The current in the circuit is used in basic meters to measure the voltage drop or current in the circuit. The current passes through the coils of wires, which is then magnetized. Inside the apparatus, there is also a permanent magnet which opposes the magnetic field of the magnetized coil of wire that is connected to the pointer therefore, as we see in meters, the pointer moves. There is also a spring connected to the pointer to counteract the torque produced by the pointer, so the pointer will move in proportionality with the current density. As the current increases, the pointer will travel farther. When the current is interrupted, the field will also be gone, and the pointer will go back to zero by the spring.
2. What device is used to extend the range of a DC ammeter and how is it connected to the basic meter?
An additional resistance, which is called shunt resistor, is placed in parallel with the galvanometer of an ammeter to extend the range of the DC ammeter. The added resistance must be less than the resistance of the galvanometer to attract more current and the ammeter itself will receive less current, but will not affect the total current of the circuit, therefore extending the full scale range of the ammeter.
3. What device is used to extend the range of a DC voltmeter and how is it connected to the basic meter?
An additional resistor, which is called multiplier resistor, is placed in series with the galvanometer of a voltmeter to extend the range of the DC voltmeter. The added resistance will cause the path of the voltmeter to draw less current without changing the voltage of the voltage source, therefore extending the full scale range of the voltmeter.
4. Why is it necessary that an ammeter be a low resistance instrument? Why must a voltmeter be a high resistance instrument?
The ammeter must be a low resistance instrument, because if the ammeter will have a decent to high resistance it will affect the total resistance of the circuits resulting to a change in current flowing throughout the circuit, which will lead to an incorrect reading of the ammeter. The ideal voltmeter has infinite resistance, because it is place parallel to the circuit, but since it is impossible to get a infinite resistance, it should be as high as possible to receive less current. Another reason is that when the current arrives in a junction of a parallel path, most current will tend to go to the least resistive path. When one of the paths is the voltmeter, no current must pass through the voltmeter, so that the voltmeter wont change anything with the circuit.
5. What type of basic meter is configured as a wattmeter?
There are two coils in a wattmeter. One coil is placed in series with the circuit and the other one is placed in parallel with the circuit. The deflection of the pointer will be proportional to both coil resulting to the magnitude of power, which follows the P=VI.
6. Two 150 V voltmeters are being compared; meter A has 5 k|/V while meter B has a total meter resistance of 750 k|. Which is the more sensitive meter? Why?
Both 150V voltmeters have the same sensitivity of 5kohms/V. The sensitivity of a voltmeter is taken by dividing the resistance of the voltmeter by the full scale voltage reading of the voltmeter. Meter B has a resistance of 750Kohms, dividing it by 150V will have a quotient of 5kohms/V, which is also the same with the meter A.