12
1.
THE CHARGE OF THE ELECTRON
-
În order to determine vd, the microscope is rotated so that its
scale is vertical and the time for the drop to fall a given distance
measured.
The drop is drawn back to its starting point by an
upward electrical attraction (whose magnitude is immaterral) and
allowed to fall again. The individual values of the free fall time
will be-found to vary appreciably around the average (vd) because
the Brownian movements sometimes increase and sometimes
decrease the time required to fall a given distance under the force
of gravity alone. ln fact, these variations from the average may
'
also be used to determine x in the Einstein equation.
(See the
work of Fletcher, reference 3.)
'
'
1—10.
X—ray‘
charge of the electron may be
measured by an indirect process using X-rays. Briey, the method
consists
:
rst, a direct measurement of the wave-length of an '
X—ray; second, a measurement of the “grating constant” of an
‘
elementary crystal; third, a calculation of Avogadro’s number, and
nally, the evaluation of 6 from the equation Ne = 9649.
The wave-length of X-rays was rst measured in a direct
-
“
manner by A. H. Compton and Doan.4 As seen in gure 1—2,
/
D
_
‘
’
.
.
°
'
__
‘
;
-
Measurement of the wave—length of x-rays by the grating method.
beam, «reected from a calcitecrystal C, is
allowed
upon the surfaceof a-difaction gràting G at a
V€fY8maglanmng
As with
parts of
-
themlesurfacerecombme at
plate “P to,
.
dlœ°YfeectedbeamR «and
orders, D, ô’
on the
th€X”WSreectedf1‘®mth€Cfilc1tecrystal1scalculauœd Bèlardeh3ä
*
,