The rest positions of the needles in a milliammeter and voltmeter not in use are as shown in Fig. A. When a student uses these in his experiment, the readings of the needle are in the positions shown in Fig. B.


Find the corrected values of current and voltage in the experiment.


OR


While experimentally verifying Ohm’s law a student observed that the pointer of the voltmeter coincides with 15thdivision when the voltmeter has a least count of 0.05 V. Find the observed reading of voltmeter.

Standard minimum values for ammeter and voltmeter readings are taken as 0. When not in use, if the readings show anything else then we can conclude that it has zero error. Zero error is how far from zero, the reading is of the instrument. It can either be positive or negative zero error.


Therefore we see that the zero error in the ammeter is negative zero error while that of the voltmeter is positive zero error.


Hence zero error in ammeter: -4milliAmpere


Zero error in voltmeter: +0.4 milliVolt


So, the final reading as seen is:


Ammeter: 34mA


Voltmeter: 3.6 V


Now correcting the reading:


Ammeter: [34-(-4)]mA= 38 mA


Voltmeter:[3.6- (+0.4)]V = 3.2 V


OR


Least Count = smallest value measured by a particular measuring instrument (given 0.05V)


Therefore correct reading: Least count × Number of divisions in the instrument


Here :


(0.05 × 15)V = 0.075 V (the observed reading of voltmeter)


27