Re: Cosmology

[Hugh Logan <HloganPHY@CFL.RR.COM>]

Brian Whatcott wrote:
> Ah another illusion shattered. I had it that Einstein actually
> participated in an experimental verification.
> The detail of the clean copper sheet was vivid.
>
> Was it  Lenard who also used copper, after all?
>
> Brian W
I am not sure what metal(s) Lenard used. According to a reference from
UCSD
<http://physics.ucsd.edu/was-sdphul/labs/2dl/exp3/exp3-BACKGROU.html>,
Elster and Geitel, teachers at Wolfenbuettel Grammar School, built the
first two-electrode vacuum phototube. According to the reference, they
stated the first law of photoelectricity in 1900, namely that "the
electrical circuit current, and hence the number of photo-electrons
ejected per unit time, is exactly proportional to the intensity of
illumination." According to the reference, the more astonishing second
law was stated by Lenard in 1902, namely that "the maximum electron
kinetic energy depends only upon the wavelength of the irradiation and
is completely independent of its intensity." He used a retarding
potential to determine the maximum kinetic energy of the photoelectrons.

I believe Lenard built the first three element vacuum phototube,
considered a forerunner of the radio tube, the photoemitter rather than
a heated filament being the source of electrons. An aperture in a second
electrode permitted a beam of "quanta of electricity" (electrons, not
photons) to be formed. These electrons were accelerated by a third
electrode, the anode. A diagram of Lenard's apparatus is shown on p. 122
[pdf p. 18] at
<http://www.nobel.se/physics/laureates/1905/lenard-lecture.pdf>. Lenard
studied these beams with a magnetic field, and he showed that the
charge-to-mass ratio was the same as J.J.Thomson obtained for cathode rays.

Elsewhere, I read that Elster and Geitel built the first practical
vacuum phototube in 1904. I don't know what metal they first used for
the photoemitter, but they determined that the "sensitivity" depends on
the electropositivity of the metal, sodium being most sensitive for blue
light. I presume this "sensitivity" pertains to what one now calls the
work function. The vacuum phototubes of Elster and Geitel were sometimes
called alkali-metal cells. As I recall, potassium cells are sensitive
only to the highest visible frequencies, whereas sodium cells work into
the red region. Other metals work only in the ultraviolet. There is a
detailed early history of photoelectricity at
<http://fisicavolta.unipv.it/percorsi/pdf/napesi.pdf>.

Lenard showed that electrons were ejected from a zinc surface in the
Hallwachs demonstration when irradiated with UV of sufficiently high
frequency. (I used to introduce the photoelectric effect by discharging
a negatively charged zinc plate attached to a Braun electroscope with UV
from a mercury source. Ordinary black light will not work, although
sunlight will.) It was claimed that irradiating a neutral plate would
cause it to become positively charged. According to my recollection, I
was unable to do this. I recall seeing an explanation that the
positively charged plate would attract the ejected electrons,
neutralizing it. Perhaps sufficiently high frequency radiation would
give the electrons enough kinetic energy to escape.

This is my third attempt to send this message, but my browser crashed
the first two times without my saving the message. I decided to send it
without looking at Bernard's link, having spent so long on it. My
apologies if there is too much overlap.

Hugh Logan