the first substance upon which electricity was produced by friction, and as the Greek name for amber is elektron, bodies so affected were said to be electrified. When a body, like ebonite, is rubbed with a flannel cloth, we say that it becomes charged with electricity. Just what happens to the ebonite is not clearly understood. We know, however, that it will attract light bodies, and then quickly repel them if they be conductors. Fig. 2 shows a piece of tissue-paper jumping toward a sheet of ebonite that has been electrified with a flannel cloth.

Fig. 3.

4. Conductors and Non-Conductors. Electricity can be produced upon glass and ebonite because they do not carry or conduct it away. If a piece of iron be rubbed, the electricity passes from the iron into the earth as fast as it is generated, because the iron is a conductor of electricity. Glass is an insulator or non-conductor. Frictional electricity resides upon the outside, only, of conductors. A hollow tin box will hold as great a charge as a solid piece of metal having the same outside size and shape. When frictional electricity passes from one place to another, sparks are produced. Lightning is caused by the passage of static electricity from a cloud to the earth, or from one cloud to another. In this case air forms the conductor. (For experiments, see "Study," Chapter VII.)

5. Electroscopes. A piece of carbon, pith, or even a small piece of damp tissue-paper will serve as an electroscope to test the presence of static electricity. The pith is usually tied to a piece of silk thread which is a non-conductor. Fig. 3 shows the ordinary form of pith-ball electroscope.

Fig. 4.

The leaf electroscope is a very delicate apparatus. Gold-leaf is generally used, but aluminum-leaf will stand handling and will do for all ordinary purposes. Fig. 4 shows a common form, the glass being used to keep currents of air from the leaves and at the same time to insulate them from the earth.

Electroscopes are used to show the presence, relative amount, or kind of static electricity on a body. (See "Study," Chapter XI.)

6. Two Kinds of Electrification. It can be shown that the electrification produced on all bodies by friction is not the same; for example, that generated with glass and silk is not the same as that made with ebonite and flannel. It has been agreed to call that produced by glass and silk positive, and that by ebonite and flannel negative. The signs + and - are used for positive and negative.

7. Laws of Electrification. (1) Charges of the same kind repel each other; (2) charges of unlike kinds attract each other; (3) either kind of a charge attracts and is attracted by a neutral body.

8. Static Electric Machines. In order to produce static electricity in quantities for experiments, some device is necessary.

The electrophorus (e-lec-troph´-o-rus) is about the simplest form of machine. Fig. 5 shows a simple electrophorus in which are two insulators and one conductor. The ebonite sheet E S is used with a flannel cloth to generate the electricity. The metal cover E C is lifted by the insulating handle E R. The cover E C is placed upon the thoroughly charged sheet E S, and then it is touched for an instant with the finger, before lifting it by E R. The charge upon E C can then be removed by bringing the hand near it. The bright spark that passes from E C to the hand indicates that E C has discharged itself into the earth. The action of the electrophorus depends upon induction. (For experiments, details of action, induced electrification, etc., see "The Study of Elementary Electricity and Magnetism by Experiment," Chapters VIII. and IX.)

Fig. 5.

The first electric machine consisted of a ball of sulphur fastened to a spindle which could be turned by a crank. By holding the hands or a pad of silk upon the revolving ball, electricity was produced.

Fig. 6.

Fig. 7.

9. The Cylinder Electric Machine consists, as shown in Fig. 6, of a glass cylinder so mounted that it can be turned by a crank. Friction is produced by a pad of leather C, which presses against the cylinder as it turns. Electric sparks can be taken from the large "conductors" which are insulated from the earth. The opposite electricities unite with sparks across D and E. If use is to be made of the electricity, either the rubber or the prime conductor must be connected with the ground. In the former case positive electricity is obtained; in the latter, negative.

10. The Plate Electrical Machine. Fig. 7 also shows an old form of machine. Such machines are made of circular plates of glass or ebonite, two rubbing pads being usually employed, one on each side of the plate. One operator is seen on an insulated stool (Fig. 7), the electricity passing through him before entering the earth by way of the body of the man at the right.

Fig. 8.

11. The Toepler-Holtz Machine, in one form, is shown in Fig. 8. The electricity is produced by the principle of induction, and not by mere friction. This machine, used in connection with condensers, produces large sparks.

Fig. 9.

12. The Wimshurst Machine is of recent date, and not being easily affected by atmospheric changes, is very useful for ordinary laboratory work. Fig. 9 shows one form of this machine.

13. Influence Machines for Medical Purposes are made in a large variety of forms. A Wimshurst machine is generally used as an exciter to charge the plates of the large machine when they lose their charge on account of excessive moisture in the atmosphere. Fig. 10 shows a large machine.

14. Uses of Electrical Machines. Static electricity has been used for many years in the laboratory for experimental purposes, for charging condensers, for medical purposes, etc. It is now being used for X-ray work, and considerable advancement has been made within a few years in the construction and efficiency of the machines.

Fig. 10.