Final answer:
Hot wire instruments are used for AC measurement because they measure the RMS current, which reflects the heating effect equivalent to that of a DC current. They are not the only method for measuring AC, but they are preferred for their directional independence. In a circuit diagram, current is measured by placing an ammeter in series with the component or before branching for total current.
Step-by-step explanation:
A.C. is not measured only by hot wire instruments, but hot wire instruments are commonly used to measure AC because they respond to the heating effect of the current, which is independent of the direction of flow. This means hot wire instruments can measure both AC and DC currents. The value of alternating current measured by a hot wire instrument is the root mean square (RMS) current. RMS current is used because it gives the value of the direct current that would produce the same heating effect as the measured alternating current.
In order to measure currents using an ammeter as depicted in a circuit diagram such as Figure 21.47:
- (a) To measure the total current, place the ammeter in series with the power source, before any branching occurs.
- (b) To measure current through R1, place the ammeter in series with R1.
- (c) For current through R2, place the ammeter in series with R2.
- (d) For current through R3, place the ammeter in series with R3.
For highly accurate measurements, null measurements can be more precise than standard measurements since they minimize the influence of the instrument on the circuit. Factors that limit the accuracy of null measurements include the resolution of the measurement tool, the stability of the components, and the precision with which the null condition can be established and identified.