Читать книгу Lightning Rod Conference - Various - Страница 14

Insulators.—When copper rope lightning conductors were first introduced, about the year 1837, a circumstance occurred which at once proved the efficiency of the conductor, and suggested the use of insulators. The late Mr. Andrew Smith, C.E., had fitted a factory chimney in the East of London with a rope conductor, which was fixed to the chimney by iron staples. In a violent storm which occurred soon afterwards, the lightning was seen to pass down the conductor, which remained unaltered in any way; but on examining the chimney it was found that the brickwork had received a concussion at most, if not all, of the staples, showing that the lightning in passing had expended part of its force on the iron staples. It is probable that, if the staples had been made of thicker iron, and had been so placed as to lead off from the conductor with easy curves inwards, instead of being driven into the wall at right angles with the conductor, the concussions would have been much more violent than was the case.

Angles and Joints.—It must be obvious to any one that lightning, as well as any other matter or thing which travels at high speed, would be greatly obstructed in having to turn corners. It must also be borne in mind that lightning is of intense heat, and while passing in a straight line the effect of its heat is lost in the velocity of its motion; but in passing an angle its momentary pause (much too brief to be calculated) is sometimes enough to create sufficient heat to fuse the conductor at the angle. For this reason all angles must be avoided, and easy curves having a downward tendency substituted.

The angles in copper tube conductors are doubly objectionable, for, having joins as well as angles, they are liable, by the effect of heat, to become disjointed. It would be difficult, if not impossible, to fit a tubular conductor, except in a straight line from end to end, without this double objection. Similar objections apply also, in a greater or less extent, to the copper band conductors, as they are made with joins, and, when fixed up, are usually carried into and over as many angles as come in their way. They do not so readily follow all the sinuosities of a building as a rope does on the curve principle.

The flat band conductors, which are composed of a number of galvanized iron and copper wires combined, are simply a frivolity.

Metal in Buildings.—Taking the conductivity of copper as from 7 to 10 times greater than that of iron, it would probably follow, that if two rods, the one of copper the other of iron, in these proportionate sizes, were brought together in one common terminal or point, and led by the same course to the earth, as much of the fluid would possibly pass down the one as the other. On this principle, we avoid contiguity with any metal in a building, especially if in large masses, such as machinery, &c.

Ships’ Conductors.—In fitting the rope conductor to a ship’s rigging it is only necessary to pass it through a hole in the truck, so that the end may stand about 6 inches above the truck. It may be held up by a pin or key passed through the rope close over the truck, and then carried down the topgallant backstay (to which it should be tied at intervals with yarn) to the gunwale, where a sufficient length of the conductor should be kept in a coil to reach well down into the sea in any position of the ship. In stormy weather the coil may be untied, and by its own weight the end will drop down into the sea as required. Sometimes the rope is shackled at the gunwale to a strip of sheet copper about 3 inches wide, which is nailed down the ship’s side till it meets the sheathing at the bottom. The strip of copper should overlay the sheathing for a few inches. It may be noticed that this kind of conductor, fitted with a coil at the gunwale, is without any join whatever, and that it takes almost a straight course direct from the truck into the water.

The copper band conductors let into the mast and carried through the hull of the ship are objectionable and unsafe, as, in passing from each portion of the mast, they require moveable joints, so as to admit the several parts of the mast being run up or down as required. These joints present angular interruptions which may become out of order, and, in passing through the hull, any rupture of the band in that part, or the contiguity of other metals, may cause serious consequences. Certainly, there can be no necessity for carrying the lightning through the ship when, by a safer and much more simple method, it may be kept altogether outside.

In the smaller vessels, where the mainmast is well above the other masts, it may be sufficient for that mast only to be fitted with a conductor, but in larger ships, particularly long steam-ships, where the masts are a considerable distance apart, each mast should have a conductor.

We do not, either in theory or in practice, know any necessity for protecting the yards with conductors, though it is not altogether improbable that, in the absence of conductors on the masts, the yards might get damaged while the masts remain uninjured.

