Sectional Proceedings: Section A.—Mathematical and Physical Science

Abstract

On the Mode of Action of Lightning on Telegraphs, and on a New Method of Constructing Telegraph Coils.—Mr. S. Alfred Varley. He remarked that when lightning storms occur in the neighbourhood of telegraph wires, although the wires may not be actually struck, powerful currents are induced in them which may be sufficiently strong to fuse the coils, but which more frequently simply demagnetise, and as often reverse the magnetism of the magnetic needles situated in the coils of needle telegraph instruments. Thus, not only is a considerable amount of damage done annually to telegraph instruments, but telegraphic communication is very liable to serious interruption. Mr. Varley mentioned a number of observations going to prove that an interval of dust separating two metallic conductors opposes practically a decreasing resistance to an increasing electrical tension, and that incandescent particles of carbon oppose about 1/60th part of the resistance opposed by a needle telegraph coil. Reasoning upon these data, he has constructed an instrument, the main feature of which is what he terms a “lightning bridge.” Two thick metal conductors, terminating in points, are inserted usually in a piece of wood. These points approach one another within about 1/18th of an inch in a chamber cut in the middle of the wood. This bridge is placed in the electric circuit in the most direct course which the lightning can take, and the space separating the two points is filled loosely with powder, which is placed in the chamber, and surrounds and covers the extremities of the pointed conductors. The powder employed consists of carbon (a conductor) and a non-conducting substance in a minute state of division. When this instrument is used, therefore, lightning which strikes a circuit finds in its direct path not a space of air but a bridge of powder, consisting of particles of conducting matter in close proximity to one another. These the lightning connects under the influence of the discharge, and the particles are thrown into a highly incandescent state. The secondary current, developed by the demagnetisation, finds an easier passage across this heated matter than through the coils. These lightning bridges have been in use since January 1866, and at the present moment there are upwards of 1,000 doing duty in this country alone. Yet not a single case has occurred of a coil being fused when protected by them. The reason why a powder consisting entirely or chiefly of conducting matter cannot be safely employed is, that although it can oppose a practically infinite resistance to the passage of electricity of the tension of ordinary working currents, when a high tension discharge occurs, the particles under the influence of the discharge generally arrange themselves so closely as to make a conducting connection between the two points of the lightning bridge. In the course of his exposition, Mr. Varley endeavoured to prove that when telegraphic circuits protected by ordinary protectors are struck by lightning, it is to the secondary current and not to the main discharge that the fusion must be attributed. He also pointed out the defects of the protector, which consists of two silk wires wound side by side upon a bobbin.