Convert galvanometer to 0–3V voltmeter, study thermistor

Experiment to convert a galvanometer into a 0–3V voltmeter and study how thermistor resistance changes with temperature using a slide wire bridge.

Apparatus

Thermistor
Test tube
Thermometer
Slide wire bridge
Resistance box
Galvanometer
Rheostat
Cell
Tapping key
Connecting wires

Theory

A thermistor is a heat-sensitive resistor made from semiconductor materials. Most thermistors have a negative temperature coefficient of resistance (NTC). This means their resistance decreases as temperature rises and increases as temperature falls.

The temperature coefficient of resistance (α) of a wire is the fraction of its resistance at 0°C by which it changes per degree Celsius:

$α=Rt−R0R0torRt=R0(1+αt)\alpha = \frac{R_t – R_0}{R_0 t} \quad \text{or} \quad R_t = R_0 (1 + \alpha t)$

Where:

$R_0$ = resistance at 0°C
$R_t$ = resistance at t°C

You can determine α by measuring the resistance at two temperatures, t₁ and t₂:

$R1=R0(1+αt1),R2=R0(1+αt2)R_1 = R_0 (1 + \alpha t_1), \quad R_2 = R_0 (1 + \alpha t_2)$

Then, dividing these equations gives:

$α=R2−R1R1(t2−t1)\alpha = \frac{R_2 – R_1}{R_1 (t_2 – t_1)}$

The slide wire bridge allows you to measure the resistance of the thermistor at different temperatures, as described in Experiment No. 1.

Procedure

Clean the ends of all connecting wires with sandpaper.
Arrange the apparatus as shown in Figure (a). Connect the battery (B), tapping key (K), and galvanometer (G). Connect one terminal of the galvanometer to point D and the other to a jockey that can touch the slide wire at any point.
Insert the resistance box in the gap LN and the thermistor in the gap MT. Place the thermistor in a test tube, which you submerge in a beaker of water. Measure the water temperature using the thermometer.
Wait about 5 minutes for the thermometer in the test tube to reach the water temperature. Record the resistance $R_{th}$ of the thermistor.
Heat the water gradually and stir continuously. Increase the water temperature in steps of 10°C, allowing it to stabilize for 5 minutes at each step. Record the resistance of the thermistor at each temperature.
Take at least five to six readings until the water boils.
Plot a graph of $R_{th}$ versus temperature $t$ , with resistance on the y-axis. The graph will not be perfectly straight.
Select two nearby readings $R_1$ and $R_2$ at temperatures $t_1$ and $t_2$ . Calculate the temperature coefficient of resistance using the formula above.
Repeat for at least three different temperature ranges.

Observations & Calculations

Temperature (°C)	Thermistor Resistance $R_{th}$ (Ω)
t₁	R₁
t₂	R₂
…	…

Temperature coefficient of resistance (α):

$α=R2−R1R1(t2−t1)\alpha = \frac{R_2 – R_1}{R_1 (t_2 – t_1)}$

Precautions

Ensure the test tube does not touch the sides or bottom of the beaker.
Take readings only after the thermometer shows a stable temperature.
Follow all precautions mentioned in Experiment No. 1.

Viva Questions

Q1. What is resistance?
A: Resistance is the property that opposes current flow. It equals the ratio of potential difference to current.

Q2. On what factors does wire resistance depend?
A: Length, cross-sectional area, material, and temperature.

Q3. Define the temperature coefficient of resistance.
A: The fractional change in resistance per degree Celsius rise in temperature.

Q4. Why is the experimental graph not perfectly straight?
A: External and self-heating effects alter the thermistor’s resistance. Only the surrounding temperature is measured by the thermometer.

Q5. What is a thermistor?
A: A temperature-dependent resistor made from semiconductor material, often with a negative temperature coefficient.