6. The end of the rope should be buried in moist earth, and carried in a curve to 5 or 6 feet from the foundation. In clay ground and on the shady side of a building about 3 feet below the surface would be deep enough; but in lighter ground, and particularly on the sunny side, it should be buried 6 or 7 feet deep, to ensure sufficient moisture at all times of the year.

7. As the course by which lightning approaches the earth is very devious, it would be difficult to determine with certainty the extent of area protected; but, viewing the absence of damage to the most remote parts of the roofs of buildings which have been properly fitted with conductors within the last 40 years, we should think the area protected may be taken as equal to from 3 to 5 or 6 times the height of the conductor.

8. When two or more terminals are used, the main rope should be somewhat enlarged; otherwise, the collective quantity of fluid received on the several points may be too great for the common channel.

WILKINS & WEATHERBY.

Dora Street, Limehouse, E.

We have to own receipt of your valued communication of the 14th ult., and with great pleasure to submit for the consideration of the Conference the following replies to their questions. We have endeavoured to make them as explicit as possible, but it is difficult adequately to describe our system on paper, and we suggest for the consideration of the Conference the advisability of showing any Committee they may appoint one or two of the numerous public buildings fitted by us.

Any further particulars or drawings you may require we shall be glad to send you; and it is with great pleasure that we add that any services we can render you in your valuable investigations are at your disposal.

1. A five-pointed copper spindle, the sharp points of which are silvered, and single points on high chimney shafts to the number of four or five.

2. Copper solid bands, or tubes, “as samples sent,” being simple, durable, cheap, and the most capacious form for the safe conduction of a heavy stroke of lightning, the bands being from 1 inch to 3 inches in width and ⅛ inch thick [A. 0·12 to 0·37 in.], and the tubes from ¾ to 1½ inches in diameter and ⅛ inch thick [A. 0·24 to 0·54 in.].

3. Yes; experience has proved that nothing less than 1½ inch bands [A. 0·18 in.] should be used for the main conductor to ordinary houses, with ¾ [A. 0·09 in.] to 1 inch [A. 0·12 in.] bands for branches, and from 2 to 3 inches [A. 0·24 to 0·37 in.] bands as main conductor to buildings of large area, with 1 to 1½ inch [A. 0·12 to 0·18 in.] for branches; or, in the case of chimney shafts,¾ inch to 1½ inch tube [A. 0·24 to O·54 in.] for main conductor, and 2 to 3 inches flat band [A. 0·24 to 0·37 in.] for tops of same.

4. The bands are in long lengths, are lapped, closely rivetted and soldered, to form a continuous band; while the tubes have patent insertion joints, the upper end being turned and fitted into the lower end, which is bored, and the tube then forms a continuous line externally and internally.

5. Copper holdfasts to suit shape and size of conductor.

6. Not less than 30 feet of 1½ inch to 2 inch copper bands [A. O·18 to 0·24 in.] in two or three branches, with forks at end of each band, and, if water is not near, the trenches half filled with carbonaceous materials and well watered, as this material will readily absorb the least moisture and retain it, while being in itself the best conductor. But much will depend upon the nature of the ground; for if chalk or rock foundation and water cannot be got at, the ground branches must be at least doubled, and the trenches deeper and made up of carbonaceous materials and earth.

7. Our experience is that no appreciable extent is protected by a single rod conductor in the presence of other influences. The chimney-stacks, lined with carbon in the shape of soot, with the heated gases, cause a rarefaction in the atmosphere, and form an easier passage for the electric fluid. Roofs and buildings having large masses of metals will be more likely to influence lightning than the single line of copper rod generally fitted. Many cases have occurred of chimney-stacks 4 feet to 9 feet across being struck opposite the conductor, and lead roofs, gutters, lead ridges, &c., from 10 feet to 20 feet from the rod conductor.

8. No; the system of conduction used by us does away with this, the lines of conduction being ample.