Question
Download Solution PDFWhich category of instruments does an ohmmeter fall under?
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFConcept:
Ohmmeter:
A device that measures the resistance directly is called an ohmmeter.
The simplest direct reading ohmmeter is the basic series ohmmeter circuit shown in Fig. below (A and B are the terminals of ohmmeter).
It consists of a permanent magnet moving coil (PMMC) instrument in series with a battery and a rheostat R.
(i) With terminals A and B shorted together, R is adjusted for full-scale deflection (FSD).
Since terminals A and B are shorted, the ohmmeter should read zero resistance.
FSD Current (Ig) = \(\frac{E}{R}\)
(ii) When terminals A and B are open-circuited, the pointer should indicate infinity. Therefore, zero deflection point on the scale is marked as infinite resistance.
(iii) When an unknown resistance Rx is connected to terminals A and B, the meter current Im is
\(I_m=\frac{E}{R+R_X}\)
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