When your clothes come out of the dryer, covered with static, do you know whether they are positively or negatively charged? In this article, I discuss a variety of devices that can determine sign of the charge on an insulator or conductor. Purposefully, none of these methods utilize comparison with a known charge. Some of these ideas have been previously published, and I am extending them, but many are original. These demonstrations provide students and teachers with an opportunity to contrast the actual flow of charge with conventional current and to compare the behavior of positive and negative charges with what we expect from protons and electrons.

1.
Not discussed in this article is the idea of “holes.” In the “sea of electrons” model of solid conductors, there are sometimes bubbles that can move in the opposite direction as electron flow. These bubbles or “holes” can be modeled as positive charge carriers, and their drift velocity even provides them an inertial value. They are not included here because they are a contrivance (not true charge carriers) and the model is typically beyond the high school level.
2.
For further ideas of the photoelectric effect, consult
Chun Xiao
Chen
and
Chang Geng
Zhang
, “
New demonstration of photoelectric effect
,”
Phys. Teach.
37
,
442
(
Oct.
1999
);
A.
Beehler
, “
Demonstrating the photoelectric effect using household items
,”
Phys. Teach.
48
,
348
(
May
2010
); and
G. F.
Robinson
, “
Photoelectric charging of an electroscope
,”
Am. J. Phys.
33
,
746
(
Sept.
1965
).
3.
Related to this is the analogue oscilloscope, which is essentially a well-calibrated cathode ray tube and it can be used in the same manner as the digital voltmeter mentioned above. Simply bring a charged object near a lead wire that is clipped to your positive terminal. You will easily recognize a positive or negative voltage. I prefer the slowest setting for voltage and time to watch the electron beam jump away from discharges.
4.
This idea has been explored before by
Chen
and
Wang
, “
Detecting the polarity of an electric charge
,”
Phys. Teach.
37
,
344
(
Sept.
1999
). However, we now see that even these tiny neon bulbs can work. I felt inclined to include this because their device is becoming less common. I also saw this demo in
Electrostatics
by
A.D.
Moore
, (
Anchor Books
,
1968
).
5.
See, for example,
M.
Dinca
, “
Charge sniffer for electrostatics demonstrations
,”
Am. J. Phys.
79
,
217
(
Feb.
2011
). The video of this device in action is very impressive: “
Electric Charge Sniffer
,” YouTube, https://www.youtube.com/watch?v=vAQ64Sv4iqY.
6.
For a very simple version, see
Ridiculously Sensitive Electric Charge Detector
, http://amasci.com/emotor/chargdet.html and for a medium challenge, try
W.
Connolly
,
G.
Benoit
, and
M.
Gould
, “
Apparatus for teaching physics: A new device for studying electric fields
,”
Phys. Teach.
29
,
182
(
March
1991
).
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