Re: [Phys-l] Which one is positive?

["Pete Lohstreter" <plohstreter@mail.hockaday.org>]

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
> There are other problems, including abuse of the terms
> "anode" and "cathode".
>  http://www.av8n.com/physics/anode-cathode.htm

The anode of a device is the terminal where current flows in from outside.
The cathode of a device is the terminal where current flows out. This is
illustrated in [
http://www.av8n.com/physics/anode-cathode.htm#fig-def-anode-cathode
]figure 1.  (shows positive charge leaving the cathode)

How does this set with 'cathode rays'?  Aren't cathode rays negative?  If
cathode rays come from the cathode, something's backwards. 

Cathode rays (also called an electron beam or e-beam) are streams of [
http://en.wikipedia.org/wiki/Electron ]electrons observed in [
http://en.wikipedia.org/wiki/Vacuum_tube ]vacuum tubes, i.e. [
http://en.wikipedia.org/wiki/Vacuum ]evacuated glass tubes that are
equipped with at least two metal [ http://en.wikipedia.org/wiki/Electrode
]electrodes to which a [ http://en.wikipedia.org/wiki/Voltage ]voltage is
applied, a [ http://en.wikipedia.org/wiki/Cathode ]cathode or negative
electrode and an [ http://en.wikipedia.org/wiki/Anode ]anode or positive
electrode. Electrons were first discovered as the constituents of cathode
rays. In 1897 British physicist [
http://en.wikipedia.org/wiki/J._J._Thomson ]J. J. Thomson showed the rays
were composed of a previously unknown negatively charged particle, which
was later named the electron. [
http://en.wikipedia.org/wiki/Cathode_ray_tube ]Cathode ray tubes (CRTs)
create the image in a classic [ http://en.wikipedia.org/wiki/Television
]television set.

I'm really confused now.  How can positive charges leave the negative
electrode?  Shouldn't they attract?
> > From the keen-grasp-of-the-obvious department:  If you 
> want to know which is positive, use a voltmeter.
>
> If there is some heretofore-unmentioned constraint that
> prevents you from using a voltmeter, you need to ask a
> different question, being careful to say what the
> constraints are.


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