90
4.
PHOTOELECTRIC EMISSION
by a metal, the anti—cathode of an X—ray tube, a continuous range
of x-rays are produced With frequencies from a comparatively low
value to a sharply dened upper limit Vmax- If V is the potential
used to accelerate the thermionic electrons, then
'
'
A»… = Ve.
(4—22) —
This is known as the laW of Duane and Hunt. The contact poten—
tial difference need not be considered except in the most precise
work as it is negligibly small in comparison to values of V (tens of
thousands of volts), This equation has been used to determine
/z/e, the ratio of Planck’s constant to the charge of the electron,
With great precision.6 Thus
/z/e = 1.376 X 10‘17 erg. sec./e.s.u.
EXPERIMENT 4—1
;
A STUDY OF PHOTOELECTRIC CELLS
currents from a high vacuum and
a gas lled cell are to be measured .for different light intensities
and applied voltages.
Âppamz‘us.—ln gure 4—14, the photo-cell is enclosed in a black
_
box.
A small hole, say’ 1.5 centimeters in diameter, adm1tshght
._
.
,
__
LAMP
%
%
PHOTO-CELL
.
—
uo
'
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_
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.
_
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‘FIG‘.
To study photoelectric cells;
_
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fromthe40to
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Thelamp and cell
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variation o£:the distance dfrom
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…
centnneters