Mechanical effects Beside the acoustical effects of the lightning there are noticeable mechanical effects. It is noticed that strike can damage a part of some building (roof, chimney…), destroy a tree or some wooden post but that is not all. It is noticed that some metal parts that conducted the lightning current were damaged and misshapen. It means that when we talk about the current of lightning we must pay attention to its mechanical effects that can be very important in certain circumstances. It is determined that the force can be caused by two reasons. First reason is a great enhancement of pressure that happens when lightning passes trough some media and heats it. That pressure is especially destructive when it is caught in some closed area, not in the open air. If lightning current passes through some crack in the wall of the house, pressure increases so those walls can't stand that. It especially happens if those cracks are humid and water in them transforms into steam. Same thing happens if lightning current is passing through the capillary of some tree. Usually the pressure is very high and the tree explodes. Second reason for appearance of force is purely electromechanical. When current passes through some metal part put in magnetic field, mechanical force acts and "tries to pull the part out of the field". Force can be determined from next expression: P = B * ig * l P stands for mechanical force, B stands for magnetic induction (magnetic field), ig stands for current and l stands for length of the conductor. It is also known that every current produces its own magnetic field. For example if current passes through two wires not far one from another, every wire produces its magnetic field and every wire is in the field made by another wire. It means that mechanical force appears and affects both wires. Direction of force depends on directions of currents in wires. For example, if the currents are parallel and have the same direction, than the force is trying to separate them. Oppositely, if currents are parallel but have the opposite direction, than the force is trying to bring them close to each other. That force can be determined from this expression in wich P stands for force, uo stands for permeability of media between wires, ig stands for current, a stands for distance between wires and l for lebght of wires. Almost all damages and deformations of electrical installations are result of this kind of forces. If the house doesn't have appropriate protection and lightning strikes into the house or transmission lines nearby, these damages could be huge.