Chap. XIV Of the Structure and Use of the Electrical Kite
In the structure of an electrical kite, the circumstances to be principally attended to are those near, and on the ground. Silk being a non-conductor, the end of the string which is held in the hand is to be of that substance—a silk handkerchief tied to the hempen twine of the kite will answer very well. An iron key is to be tied on the hempen string, an inch or two above its junction with the silk, and from this key, when the kite is electrified, the sparks are to be received into a Leyden phial, to be used in the same manner as if it had been charged from the electrical machine. As curiosity may prompt many to repeat the experiments made with this kite, and as no experiments with atmospheric electricity can be made without some danger,16 we shall give the substance of Mr. Cavallo’s directions (the best we are acquainted with) relative to the forming and using of this instrument.—He observes that the whole power of the machine lies in the string: and that in other respects a common school boy’s kite, will answer the purpose as well as any other. The string is made by twisting two threads of twine with one of brass wire or copper, such as is commonly used for trimmings. When a kite constructed in this manner was raised, the string always gave signs of electricity except once, when the weather was warm, and the wind so weak that the kite could not be kept up for a few minutes; afterwards, however, when the wind increased, he obtained as usual a considerable quantity of electricity.
Concerning the management of this kite he gives the following directions.—
In raising the kite when the weather is very cloudy and rainy, at which time there is much danger of meeting a great quantity of electricity, I usually hang upon the string a chain with one extremity touching the ground; and sometimes I use another caution besides, which is, to stand upon an insulated stool; in which situation, I think that if any quantity of electricity, suddenly discharged by the clouds, strikes the kite, it cannot much affect my person. Although I have raised my electrical kite a hundred times without any caution whatever, I have very seldom received a few exceedingly slight shocks in my arms. In time of a thunder storm, if the kite has not been raised before, I would not advise a person to attempt it while the stormy clouds are over head, the danger at such time being very great, even when every caution is used. At that time the electricity of the clouds may be observed by means of a cork ball electrometer, placed in an open situation.
But Mr. Cavallo with all his caution could not avoid danger in making experiments on atmospheric electricity, as appears from the following account of his observations on the 13th of October 1773. “After having rained a great deal in the morning and the night before, the weather became a little clear in the afternoon, the clouds appearing separated and pretty well defined; the wind was west and pretty strong; the atmosphere was in a temperate degree of heat. In these circumstances, at three o’clock P. M. I raised my electrical kite, with 360 feet of string. After the end of the string was insulated, and a leather ball coated with tin foil, hung to it, I tried the power and quality of the electricity, which appeared to be positive and pretty strong; in a short time a small cloud passing over, the electricity increased a little; but the cloud being gone it returned pretty soon to its former degree.
The string of the kite was now fastened by a silk string to a post in the yard of the house; I was repeatedly charging two phials, and giving shocks with them: while I was so doing, the electricity, which was still positive, began to decrease, and in two or three minutes it became so weak, that it could hardly be perceived, with a very sensible cork ball electrometer.—Observing at the same time that a large black cloud approaching the zenith, (which no doubt caused the decrease of electricity) indicated rain, I introduced the end of the string through the window on the first floor, where I fastened it by the silk to an old chair.—The quadrant electrometer was set upon the same window, and was, by means of a wire, connected to the string of the kite. Being now three quarters of an hour after three, the electricity was actually imperceptible, however in about three minutes it returned, but now upon examination, it was found to be negative, which was evidently occasioned by the approach of the cloud, which by this time had reached the zenith of the kite; the rain also began to fall in large drops. The cloud came farther on, the rain increased and the electricity keeping pace with it, the electrometer soon arrived at 15°. Seeing now that the electricity was strong, I began again to charge the phials and to give shocks with them; but the phials had not been charged more than three or four times, before I perceived that the index of the electrometer was arrived at 35°, and was still rising. The shocks now being very smart, I desisted from charging the phials, and considering the rapid advance of the electricity, thought to take off the insulation of the string, that if it should farther increase it might be conducted silently to the earth, without occasioning any bad accident.
To effect this, as I had no proper apparatus near me, I thought to remove the silk string, and to fasten the twine itself to the chair. I disengaged the wire which connected the electrometer with the string; untied it from the silk, and fastened it to the chair: but while I was effecting this, which took up less than half a minute, I received twelve or fifteen very strong shocks, which I felt all along my arms, in my breast, and legs, shaking me in such a manner that I had hardly power to effect my purpose, or to warn the people of the room to keep their distance. As soon as I took my hands from the string, the electricity (in consequence of the chair being a bad conductor) began to snap between the string and the window shutter, which was the nearest conductor. The cloud was now just over the kite; it was black, well defined, and nearly of a circular form, its diameter appearing to be about 40°; the rain was copious but not remarkably heavy.
As the cloud was going off, I went near the string, and finding the electricity weak, but still negative, I insulated it again, thinking to keep it up some time longer; but observing that a larger and denser cloud was approaching, I resolved to pull the kite in; accordingly a gentleman, who was near me, began pulling it while I was winding up the string, he told me he had received two or three slight shocks in his arms, and if he should feel one more, he would let the string go; upon which, I pulled the kite in as fast as I could myself, without any further observation, being ten minutes after four o’clock.
N. B. There was no thunder or lightning perceived that day, nor for some days before, nor afterwards.
The general laws which Mr. Cavallo deduced from a variety of experiments made by means of electrical kites, are the following:
1st. The air appears to be electrified at all times; its electricity is always positive and much stronger in frosty than in warm weather; it is by no means less in the night than in the day time.
2d. The presence of the clouds generally lessens the electricity of the kite, sometimes it has no effect upon it, and it sometimes, though rarely, increases it a little. To this the above mentioned instance is a remarkable exception.
3d. When it rains, the electricity of the kite is generally negative, and very seldom positive.
4th. The aurora borealis seems not to affect the electricity of the kite.
5th. The electrical spark taken from the string of the kite, or from an insulated conductor connected with it, especially when it does not rain, is seldom longer than the quarter of an inch; but it is exceedingly pungent. When the index of the electrometer is not higher than 20° the person who takes the spark will feel it in his legs; it appearing more like the discharge of an electrical jar, than the spark taken from the prime conductor of an electrical machine.
6th. The electricity of the kite is generally stronger or weaker, according as the string is longer or shorter; but it does not keep any exact proportion to it; the electricity, for instance, brought down from a string of an hundred yards, may raise the index of an electrometer to 20°, when with double the length of string, the index of an electrometer will not go higher than 25°.
7th. When the weather is damp, and the electricity pretty strong, the index of an electrometer, after taking a spark from the string, or being presented to the knob of a coated phial, rises surprisingly quick to its former place; but in dry and warm weather it rises exceedingly slowly.