According to the classical dynamics, the molecules in the path of a beam of electrons will, by virtue of their electric fields, deflect the electrons constituting the beam. This deflection, while not perceptible for the electrons which pass at large distances from the molecule, may cause those which approach more closely to disappear from the beam. The effective area, within which an electron will be deflected from the beam, can be calculated from the equation I = I
0
e
-α
xp
, where I
0
is the number of electrons initially present in the beam, I the number remaining at the end of the path
x
,
p
the pressure of the gas, and α the absorption coefficient or the effective stopping area of all the molecules in a unit volume of gas at unit pressure. The mean effective area of a single molecule is obtained by dividing a by 3·56 × 10
16
, when the units chosen are millimetres of Hg and centimetres. Using this equation, Lenard and others have determined the absorption coefficients for most of the common gases.
The absorption coefficient for slow electrons in mercury vapour