Series Ohmmeter
If the resistor’s value is unknown and has to be measured by placing it in series with the ohmmeter, then that ohmmeter is called series ohmmeter. The circuit diagram of series ohmmeter is shown in below figure.
The part of the circuit, which is left side of the terminals A & B is series ohmmeter. So, we can measure the value of unknown resistance by placing it to the right side of terminals A & B. Now, let us discuss about the calibration scale of series ohmmeter.
If Rx=0ΩRx=0Ω, then the terminals A & B will be short circuited with each other. So, the meter current gets divided between the resistors, R1 and R2. Now, vary the value of resistor, R2 in such a way that the entire meter current flows through the resistor, R1only. In this case, the meter shows full scale deflection current. Hence, this full scale deflection current of the meter can be represented as 0Ω.
If Rx=∞Ω Rx=∞Ω, then the terminals A & B will be open circuited with each other.
So, no current flows through resistor, R1. In this case, the meter shows null deflection current. Hence, this null deflection of the meter can be represented as ∞Ω.
In this way, by considering different values of Rx, the meter shows different deflections. So, accordingly we can represent those deflections with the
corresponding resistance value.
The series ohmmeter consists of a calibration scale. It has the indications of 0 Ω and ∞Ω at the end points of right hand and left hand of the scale respectively. Series ohmmeter is useful for measuring high values of resistances.