Physics Demo Number: 012

Approximate Run Time: 8 min

Temperature Coefficient of Resistivity

Demo Description

Measure the resistance of incandescent light bulb filaments, in both their room temperature state and as a function of time immediately after they are switched off.

Scientific Principles

  • Resistance of conductors changes with temperature.

  • Some conductors, such as carbon, decrease in resistance as they heat up, while others such as metals increase in resistance as they heat up.

 

Equipment Location

All the components live in Kit (012) on [B-2-4].

On occasion the digital multimeter may be in Kit (051) on the middle shelf of cart at end of [A-B] row next to library wall.



 

Equipment

  • Lamp Fixture with incandescent light bulb

  • Double-pole-double-throw switch

  • Overhead Projector Modified Radio-Shack digital multimeter

  • A carbon-filament bulb and a tungsten filament bulb

  • Special cheater block for safely connecting lamp socket to 120V AC outlet




Instructions

Photos shows apparatus for measuring resistance of incandescent light bulb filament both in its room temperature state and as a function of time immediately after it is switched off.

The bottom photo shows the basic hookup of the multimeter.

The top left photo gives a view of the lamp hookup.

The top right shows the dedicated switching hardware.

The bottom photo shows the output stage of the multimeter on the right , which may be placed on the overhead in the lecture room to let the class see the resistance values of the bulbs as a function of time after being switched off.

There is a carbon-filament bulb (whose resistance goes down when heated) in the brown cardboard carton in the bottom left photo.

For a given bulb, when the room temperature resistance is measured, one should slide the meter switch to" range- hold", so that a larger range of the transient resistances during the cooling cycle can be seen on the overhead projection screen.

SAFETY WARNING:

Exercise great caution in showing this demo as there are bare conductors somewhat available with 120 VAC on them at times!


   
Writeup created by David A. Burba
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