Open circuit armature voltage of dc machine?

[theone]

Homework Statement

The book says that "the induced voltage E_a can be found by measuring the open circuit voltage at the armature terminals"

Homework Equations

The Attempt at a Solution

But isn't there a voltage drop across the resistor so that they can't be equal? or is this ignored

 

[andrevdh]

There will only be a voltage drop over the resistor if current
is drawn from the circuit. The voltmeter draws very little current
so no voltage is lost over the resistor and the full induced voltage
is measured. If the current is zero then the IR product is zero.