function Boltzmann_probability {of charge qe on a particle of diameter d}
        (q {number of elementary charges}: integer;
         d {particle diameter : nm},
         T {air temperature : K} : real) : real;
   const   k = 1.38e-23; {Boltzmann constant}
           e = 1.6e-19;  {elementary charge}
         eps = 8.85e-12; {electric constant}
   var     i : integer;
     a, x, s : real;
   begin
   a := e * e / (4 * pi * eps * d * 1e-9 * k * T);
   i := 0; s := 0;
   repeat i := i + 1;
      x := exp (-i * i * a);
      s := s + x;
   until x < 1e-9;
   Boltzmann_probability := exp (-q * q * a) / ( 1 + 2 * s);
   end;