ELECTRICAL MACHINE. ELECTRICAL MACHINE. by the friction ol the rubber ami the silk llaps, and a po.sitive eharj^e on the glass is jiro- duced. which cscaix-s at the rinjrs or points to the prime eondiutor, where it can be used to charge a Leydcn jar or perform other experi- ments, while the negative charge produced on the rubber passes to the ground. Nearly all forms of frictional machines have l)een rendered obsolete and their place taken by inlluence ma- chines, whose action is far more effective, and is less dependent on the dryness of the atniosi)here. Of the inlluence niadiines which are extensively tised, those of Tiipler. Wimsliurst. and Uoltz, with improvements added by other physicists, are the leading types. Their action is based on the principle of the electrophorus (q.v.), where a charged body is used to charge a conductor which is under its influence. If a conducting body is brought near a substance which is elec- trified and then is touched so that a path of escape is provided for electricity of the same nature as the original charged body, a charge is communicated to the conductor. The influence machine accomplishes this automatically instead of requiring contact to be made with the finger, as in the case of the electrophorus. In the Tiipler machine there are two glass plates, one of which is capable of revolution and is pro- vided with a number of pieces of tinfoil attached to its surface, with small buttons of metal in the centre of each, while the other disk is fi.ved and has cemented to it two pieces of tinfoil, FlO. 3. WIMSBrEST INFLUENCE MACHINE. which are termed field-plates. A slight charge is given to one of these fielil-plates and the mova- ble disk is then revolved so that when one of its tinfoil disks, or carriers, comes opposite the field- plate, it is charged inductively, since contact is made by a brush of fine brass wire which per- mits the escape of the electricity of the same kind as the charge of the field-plate. The car- rier thus charged then passes the collecting combs, where it communicates to them a portion of its charge, and is then discharged by a neu- tralizing brush similar to that with which con- tact was first itade, and connecting with the Vol. VI.— 18. field-plate, thus increasing its charge. The same process takes place at the opposite side of the plates; only the kind of electricity is changed, with the result that the process is continuous. T he electricity from the collecting combs passes to Leyden jars, where it is collected. In the Wimshurst machine, which is shown in Fig. 3, Fig. 4. noLTz electrical machine both plates are movable, revolving in opposite directions. Each plate carries a series of tinfoil sectors which not only serve as carriers of the charges, but also as inducting plates. The brushes and collecting combs possess essentially the same functions as in the Topler machine. In the Holtz machine, shown in Fig. 4, there are two glass plates, one of which is solid and mounted on an axle, capable of revolution, while the second plate is fixed and has a central open- ing through which the axle passes, and two win- dows through which pointed strips of paper pro- ject so that they are in contact or nearly in contact with the revolving plate. Below the window on the fixed plate are attached two pieces of varnished paper connected with the points, which serve as field-plates, and opposite to these plates on the farther side of the re- volving disk are the collecting combs. One field- plate is charged by contact with an electrified body, such as a piece of hard rubber, and the electricity of the same kind, in this case nega- tive, is driven from the opposite side of the re- volving plate at the combs, leaving the glass plate with a positive charge. The plate thus charged revolves and the charge is brought oppo- site the field-plate on the other side. Here nega- tive electricity is drawn from the combs, leaving that knob of the prime conductor with a positive charge, while a positive charge is also given to the field-plate. The inductive effect becomes greater as the disk is revolved, and as a result we have one set of field-plates and the combs and conductors opposite them charged positively, hile the others are negatively charged. The knobs of the conductors are at first in contact, but after a few revolutions of the disk they may be separated and a series of sparks produced whose length can be increased as long as the difference of potential is sufficient to overcome the air-gap. In a number of these electrical machines, plates of rubber or vulcanite are substituted for glass disks: and the effect is also intensified by increasing the number of disks carried on the same axle, together with the combs, conductors.
