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.) [Illustration: 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. [Illustration: Fig. 6.] [Illustration: 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. [Illustration: 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. =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. [Illustration: Fig. 9.]