The scattering formula of Rutherford gives an expression for the number
n
1
d
Ω of electrons in a gas which are scattered from a beam of electrons over the solid angle
d
Ω by impacts with atoms, which are to be found along a certain length
l
of this beam. If + Z
e
is the charge of the nucleus of the atoms, —
e
and
m
the charge and the mass of the electron, V the potential difference through which the electrons are accelerated, N the number of atoms in unit volume and
n
0
the total number of electrons which pass a certain cross-section of the beam, we have the well-known formula:
n
1
d
Ω =
n
0
N
l
(Z
e
/4V)
2
d
Ω/sin
4
½Θ, (1) where Θ is the angle of scattering. When
n
0
= 1, N = 1, and
l
= 1 the scattering is usually expressed by I
θ
d
Ω, where I
θ
is the so-called “scattered intensity.’’ According to Rutherford’s formula we get for the classical scattering due to the nucleus: I
θ
= (
e
/4V)
2
Z
2
/sin
4
½Θ. (2) Taking into consideration the electrons around the nucleus Mott and Bethe find: I
θ
= (
e
/4V)
2
(Z -F)
2
/sin
4
½Θ, (3) where F is the atomic form factor, known from the scattering of X-rays, and also a function of (V sin
2
½Θ). The values calculated for helium by James have been used for F in this paper.
Electron scattering in helium. Absolute measurements at 90° and 45°
