Category Archives: Resistance

Resistance vs. Temperature

Demo: EM-D-RT
Pira Index: 5D 20.30

 Lab Equipment  Directions
  1.  Variac
  2. 2 Fluke DMM
  3. Ohm’s Law lightbulb apparatus
  4. 100 and 60 watt light bulb
  5. 2 pairs of probe leads
  1.  Set up equipment as shown.
  2. For several different settings on the variac, measure the voltage and current in the circuit.
  3. Calculate the R of each voltage setting. (R=V/I)
  4. Repeat for a bulb of different wattage.


Resistance of Bulb Filament

Demo: EM-D-RF
Pira Index: 5D 20.30

 Lab Equipment  Directions
  1.  Demo multimeter (large display)
  2. Special lamp box with switch
  3. 100 watt blue flood lamp
  4. Probe leads
  5. Plug strips
  1.  Set up equipment as shown with the lamp box plugged into an electrical outlet.
  2. Plug the probe leads from the lamp box into the V/Ω and Com receptacles. Set the demo meter on the left side to read ohms.
  3. Notice the resistance reading with the cool light bulb.
  4. Push the switch on the lamp box to momentarily turn the light on. Release the switch and notice the resistance reading of the hot light bulb.


Measuring Resistance

Demo: EM-D-MR
Pira Index: 5D 10.20

 Lab Equipment  Directions
  1.  Demo multimeter (use CPK #90259)
  2. Resistance coil set
  3. 2 B-B leads
  1.  Connect B-B leads to the meter at Ω and com. Push the button just above these connections to read ohms.
  2. Connect the leads to the 10m copper wire and read the resistance. Move the lead to the 20m copper wire for comparison.
  3. Compare the 10m 22 gauge copper wire with the 10m 28 gauge copper wire.
  4. Compare the 10m 22 gauge copper wire and the 10m 22 gauge nickel silver wire.


Jacob’s Ladder

Demo: EM-D-JL
Pira Index: 5D 40.10

  1.  Jacob’s ladder apparatus
  2. Plexiglas tube
  3. Variac
If it doesn’t work adjust the bottom gap between the 2 uprights to approximate 1cm 140V on variac may work a little better than the 120V.
  1.  Place the plexiglas tube over upright posts to prevent accidentally coming in contact with them
  2. Plug the Jacob’s ladder into an AC source and watch the climbing spark.
How it works: A transformer steps the 115V AC up to 10,000 V. The current arcs across the shortest distance between two upright conductors. Once started, the arc rises, owing to the heated air, and jumps over a distance of several inches.


Drude Model of Conduction

Demo: EM-D-DM
Pira Index: 5D 20.00

  1.  Bed of nails
  2. Large cart
  3. Box of ball bearings (1 cm diameter)
  4. box of paper to use for ramping
  1.  Set up as shown
  2. Pour ball bearings down from the top of the board, noting how long it takes for the balls to reach the bottom.
  3. Repeat, this time shaking the board side to side to simulate thermal activity. Note the rate of the balls now


Conductivity of Solutions

Demo: EM-D-CS
Pira Index: 5D 30.10

  1.  Light bulb apparatus
  2. Table salt – 5 mL container
  3. Stirrer
  4. Variac
  5. Distilled water, half-liter bottle
  6. 4″ plastic nalgene container
  1.  Set up the apparatus as shown in the figure.
  2. Fill the glass with distilled water and turn on variac. Observe what happens, turn the variac off.
  3. Add some salt to the water, stir with the spoon and turn the variac on again. What happens now?


Conductivity of Glass

Demo: EM-D-CG
Pira Index: 5D 20.60

  1.  250W flood lamp and 40W light bulb on a 110V AC series circuit board
  2. Hammer
  3. Nail
  4. Glove
  5. Propane torch
  6. Large plastic tray
  1.  Set the circuit on a plastic tray to catch broken glass. Plug the unit in and the incandescent bulb should come on. Place the plastic bottle over the light bulb to act as a shield
  2. Hold the nail on the top of the shield and hit it lightly with the hammer to break the bulb glass. Carefully lift the shield and using glove, remove any shards of glass from the base of the bulb.
  3. Heat the glass at the stem of the filament until it liquefies. The spotlight begins to glow, showing conductivity.


Resistance of a Light Bulb-Cooling

Demo: EM-D-BR
Pira Index: 5D 20.00

  1.  Bulb and bare filament apparatus
  2. 2 B-A leads
  3. Edison cell battery
  1.  Connect as shown. Start with the alligator ends across 4 cells of the battery to see if you can only dimly see the small bulb light, if not try 5 cells.
  2. When bulb glows dimly, blow gently across the exposed filament.
  3. As that filament cools, decreasing the resistance, the small bulb glows more brightly.