[Illustration: Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other.] This experiment indicates that there are two kinds of electrification: one developed by rubbing glass with silk and the other developed by rubbing sealing-wax with flannel. In the first instance, the glass rod is said to be _positively_ electrified, and in the latter case the sealing-wax is _negatively_ electrified. The same law that applies to magnetism also holds true in the case of static electricity, and similarly electrified bodies will repel each other and dissimilar ones attract. *The Electrophorus* is an instrument devised by Volta in 1775 for the purpose of obtaining static electricity. [Illustration: Fig. 27.—The Electrophorous] It is easily constructed and will furnish a source of electricity for quite a number of interesting experiments. An electrophorus consists of two parts, a round cake of resinous material cast in a metal dish or pan, and a round metal disk which is provided with an insulating handle. To make an electrophorus, first procure an old cake or pie tin, and fill it with bits of resin or sealing-wax. Place the pan in a warm spot upon the stove where the resin will melt, taking care not to overheat or it will spatter and possibly take fire. As the resin melts, add more until the pan is nearly full. When all is melted, remove from the fire and set it away where it may cool and harden in the pan without being disturbed. Cut a circular disk out of sheet tin, zinc, or copper, making the diameter about two inches less than that of the pie pan. Solder a small cylinder of tin or sheet brass to the center of the disk to aid in supporting the handle. The latter is a piece of glass tubing about three-quarters of an inch in diameter and four or five inches long, placed in the center of the cylinder and secured with molten sealing-wax. In order to use the electrophorus the resinous cake must first be beaten or briskly rubbed with a piece of warm woolen cloth or flannel. Then place the disk on the cake holding the insulating handle with the right hand. Touch the cover or the disk momentarily with the forefinger of the left hand. After the finger is removed, raise the disk from the cake by picking it up with the glass insulating handle. The disk will now be found heavily charged with positive electricity, and if the knuckles are presented to the edge, a spark will jump out to meet them. [Illustration: Fig. 28.—An Electric Frog-Pond.] The cover may then be replaced, touched, and once more removed. It will yield any number of sparks, the resinous cake only needing to be recharged by rubbing once in a long while.