Comprehension (Q. 15 to 17): A simple ohmmeter consists of a battery connected in series with a galvanometer and a resistor, as shown in figure. The resistance \(R_b\) is chosen such that when the terminals \(a\) and \(b\) are shorted (put in electrical contact, with negligible resistance between them), the current through the galvanometer gives a full-scale deflection Thus, a full scale deflection indicates no resistance between terminals \(a\) and \(b\). A zero deflection indicates an infinite resistance between the terminals. When the terminals are connected across an unknown resistance \(R\), the current through the galvanometer depends on \(R\), so the scale can be calibrated to give a direct reading of \(R\), as shown in figure. Because an ohmmeter sends a current through the resistance to be measured, some caution must be exercised when using this instrument. For example, you would not want to try to measure the resistance of sensitive ammeter with an ohmmeter, because the current provided by the battery in the ohmmeter would probably damage the ammeter. Let us use a galvanometer with resistance of \(20 \Omega\) and maximum current of \(10 \mathrm{~mA}, \varepsilon=1 \mathrm{~V}\). For a current more than \(10 \mathrm{~mA}\), galvanometer would be damaged.
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