Name: Date: Time:

Course:

METERS

Purpose: a. To measure the internal resistance (R_M) and the full scale current (I_M) of a galvanometer.
b. To construct a voltmeter and an ammeter using the above galvanometer.

Apparatus: Galvanometer, decade resistance box -2 (999 and 999 K ohms), digital multi-meter (DMM), 9-V DC source, switch, 350-ohm rheostat, connecting wires, meter stick, wire cutter, and # 30 Cu wire.

Theory: The essential parts of a dc galvanometer are shown below.

It consists of a magnet, a coil of wire, a spring, a pointer, and a calibrated scale. The coil is mounted in such a way that it can rotate, which causes the pointer to move in relation to the scale. The coil of wire and pointer rotate when there is a current in the wire. The coil rotates in response to the torque applied by the magnet when there is a current in the coil. The coil stops rotating when this torque is balanced by the torque of the spring.

A galvanometer has two important characteristics that must be considered when it is used as part of a measurement device. First, is the sensitivity of the galvanometer. It is the amount of dc current that causes a full-scale deflection (I_M) of the pointer. The second important characteristic is the resistance R_M of the wire in the coil. Figure below shows how a galvanometer with a coil resistance of R_M is represented in a circuit diagram.

In this lab, first we will measure the internal resistance (R_M) and the full scale current (I_M) of a galvano meter. Then we will construct and check a voltmeter and ammeter.

Procedure:

A) To measure the internal resistance (R_M) and the full scale current (I_M) of a galvanometer:

1. Set up the following circuit and get the construction checked by the constructor.

2. Set R₁ to 2999-ohm. With S open, adjust R₁ by changing 100, 10, and 1 dials, to get a full scale deflection in the galvanometer. Record the reading of the DMM, which is I_M.

3. Close S and adjust R₂ until the galvanometer deflects half scale reading. Record the value of R₂, which is equal to R_M.

Full scale current of the galvanometer, I_M =

Internal resistance of the galvanometer, R_M =

4. Explain why R₂ = R_M in procedure 3. __________________________________________________________________________________________________

__________________________________________________________________________________________________

B) Construction of a voltmeter: A resistance is placed in series with a galvanometer to convert it to a voltmeter as shown below.

The potential difference or voltage (V) across the meter and series resistance is given by,

V = I_M(R_M + R_S).

1. Calculate the series resistance R_s for a 10-volt voltmeter. R_s= _______________.

2. To construct the voltmeter; connect the resistance, R_s in series with the galvanometer. Use a decade box for R_s.

3. Set up the following circuit to measure voltages. Get the circuit checked by the instructor.

4. The constructed voltmeter should read full scale for 10 volt.

5. Change the voltage and complete the following table.

Voltmeter reading ( V )	Constructed voltmeter reading
0
2
4
6
8
10

C) Construction of an ammeter: A resistance (R_p) is placed in parallel with the galvanometer to convert it to an ammeter as shown below.

(I - I_M) R_p = I_M R_M

1. Calculate the parallel resistance, R_p for a 500-mA ammeter. R_p= _____________

2. Calculate the length (L) in cm of #30-Cu wire required to obtain R_p. L = _____________

Diameter (#30) = 0.02548 cm, Resistivity of Cu = 1.72 x 10^-6ohm. cm.

3. Cut a piece of #30-Cu wire of length (L+5) cm.

4. Construct the ammeter by connecting the Cu wire in parallel with galvanometer, as shown below.

.5. Set up the following circuit to measure currents.

6. Get the circuit checked by the instructor.

7. The constructed ammeter should deflect full scale for 500 mA. If not adjust the length of Cu wire to get full scale deflection.

8. Change the current and complete the following table.

amtmeter reading (mA)	Constructed amtmeter reading

Problems:

1. A galvanometer has a full-scale current of 0.100 mA and a coil resistance of 50.0 ohm. This instrument is used with a shunt resistor to form an ammeter that will register full scale for a current of 60.0 mA. Determine the resistance of the shunt resistor.

2. A voltmeter utilizes a galvanometer that has a 180- ohm coil resistance and a full-scale current of 8.30 mA. The voltmeter measures voltages up to 30.0 V. Determine the resistance that is connected in series with the galvanometer.