=Condensers; Leyden Jar.=--A _condenser_ is an apparatus for condensing a large quantity of electricity on a comparatively small surface. The form may vary considerably, but in all cases it _consists essentially of two insulated conductors, separated by an insulator and the working depends on the action of induction_. A form of condenser generally used in making experiments on static electricity is the Leyden jar, so named from the town of Leyden where it was invented. It consists of a glass jar coated inside and out to a certain height with tinfoil, having a brass rod terminating in a knob passed through a wooden stopper, and connected to the inner coat by a loose chain, as shown in fig. 30. The jar may be charged by repeatedly touching the knob with the charged plate of the electrophorus or by connecting the inner coating to one knob of an electrical machine and the outer coating to the other knob. The discharge of a condenser is effected by connecting the plates having an opposite charge. This may be done by use of a wire or a discharger, as shown in fig. 31; the connection is made between the outer coat and the knob. When the knob of the discharger is sufficiently close to the knob of the jar, a bright spark will be observed between the knobs. This discharge occurs whenever the difference of potential between the coats is great enough to overcome the resistance of the air between the knobs. Let a charged jar be placed on a glass plate so as to insulate the outer coat. Let the knob be touched with the finger. No appreciable discharge will be noticed. Let the outer coat be in turn touched with the finger. Again no appreciable discharge will appear. But if the inner and outer coatings be connected with the discharger, a powerful spark will pass. =Electric Machines.=--Various machines have been devised for producing electric charges such as have been described. The ordinary “static” or electric machine, is nothing but a continuously acting electrophorus. Fig. 32 represents the so-called Toepler-Holtz machine. Upon the back of the stationary plate E, are pasted paper sectors, beneath which are strips of tinfoil AB and CD called _inductors_. In front of E is a revolving glass plate carrying discs _l_, _m_, _n_, _o_, _p_ and _q_, called _carriers_. To the inductors _AB_ and _CD_ are fastened metal arms _t_ and _u_, which bring _B_ and _C_ into electrical contact with the discs _l_, _m_, _n_, _o_, _p_ and _q_, when these discs pass beneath the tinsel brushes carried by _t_ and _u_. A stationary metallic rod _rs_ carries at its ends stationary brushes as well as sharp pointed metallic combs. The two knobs _R_ and _S_ have their capacity increased by the Leyden jars _L_ and _L′_. [Illustration: FIG. 32.--The Toepler-Holtz electric machine.] [Illustration: FIG. 33.--Principle of Toepler-Holtz electric machine.]