112 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



ordinary galvanized iron wire known as No. 4, which is one 

 fourth of an inch in diameter, is amjDly sufficient for any 

 dwelling-house. His reason for recommending telegraphic 

 Avire is based on the experience of the English companies in 

 protecting the poles of their telegraph lines. These are in- 

 variably supplied with lightning-conductors of No. 8 wire, 

 running from the upper end of the pole to the ground. No 

 pole so protected has ever been known to suffer from light- 

 ning. The precautions necessary in fixing conductors to 

 houses are. First, that the conductor must be solid and con- 

 tinuous. There should be no joint, unless it be a well-solder- 

 ed one. Chain-link rods, braided rope, tubing, etc., should 

 be avoided. Second, the ground connection must be sound 

 and good. Third, each conductor, if there be more than 

 one, should have a separate ground connection, but they 

 should all be also connected together, and connected with 

 the lead roofing, and all masses of metal in their neighbor- 

 hood. It is unnecessary to insulate these conductors. There 

 should be no points or acute angles in the conductor the 

 straighter and more direct its course to the earth the better. 

 It is safest to consider that the area protected by the con- 

 ductor is equal to its height above the ground. 



Besides considering many details in the protection of tele- 

 graphic instruments, Preece explains the theory of the action 

 of fine points in dissipating or preventing discharges of light- 

 ning. The lightning-conductor should terminate at its ujoper 

 end in a fine gilded or polished point. When this is proper- 

 ly attended to, a glow or brush of light may be seen at the 

 point when lightning approaches it, but a destructive dis- 

 charge of electricity can scarcely ever occur. These points 

 are sometimes fused by powerful discharges, and require to 

 be* renewed. The ground connection should be with as large 

 a mass of metal as possible, such as the gas or water pipes 

 of a city. If the " ground " is too small, it will be corroded 

 and destroyed by electrolytic action. Lightning-conductors 

 of all kinds require periodical examination. If this is neg- 

 lected, that which was erected for our protection may be- 

 come a source of danger. Jour. /Soc. Telegraph Engineers^ 

 I, 337. 



