Measuring Instruments 16. 5
Galvanometer: Galvanometer is a very sensitive instrument by which we can detect the presence of current in a circuit. Apparently it consists of a dial on which a needle can rotate (Fig. 16. 12). In order to detect current in a circuit, the galvanometer is connected in the circuit in series (Fig. 16. 13). If its needle shows some deflection, it would indicate the presence of current in the circuit.
Fig. 16. 12: A galvanometer Fig. 16. 13
Galvanometer is also used to indicate potential difference between two points of a circuit. Fig. 16.14 shown an electrical circuit. If we have to find whether its points A and B are at the same potential or not, a galvanometer is connected between these points (Fig. 16. 14). If the galvanometer shows a deflection, it would indicate that a current is passing through the galvanometer, this would happen only in the case when there is a potential difference between the points A and B. if the galvanometer shows no deflection the points A and B would be at the same potential.
Fig. 16. 13
Ammeter: Galvanometer is a very sensitive instrument. Only a current of few milli-amperes is sufficient to cause full scale deflection in it. Therefore, it cannot be used directly to measure current because usually during experiments, current in a circuit is quite large. After suitable modification, a galvanometer is used to measure large values of current. This modification is accomplished by connecting a suitable low resistance in parallel with the galvanometer (Fig. 16.15). This resistance is known as shunt. Shunt provides an alternate path for the current to flow. The major part of the total current passes through the shunt and small fraction of it flows through the galvanometer. In this way the range of the galvanometer for current measurement is considerably increased. This modified galvanometer is known as ammeter. Usually the resistance of the shunt is very small. As in ammeter it is connected in parallel with the galvanometer, hence the resistance of an ammeter is very low.
Fig. 16. 15
In order to measure current in a circuit, the ammeter is connected in series, so the current flowing in the circuit also passes through the ammeter (Fig. 16.16). As the resistance of the ammeter is very small, so its introduction into the circuit does not affect the current of the circuit.
Fig. 16. 16
Voltmeter: When current I passes through a resistance R, there would be a potential across its two ends, the value of which is equal to V = IR. This potential difference can be directly measured by an instrument known as voltmeter. Galvanometer is converted into a voltmeter. Galvanometer is converted into a voltmeter by connecting a suitable resistance in series with it a voltmeter by connecting a suitable resistance in series with it (Fig. 16. 17). The value of this series resistance depends upon the range of the voltmeter. Usually its value is several thousand ohms. Thus the resistance of a voltmeter is very high.
Fig. 16. 17
Voltmeter is always connected in parallel with the resistance across which the potential difference is to be measured (Fig. 16. 18). Higher the resistance of the voltmeter, more reliable would be its readings. If the resistance the voltmeter is comparatively low, it will draw more current from the circuit. Due to this the potential difference across the resistance for the measurement which of the voltmeter was connected, would drop. Therefore a good voltmeter should have such a high resistance so that no or very little current passes through it.
Fig. 16. 18
When a voltmeter is connected in a circuit, care should be taken to connect its terminal marked positive (+) to that end of the resistance which is at a higher potential.