38
2.
THE MASS OF THE ELECTRON
The relativity theory also states that mass and energy are
_theoretically interchangeable. ’Thus the energy change éÎ 1n
ergs, equivalent to a mass change of 772 grams is gwen by the
,
expression
_
=
77162
.
(2—27)
where 6 is the velocity of light. From this, the mass of an electron
at rest is the equivalent of 0.813 micro—ergs, the mass of 0.001 units
’
of atomic Weight is the equivalent of 1.484 micro-ergs or, approximately, one million electron-volts (=1 M.e.v.). More accurately
'
…
1.000 Me.-v. = 0.001074 mass units and 1.000 mass units =
931 M.e.v.
The quantity 77252 is called the self—energy of the
_
particle whose mass is m.
.
EXPERIMENT 2—1
MEASUREMENT OF e/m BY THE HELICAL METHOD ”
Âppamius.—'—The Vacuum tube,_solenoid and associated cir—
cuits) are shown in gures 2—10 and 2—11. In gure 2—10, the
electron gun consists of an indirectly heated (2% volt a.c.) cathode,
K, a control grid G (0—40 volts negative With respect to K) which
serves to vary the number of electrons emitted by the-gun and
,
'
!
“
€
-
G A. A;
5æ—-———.
___
_)
‘g
.
‘
=
_
.
1
,
:
Fm2—10
tube and
the
method
f
“
3Ï -‘ Ë
'
'
—
250 volts—
=œrgçathoge. potential) and a
.
posmvely charged anode ZÎ2 (about1000voltsaboveK)The ;
transformer T,aïÏrëCtier
,
umtsA changeof
potentiometeralters thebrlghtnessof
?