An epitome of electricity & galvanism

by Jacob Green; Erskine Hazard

At the same time its power as a conductor of electricity is perhaps equal, or but little inferior, to that of any of the metals. A line drawn on a piece of paper by a black lead pencil, will as I have often experienced, conduct an electric explosion seemingly as well as a similar line of gilding would do, and that without ever loosing its conducting power, which is not the case with gilding.

The second intention is, to facilitate the escape of the electric fluid from the lower part of the rod into the surrounding earth.

It is in many cases impracticable, from the interruption of rocks or other obstacles, to sink the rod so deep as to reach moist earth, or any other substance which is a tolerably good conductor of electricity. Nor, even if this were practicable, would it, I presume, be alone sufficient to answer the desired intention. Iron, buried in the earth, and especially in moist earth, will presently contract a coat of rust, which will continually increase till the whole is converted into rust, but rust of iron, and indeed the calx of all metals is a non-conductor, or at most but a very imperfect conductor of the electric fluid. Hence it is easy to see, that in a few years after a lightning rod has been erected, that part of it which is under ground will contribute little or nothing towards the safety of the building. Besides, the surface of this part of the rod is too small to afford an easy and copious discharge of the electric fluid into the surrounding earth, when this is but an imperfect conductor.

As a remedy for these defects I would propose, that the parts of the rod under ground be made of tin, or copper, which are far less liable to corrosion or rust, by lying under ground than iron.—Or, which perhaps would answer the purpose better, let this end of the rod, of whatever metal it be made, be coated over with a thick crust of black lead, previously formed into the consistence of paste, by being pulverised and mixed with sulphur (as in the manufactory of the ordinary kind of black lead pencils) and then applied to the rod while hot. By this means, the lower part of the rod would, I apprehend, retain its conducting power for ages, without any diminution.

In order to increase the surface of the lower part of the conductor, let a hole or pit, of sufficient extent, be dug as deep as convenient; and into this pit let there be put a quantity of charcoal, round the lower extremity of the rod. Charcoal possesses two properties, which, in a peculiar manner, fit it for answering the purpose here in view.—(1st.) It is a very good conductor of electricity and, (2d.) It will undergo little or no change of property by lying ever so long in the earth. Thus might the surface of that part of the conductor, in contact with the earth, be increased, with little trouble or expense to any extent at pleasure; a circumstance which every one acquainted with electrical experiments, must acknowledge to be of great importance to the end here proposed.”

The following experiments with a thunder-house, shew the utility of lightning rods, and ascertain what termination of the rod best answers the end proposed.

To shew the effect of lightning on a house not furnished with a conductor, or when the conductor is discontinued.

Provide yourself with the model of a house made of tin, four inches in breadth, six long, and about five in height. Let there be a chimney placed in the roof equidistant from both ends, and let a glass tube pass through it, the upper extremity of which must reach a little above the chimney, and the lower one come within an inch of the floor of the house.—Let a small wire pass through the bore of the glass tube, the upper end of which must extend a small distance above the orifice of the tube, having its extremity, which must be pointed, furnished with a screw, on which a metallic ball is to be fastened. The other end must likewise have a ball fixed upon it.—The instrument being thus prepared, fill the house with cotton, and sprinkle a little powdered rosin on that part of it, which is immediately between the lower knob of the wire, and the floor of the house. Then connect the lower part of the instrument with the outside coating of a pretty large jar.—From the prime conductor, in order to represent the clouds, suspend a small scale beam, having two balls of metal or wood coated with tin foil, in the place of the scale dishes, nicely balanced. The knob of the jar being connected with the prime conductor; bring the ball on the wire extending through the glass tube, under one of the balls representing a cloud.—Now charge the jar. The cloud will be attracted by the ball on the wire—the electricity of the cloud will be discharged—and if the experiment succeeds, the contents of the house will be set on fire.

The effects of lightning, when a house is furnished with a pointed conductor.

Repeat the above experiment with this variation: unscrew the ball from the upper extremity of the wire of the house, so that it may remain pointed. Place the house under the cloud as in the former experiment.—You will now find it impossible to charge the jar: or if you charge the jar before the house is placed under the cloud; the cloud, instead of being attracted by it, will be repelled, and the jar will be discharged without any explosion, and without firing the cotton.