During the early days of telegraphy, overhead lines were a source of considerable danger when thunderstorms were taking place. Lightning flashes often completely wrecked the instruments, giving severe shocks to those in the vicinity, and in a few cases operators were killed at their posts. Danger of this kind is now obviated by the use of contrivances known as lightning arresters. There are several forms of these, but only one need be mentioned. The main features of this are two metal plates separated slightly from one another, so that there is a small air gap between them. One plate is connected to the line wire, and the other to earth. Almost all lightning flashes consist of an oscillatory discharge, that is one which passes a number of times backwards and forwards between a cloud and the earth. A very rapidly alternating discharge of this kind finds difficulty in passing along the line wire, being greatly impeded by the coils of wire in the various pieces of apparatus; and although the resistance of this air gap is very high, the lightning discharge will cross the gap sooner than struggle along the line wire. In this way, when a flash affects the line, the discharge jumps the gap between the plates of the arrester and passes away harmlessly to earth, without entering the telegraph office at all. As was mentioned in Chapter III., the prevalence of magnetic storms sometimes renders telegraph lines quite unworkable for a time, but although such disturbances cause great delay and general inconvenience, they are not likely to be at all dangerous. It is often possible to maintain telegraphic communication during magnetic disturbances by using two lines to form a complete metallic loop, so that there is no earth return. CHAPTER XVII SUBMARINE TELEGRAPHY