Wednesday, July 02, 2014

The Lightning Bridge

 Mr. Samuel Alfred Varley was born in London in 1832. He wrote some of the very first treatise on submarine cables, including one page-tuner in 1858 titled "On the Electrical Qualifications requisite in Long Submarine Cables." That one got him a membership with the Institute of Civil Engineers at the age of 26. His later work, "On the mode of action of lightning on telegraphs, and on a new method of constructing telegraph coils" in 1870 was much more important. He was praised in writing by Faraday, David Hughes, Oliver Lodge, and many others in an 1892 testimonial in Nature magazine. But today both Varleys have fallen into total obscurity.

That paper in 1870 got to the heart of a basic problem in early telegraphs. I'm going to quote the 1870 description here because it's nice and timely.
"He [Varley] remarked that when lightning storms occur in the neighborhood 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 demagnetize, 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."
His solution was what he called a "lightning bridge."  He debuted the design in January of 1866 and in 1870 it was estimated that upwards of 1000 were in use. That same article described a 100% success rate which I find somewhat more doubtful. The lightning bridge was composed of two thick metal conductors, which where nailed into a peice of wood. That piece of wood contains a chamber in which their points are separated by about 1/8th of an inch. The space between was filled loosely with a powder made of carbon and "a non-conducting substance." The level of the powder covered the metal points.

This bridge was installed along the direct path between the telegraph wire and the telegraph and leads to ground. When lightning struck the current jumped across this air gap and the particles are thrown into an incandescent state. This generates a secondary current, and produces a more attractive path to ground than across the telegraph coils. The carbon provided a conduction path in the event of discharge that was not sufficient to jump across the spark gap, removing any residual charge. It was the first functional a lightning arrestor.

In 1875 Varley became assistant manager at the British Telegraph Manufactory Ltd. there he studied the magnets in it's armature and he devised a compound winding. He patented this in 1876 as a series-shunt or compound-wound dynamo. This did get him into a but of hto water with Wheatstone who had more-or-less invented the same thing. One writing of the era called Varley's work on the dynamo hi "magnum opus."   But that lightning bridge...  It exploited the basic principle of the coherer. By 1878 David Hughes was experimenting with a glass tube willed with zinc and silver filings he discovered was sensitive to electric sparks at a distance. Edouard Branly experimented with a similar coherer in 1890 and determined they were sensitive to radio waves. Oliver Lodge repeated the experiment the same year... and that was the device Marconi used to detect radio waves.

*His brother Cromwell Fleetwood Varley is just as interesting