Bore two small holes through the wall of the mortar, near the bottom and exactly opposite to each other Insert two short pieces of coper wire, W, W, in the holes and fasten them tightly in position. The ends of the wires should be about one-eighth of an inch apart.
A small pinch of gunpowder is then placed in the bottom of the mortar.
Charge the Leyden jar and then discharge it through the mortar by connecting it to the two wires W, W. As soon as the spark passes, the powder will explode. An experiment such as this should be performed cautiously and the face and hands should be kept away from the powder. Do not put more than a pinch of powder in the mortar at a time and by all means keep the reserve supply out of the way so that there will be no danger of exploding it by accident.
*An Electric Whirligig* is an interesting piece of apparatus which may be built by following the suggestions contained in Figure 25.
[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.]