Demonstrating the sign of a static charge using an inexpensive oscilloscope

摘要

If you have an inexpensive oscilloscope, such as the Hameg HM 203-4 and Hitachi V-222 oscilloscopes I happen to have in my lab (expensive models are well-shielded from external fields and will not work), you can use them for a vivid determination of the sign of the static charge on an object. To do this, turn the instrument on and adjust the settings so that the beam is not moving, is near the center of the CRT screen, and is quite dim. With a means of putting a static charge on an object-an electrophorus, silk and a glass rod, fur and amber, a comb and your hair (I should note that it is a law of nature that combs become negatively charged when run through dry hair), etc.— have your students gather around the front of the oscilloscope so that each student can see the point of light on the screen (for large classes, a videocam can be used to display what happens on the CRT screen), then darken the room enough that the beam can be seen easily.