Rc be resistane of the coil and V be the potential difference applied to the voltmeter, the current in the coil is

i = V/ Rc +R 

The deflection is proportional to the current i and hence to V. The scale is graduated to read the potential difference directly.

figure1.2

When the voltmeter is used in a circuit, its resistance Req = Rc + R is connected in parallel to some element of the circuit. This changes the overall current in the circuit and hence, the potential difference to measured is also changed. To minimise the error due to this, the equivalent resistance Req of the voltmeter should be large. (When a large resistance is connected in parallel to a small resistance, the equivalent resistance is only slightly less than the smaller one.) That is why, a large resistance R is added in series with the coil of a voltmeter.