Page 818 - Basic Electrical Engineering

Current drawn by the voltmeter,

Actual value

Percentage error

It is seen that when we use a voltmeter of higher input impedance, the
measurement error gets reduced considerably. Thus, we can conclude that

while measuring a high value of resistance by the voltmeter–ammeter
method, the resistance of the voltmeter should be very very high. Otherwise

there will be considerable measurement error due to the loading effect of the
voltmeter. In the ideal case the voltmeter used should not draw any current

while measuring the voltage across the terminals.
Carelessness in taking readings and their recording, incorrect adjustment of

the instrument, incorrect choice of instruments, etc. are attributed to gross
error. These errors can be avoided by a careful choice of instruments and
recording of measured data. Normally, a number of readings are taken instead

of depending upon one or two readings.
Systematic errors are due to instrumental error or due to the effect of

environment on measurement.
Random errors are due to unknown causes. While measuring some

quantity through an instrument we may find that the readings vary for reasons
not known to us. Such variations are not attributed to reasons mentioned

under gross error and systematic error. While we are particular about the
accuracy of measurement, we make corrections in the measured value by
using statistical methods of calculations. For this purpose a large number of