Fig. 1.—Electric Windmill or Tourniquet.
The following experiments are given as a means of amusement to those who know little or nothing of electrical phenomena. Some of them may be recognized by some readers as being standard experiments, others may possess the charm of novelty. To many, however, the whole series will be new, and it is hoped that these will find a new source of interest opened to them, and that they may possibly be impelled thereby to investigate further concerning the causes of what they see. Frictional electrical machines can be purchased from any electrical instrument makers, at a small price, and with these experiments mentioned are more readily performed. In this article I only mention experiments that can be performed with materials to be found in every house, or the necessaries for which can be procured from a shop for a nominal sum. Friction between two substances of any sort probably always produces electricity; but it can only be made visible under certain circumstances.
For instance, if a stick of sealing-wax is warmed and rubbed with a piece of flannel also warm, they both become electrified. This may be proved by holding the wax near an electrometer, which is simply a bottle through the cork of which a wire is passed which has two pieces of gold leaf fastened to its extremity, when the leaves at once diverge owing to the repelling force of the electricity. The flannel is also electrified, but the electricity soon escapes, through the hand of the operator to the ground.
We now proceed to make a simple experiment on the production of electricity on a larger scale. Take a piece of stout brown paper and hold it in front of a hot fire till all the moisture inherent in it is expelled, and the paper is dry and quite hot. Now take it away suddenly, and holding it against the side of the coat rub it briskly with the sleeve by holding the sleeve in the hand. Take it away and hold it against the wall of the room, to which it will instantly adhere firmly, this adherence being caused by the development of electricity over the surface of the brown paper by the friction it has undergone. The paper can be removed from the wall, and on holding it at a short distance will fly towards it and adhere again. In a short time, however, the electricity departs, and the paper falls to the ground. If the hand is spread open upon the paper as it sticks, the electricity departs at once and the paper falls. A spark can be obtained from the paper, but it is hardly strong enough to be visible. In the next experiment, however, it is plainly to be seen.
Take an ordinary tea-tray and place it on the top of four glass tumblers, which must have previously been made quite hot and dry at the fire. They must also be scrupulously clean, as dirt is a good conductor of electricity. Now take a sheet of foolscap paper, and heat it strongly at the fire until perfectly dry, as the brown paper was. Place it while hot flat on the table and rub it from side to side, from the top to the bottom, with a piece of thick india-rubber. It will now adhere firmly to the table on account of the electricity developed. Take hold of two corners, pull it up, and quickly place it on the tray. On approaching the knuckle of your closed hand to the edge of the tray you will now obtain a brilliant spark, which, if the room is dark, will appear vivid. On removing the paper from the tray, and again approaching the knuckle, another spark will pass, but not so bright as the former. The experiment can be repeated as often as wished by heating and rubbing the paper again.
Now take four more tumblers, heat them as before, and place them on the floor with a board on the top of them. Let someone stand on this board, taking care that he is completely separated from all surrounding objects of furniture, etc., and that his clothes do not touch the table while the experiment is performed. Let him place his hand on the tray while the paper is heated, rubbed, and placed thereon.
He will then become charged with electricity, and if he approaches his hand to any one else’s a spark will pass between them. (This should not be done with susceptible parts of the body, the eyes for example, as it would be rather painful.) Let some one be provided with a spoon in which a little methylated spirit is heated; if the charged person holds his knuckle to this spirit it will instantly be ignited. Small pieces of paper—comic paper figures, etc.—will dance up and down briskly if he holds his hand outspread over them while lying on the table. The same thing will happen if the pieces of paper are placed between the tray and the table when the former is charged by the hot paper, or if the brown paper in the first experiment is held above them when excited.
Now take a needle and place it on the tray, its point projecting over the edge. If the room is now darkened, on placing the excited paper on the tray, the point of the needle will be seen to glow brilliantly for some seconds. This is caused by the electricity escaping into the air from the point of the needle, and is known as the “brush discharge.” The tray will consequently speedily lose its electricity. It will be found to be impossible to get a spark from the tray as long as the needle is on it, as the electricity vastly prefers to escape by the point. The escape of the electricity may be rendered still more evident by means of the following piece of apparatus.
Take two pieces of thin wire about two inches long, and bend each at right angles about an eighth of an inch from each end, both the bent portions being in the same direction. These two pieces of wire are now to be joined together at the middle at right angles by means of a piece of finer wire twisted around them. This finer wire can, with a little care, be caused to form a small cap, in which the point of a needle is inserted, the needle acting as a pivot, so that the bent wires turn freely on top of it (Fig. 1). The needle is supported by thrusting it into a large cork to act as a stand.