Objective 4-Be able to determine the reading of ammeters and voltmetersa) Find the current...

Objective 4-Be able to determine the reading of ammeters and voltmeters

a) Find the current in the circuit shown.

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b) If the ammeter in Example 1 is used to measure the current, what will it read?

Example 1

A 50 mA, 1 mA d’ Arsonval movement is to be used in an ammeter with a full-scale reading of 10 mA. Determine RA.

Solution

From the statement of the problem, we know that when the current at the terminals of the ammeter is 10 mA, 1 mA is flowing through the meter coil, which means that 9 mA must be diverted through RA. We also know that when the movement carries 1 mA, the drop across its terminals is 50 mV. Ohm's law requires that

9 × 10-3RA= 50 × 10-3,

or

Ra = 50/9 = 5.555 Ω.