Calibration of Voltmeter:
Observations measured from voltmeter is errorful due to different reasons:
(i) In built mechanical error,
(ii) Parallex error,
(iii) Unsymmetricity in the spring of voltmeter etc. Potentiometer can increase precise value of potential difference.
Checking the errors of observations of voltmeter by potentiometer is known as Calibration of Voltmeter.
In figure necessary circuit diagram is shown for calibration of voltmeter. Primary circuit is usually connected as section before.
In secondary circuit positive terminal of standard cell is connected at point A (high potential end) while negative terminal is connected with the terminal 1 of two way key. In this circuit other cell ε, rheostat Rh2, key K2 and resistance box (R.B.) is connected in series as shown in figure. The high potential end of R.B. is connected at A while lower potential end is connected at the terminal 3 of two way key. The voltmeter which is to be calibrated should be connected across the ends of R.B. The middle terminal of two way key is connected with the one end of galvanometer (G) and other end of (G) is connected with jockey.
Working: Firstly we completed the primary circuit and insert the plug between terminal 1 and 2 and by the help of jockey finding balancing length l0for the standard cell εs
\(\varepsilon_{s}=k l_{0} \quad \text { or } \quad k=\frac{\varepsilon_{s}}{l_{0}}\) …………….. (1)
k is potential gradient.
By the standardisation of cell, then find out k.
Now eliminate the plug between 1 and 2 and insert it between 2 and 3. Now key K2 be closed and suitable resistance out from R.B. By the help of Rh2 necessary current pass through the resistance and takes the deflection in voltmeter, which is V. It is errorful observation, corresponding to V. Find out the real value of potential difference by potentiometer. Find the balancing length l2. Now by the principle of potentiometer the real value of potential difference
V’ = kl2
V’ = \(\left(\frac{\varepsilon_{s}}{l_{0}}\right)\)l2 ………… (2)
Error in the observation of voltmeter
ΔV = V – V’
By the help of R.B. and Rh2, observe different values of voltmeter and for it find corresponding values of potential difference by potentiometer. Now taking difference of V (observations of voltmeter) and V’ (potential difference by potentiometer) we can find error in it
ΔV = V – V’
Calibration Curve: A graph is plotted between the readings of voltmeter and the corresponding errors. This graph is called calibration curve of the voltmeter. The graph has been no definite shape. The any graph of calibration is shown in figure.
