…
“2—2.
]. ]. «THOMSON METHOD
23
one of the quantities, @ or e/m in equation 2—1, can be deter'
'
'mined the other can then beevali1ated.
'
In order to determine the velocity 0 of the electrons, Thomson
applied an_ electrostatic potential to the metal plates
in such a —
direction that the upper plate was positive and the lower was
negative, and-Of such an amount that the electrons were brought
back to the point 8. If, then, E is the electric eld intensity or
force on unit charge in the cohdenser BC, then Ee is the electrical
force on each
When the combined electric and magnetic
forces are equal
.
—
-
._
,.
Ee = Hev
or
if) = E/H.
(273)
The electric eld, E, is equal to the potential di'erence between
the Condenser ‘plates divided by the distance, d, between their
Thus, if the potential difference, V…, is measured in
electromagnetic units, E‘= V…/d and if measured in volts V,
.
E = 10gV/d. Then
,
'
{
8
,
’
'
v
%
%
=
.
_
Ï
The iœlocities found Were surprisingly high, of the order of One— —
»:,third the velocity of light.
“
‘
‘
=
Substitution of equation
equation 2—1 gives
'
_«
Ÿ
_
.
&
H
;
_
.
'
'
-
‘
=
.
'
Inth1spioneering work, Thomson obtained a value for e/mof
_
order of magnitude
the
.
€:
metals®ftheelectrodesthusdem0nstratmthatelectmnsaœt@
efoundWldœpreadthroughNawreandthattheycanbeebtamed_
frommatterwhlchls©rdmaryelectncallyneuf—ralSmœmattel‘=