**Objective:**

To determination and Verification of Stefan’s Constant.

**Theory:**

The experimental set up consists of a vacuum diode with a cylindrical cathode made up of nickel. Closely fitted with the cathode sleeve is the tungsten heater filament. Temperature of the filament can be determined using the known resistance (R) – temperature (T) relationship for tungsten. Temperature coefficient of resistance for tungsten remains constant up to very high temperature and the R-T relation for tungsten is expressed in the form:

Applying Stefan’s law to the heated cylindrical cathode we can determine Stefan’s Constant from the knowledge of the surface area and the emissivity of the cathode.

**Formula Used:**

From Stefan’s law we have,

**Experimental Results:**

*Plot a graph *with log P along* Y-axis *and log T along* X-axis.*

**Calculation:**

We know That,

**Result:**

The measured value of Stefan’s constant (σ) is ………

**Discussion:**