A SERIES of articles on lightning by J. F. Shipley which is being reprinted in Distribution of Electricity, a paper published by W. T. Henley's Telegraph Works Co., gives a resume of what has been accomplished in recent years in protecting transmission lines and engineering plant connected with them from damage from lightning. The effect of a lightning flash on a transmission line is to puncture the insulators or make them flash over, sometimes causing a short circuit which shuts down the supply. During the last forty years a very large number of devices have been employed to protect the lines, such as air-gaps, water-jets, oxide films, etc. These have been found only partially effective. The ideal arrester would be some link between the line and earth which would have infinite resistance at the normal pressure, but when for any reason that pressure increased by 10 or 20 per cent, the resistance should become practically zero, thus furnishing the impulsive rush of electricity with a safe path to earth. A recent device consists of a solid block of a material consisting of conducting particles of metallic oxide diffused in a baked clay which is microscopically porous. It is similar to porcelain in texture and mechanical strength and normally has almost infinite resistance. As soon as the electrical pressure across a block of it rises above a definite value its resistance decreases at a very rapid rate. If we double the voltage, the current it will pass increases more than twelve times. The material seems to have two names, ‘thyrite and ‘ocelit’. As it is an artificial product and can be accurately controlled, it looks as if a real step forward has been made in the design of these arresters, or ‘safety-valves’ as they are often called.