An ohm meter measures the resistance placed between its leads. This resistance reading is indicated by a galvanometer that operates on current. The ohm meter has an internal source of voltage to create the necessary current to operate the galvanometer and also has approproate resistor to allow just right amount of current through galvanometer. A simple ohm meter is shown here.

When there is an infinite resistance, there is zero current through galvanometer and it points in middle. If the test leads of this meter are directly shorted, (zero resistance) galvanometer will give full deflection.
Galvanometer specification
Resistance of galvanometer `= 100 Omega`
Fullscale current `= 0.5 mA`
Total number of division `=40` .

Options

A. `20 k Omega`
B. `19.9 k Omega`
C. `19 k Omega`
D. `9 k Omega`

Answer

B

Explanation

b. When ends of ohm meter are shorted, the ohm meter will show zero
reading at that time the galvanometer will show full scale deflection.
Full scale current `i_g = 0.5 xx 10^(-5) A`
`i_g = 10/(G+R) or 0.5 xx 10^(-3) = 10/(100+R)` ,brgt or `(100+R) = (10/(0.5 xx 10^(-3))) = 20000` ,brgt or `R = 20000 - 100 = 1990 Omega = 19.9 kOmega`
If needle deflect to middle of scale the current through the galvanometer
will be `(1//2) xx 0.5 mA i.e., 0.25 mA. `
Hence `0.25 xx 10^(-3) = 10/(100+19900+R')`
`20000 +R' = 10/(0.25 xx 10^(-3)) = 40000`
or `R' = 20000 = 20kOmega`
If `5Omega` is measured with this ohm meter, the needle will be near `0 Omega`
mark, hence the needle will be between C and zero.