=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. [Illustration: Fig. 10.] With the modern machines large sparks are produced by merely turning a crank, enough electricity being produced to imitate a small thunderstorm. The sparks of home-made lightning will jump several inches. Do not think that electricity is generated in a commercial way by static electric machines. The practical uses of static electricity are very few when compared with those of current electricity from batteries and dynamos. =15. Condensation of Static Electricity.= By means of apparatus called _condensers_, a terrific charge of static electricity may be stored. Fig. 11 shows the most common form of condenser, known as the _Leyden jar_. It consists of a glass jar with an inside and outside coating of tin-foil. [Illustration: Fig. 11.] [Illustration: Fig. 12.] _To charge_ the jar it is held in the hand so that the outside coating shall be connected with the earth, the sparks from an electric machine being passed to the knob at the top, which is connected by a chain to the inside coating. _To discharge_ the jar, Fig. 12, a conductor with an insulating handle is placed against the outside coat; when the other end of the conductor is swung over towards the knob, a bright spark passes between them. This device is called a discharger. Fig. 13 shows a discharge through ether which the spark ignites. [Illustration: Fig. 13.] =16. The Leyden Battery=, Fig. 14, consists of several jars connected in such a way that the area of the inner and outer coatings is greatly increased. The battery has a larger capacity than one of its jars. (For Experiments in Condensation, see "Study," Chapter X.) [Illustration: Fig. 14.] =17. Electromotive Force of Static Electricity.= Although the sparks of static electricity are large, the _quantity_ of electricity is very small. It would take thousands of galvanic cells to produce a spark an inch long. While the quantity of static electricity is small, its potential, or electromotive force (E. M. F.), is very high. We say that an ordinary gravity cell has an E. M. F. of a little over one volt. Five such cells joined in the proper way would have an E. M. F. of a little over five volts. You will understand, then, what is meant when we say that the E. M. F. of a lightning flash is millions of volts.