All experiments with static electricity perform better in the winter time, when it is cool and clear, than in the summer. The reason is that the air in winter is drier than in summer. Summer air contains considerable moisture and water vapor. Water vapor is a _partial_ conductor of electricity, and the surrounding air will therefore conduct the static electricity away from your apparatus almost as fast as it can be produced in the summer time. [Illustration: Fig. 20.—A Surprise for the Cat.] Some day during the winter time, when it is cool and clear, and the cat is near a fire or a stove, stroke the cat rapidly with the hand. The fur will stand up towards the hand and a faint crackling noise will be heard. The crackling is caused by small sparks passing between the cat and the hand. If the experiment is performed in a dark room, the sparks may be plainly seen. If you present your knuckle to the cat’s nose a spark will jump to your knuckle and somewhat surprise the cat. If the day is brisk and cool, so that everything outside is frozen and dry, try combing the hair with a rubber comb. Your hair will stand up all over your head instead of lying down flat, and the faint crackling noise, showing that sparking is taking place as the comb passes through the hair, will be plainly heard. The electricity is produced by the friction between the hair and the comb. Electricity may be produced by friction between a number of substances. A hard rubber rod, a glass rod, a rubber comb or a stick of sealing-wax may be very easily electrified by rubbing them briskly with a piece of dry, warm flannel. *Electroscopes* are devices for detecting the presence of static electricity. [Illustration: Fig. 21.—A Paper Electroscope.] A very simple form of electroscope may be made in much the same manner as the paper compass described in the last chapter. It may be cut out of writing-paper and mounted on a pin stuck through a cork. If an electrified rod is held near the electroscope it may be made to whirl around in the same manner as a compass needle when a bar magnet is brought to it. *The Pith-Ball Electroscope* is a very simple device, in which a ball of cork or elder pith is hung by a fine silk thread from an insulated support. A suitable electroscope may be made from a glass bottle having a piece of wire thrust into the cork to support the pith ball. When the electrified rod is presented to the pith ball, it will fly out towards the rod. [Illustration: Fig. 22.—A Pith-Ball Electroscope.] If the pith ball is permitted to touch the glass rod, the latter will transfer some of its electricity and charge the ball. Almost immediately the pith ball will fly away from the glass rod, and no matter how near the rod is brought, it will refuse to be touched again. This action is much the same as that of the magnetized needle suspended from a thread when the similar pole of the magnet is presented to it.