It should be particularly noted that, in the potentiometer method of
measurement of an EMF, there is no current being drawn from this EMF when
balance is reached.
Thus, the measurement of a source of voltage is
completely independent of its internal resistance.
Also of particular
significance is the fact that the potentiometer operates on a ratio
The EMF to be measured is determined from the known standard
cell voltage by the ratio of the resistances which are established to
achieve the two conditions of balance.
Therefore, the accuracy of the
potentiometer is more dependent upon the correct relationship of the
resistors used than the actual values of resistance employed.
Learning Event 5:
DESCRIBE THE VOLT BOX
For measurement of voltages above the upper potentiometer voltage
limit, it is customary to use another ratio device called a "volt box" or
voltage divider, (fig 1-5).