Measurement of Thermal Conductivity of a Bad Conductor by Lee's Method
Aim:
To determine the co-efficient of thermal conductivity of a bad conductor (bakellite) in form of a
disc using Lee's method.
Apparatus Required:
(1) Lee's Apparatus and the experimental specimen in the form of a disc,
(2) Thermometer, (3) Stop watch, (4) Boiler, (5) Heater, (6) Special lamp stand, (7) Mirror.
Theory:
On passing steam through the cylindrical vessel a steady state is reached soon. In this condition the
rate at which heat is conducted across the specimen disc (B) is equal to the rate at which heat is
emitted through the exposed surface of the lower disc (C). If K is the co-efficient of thermal
conductivity of the material of bad conductor, d its thickness and A its area; θ 1 and θ2 the
temperature readings of A and C respectively, then rate at which heat is conducted across the disc of
the material
If m is the mass of the metal disc, s is the specific heat of that material, then rate of coling at θ2 is
equal to
…..........................(2)
Where dθ/dt is the rate of fall of temperature at θ2. Equating Eqn. (1) and Eqn. (2), we get
This is the working formula to calculate co-efficient of thermal conductivity of a bad conductor.
T1 (θ1)
Cu base (A)
Sample Sheet
T2(θ2)
Bakellite (B)
Cu disc (C)
Fig. 2
d
About Lee's Apparatus:
The apparatus shown in Fig. 2 consists of two parts. The lower part C is circular metal disc. The
experimental specimen B, (here it is Bakelite) is placed on it. The diameter of B is equal to that of C
and thickness is uniform throughout. A steam chamber A is placed on C. The lower part of the steam
chamber, A, is made of a thick metal plate of the same diameter as of C. The upper part is a hollow
chamber in which two side tubes are provided for inflow and outflow of steam. A thermometer T is
inserted into the hole in C. There are three hooks attached to C. The complete setup is suspended
from a clamp stand by attaching threads to these hooks.
Procedure:
1. Fill the boiler with water to nearly half and heat it to produce steam.
2. Put the specimen, steam chamber etc. in position and suspend it from the clamp stand. Insert the
thermometer. Check if both of them are displaying readings at room temperature. If not, note the
difference θ (T2 - T1).
3. Now stem is ready. Connect the boiler outlet with the inlet of the steam chamber by a rubber
tube.
4. Temperatures recorded in the thermometer (T 2) will show a rise and finally will be steady at a
particular temperature (θ2).
5. Wait for 5 minutes and note the steady temperature (θ2). Stop the inflow of steam.
6. Remove the steam chamber and the specimen B. C is still suspended. Heat C directly by the
steam chamber till its temperature is about θ2 + 20 °C.
7. Remove the steam chamber and wait for 2 – 3 minutes so that heat is uniformly distributed over
the disc C.
8. Place the insulating material on C. Start recording the temperature at 1⁄2 minute intervals.
Continue till the temperature falls by 200.
(9) A graph is plotted between temperature of C on Y axis and time on X axis. It gives a cooling
curve and at point θ2 at tangent is drawn and (dθ/dt)θ2 is measured.
Observation Table and Calculation:
Given parameters: Disc mass (m) = 870 mg,
specific heat of Cu (s) = 0.09 cal.gm/0C,
Area of Disc (A) = 96 cm2, and thickness of bakellite
sheet (d) = 3mm.
Time (in second)
Temperature (in degree
centigrade)