To test a light bulb: 1. Click on the light bulb rack, then click on the bulb you wish to test. 2. Set the vacuum level by moving the mercury bottle up or down to different positions on the ladder. 3. To test your experiment, click on the switch located in front of the light bulb rack on the table. 4. When you are finished with a test, click on the light bulb you tested. Related Investigations: Circuits Resistance