[Illustration: FIG. 24.—The Electric Mortar. C is the Mortar, P the Powder, B a Small Ball and W W the two Wires between which the Spark igniting the powder takes place.] Mount four pieces of dowel about three inches long at the corners of a wooden base about eight inches long and two inches wide so that they form four vertical posts as shown by A, B, C, D. The dowels, as well as the wooden base should be carefully dried and shellaced so that they will not absorb any moisture. Stretch two pieces of straight stiff wire between the posts A C and B D, near the top. The wires should be perfectly straight and level. The whirligig itself is made by passing a sewing needle through the axis of a small cork. Four small wires having the ends bent over at right angles should then be stuck in the cork as shown in the upper left hand part of Figure 25. All of the wires should point in the same direction. [Illustration: FIG. 25.—An Electric Whirligig.] The four wires should all be the same length so that the whirligig is perfectly balanced. The cork is then covered with tinfoil so that there will be an electrical connection between the four small wires and the needle forming the shaft. The two wires A C and B D are connected together by a wire A B and a piece of flexible wire led to the Wimshurst machine. The opposite side of the Wimshurst machine is then grounded or touched with the hand. If the whirligig is laid on the wires A C and B D as shown in the illustration and it is perfectly balanced it will commence to revolve and roll along the wires just as soon as the Wimshurst machine is set in operation. It is the escape of the electricity from the points of the four wires on the whirligig which causes this. Other interesting experiments in static electricity may be performed with the aid of a Wimshurst machine and the experimenter who is sufficiently interested to continue farther is referred to any good book on physics or some such volume as "The Boy Electrician". CHAPTER II. CELLS AND BATTERIES. Sources of Current. One of the chief difficulties of the average young experimenter is to secure a satisfactory source of current for operating his apparatus. There are three means at his disposal and he may draw his electricity from 1. A power or lighting circuit; 2. A dynamo; 3. Batteries. [Illustration: FIG. 26.—A Voltaic Cell. A Voltaic Cell consists of a Strip of Copper and a Strip of Zinc immersed in a dilute solution of Sulphuric Acid.] Only those who are so fortunate as to live in a house wired for light and power service and supplied from the street mains, are likely to be able to utilize the first named. Those experimenters who live in towns where there are no commercial power wires or whose homes are not wired for such service will have to therefore depend upon a dynamo or a battery.