The dispersion of random movement of HI clouds for 175 complexes of size 50x200 pc inside Perseus spiral arms was measured for 14 regions with careful subtraction background radiation and check the uniformity of the group. The average dispersion is 2.9+-0.1 km/sec and its dependence from angle between line of sight and spiral arm for angles from 20 to 90 degree is not found : the amplitude of the first harmonics < 0.4 km/sec, i.e. delta sigma/sigma < 15% (figure 1). This fact may be used for evaluation of large-scale magnetic field in arm.

Figure 1. The dispersion of chaotic movement of HI clouds in Perseus spiral arm in depending on distance from Sun and angle between beam of sight and axis of arm.