*An Electric Frog-Pond* may be experimented with by cutting out some small tissue-paper frogs. Moisten them a little and lay them on the cover of the electrophorus. Touch the electrophorus with the finger and then raise it with the insulating handle. If the "frogs" are not too wet they will jump from the cover upon the table as soon as the cover is raised. CHAPTER III STATIC ELECTRIC MACHINES A Cylinder Electric Machine The electrophorus described in the last chapter is capable of furnishing sufficient electricity for many interesting experiments, but for the purpose of procuring larger supplies of electricity, a static electric machine is necessary. An electric machine is composed of two parts, one for producing the electricity by the friction of two surfaces rubbing against each other, and the other an arrangement for collecting the electricity thus formed. The earliest form of electric machine consisted of a ball of sulphur fixed upon a spindle which could be rotated by means of a crank. When the dry hands were pressed against the sulphur by a person standing on a cake of resin, which insulated him, sparks could be drawn from his body. Later a leather cushion was substituted for the hands, and a glass cylinder for the ball of sulphur, so that the frictional electric machine now consists of a cylinder or a disk of glass mounted upon a horizontal axis capable of being turned by a handle. A leather cushion, stuffed with horsehair and covered with a powdered amalgam of zinc or tin, presses against one side of the cylinder. A "prime" conductor in the shape of an elongated cylinder presents a row of fine metal spikes, like the teeth of a rake, to the opposite side. A flap of silk attached to the leather cushion passes over the cylinder and covers the upper half. [Illustration: Fig. 29.—Front View of a Cylinder Electric Machine.] When the handle of the machine is turned, the friction produced between the leather cushion and the glass generates a supply of positive electricity on the glass, which is collected, as the cylinder revolves, by the row of sharp points, and transferred to the prime conductor. The first thing required in the construction of an electric machine is a large glass bottle having a capacity of from two to four quarts. The insulating power of glass varies considerably. Common green glass (not white glass colored green by copper, but glass such as the telegraph insulators are made from) generally insulates the best. Some sorts of white glass, the Bohemian especially, are good insulators, but this quality will not usually be found in ordinary bottles. [Illustration: Fig. 30.—Method of Finding the Center of a Circle.]