ELECTRICAL MACHINE (from eig rie, from Lat. electrum. (1k. li\o:rpoe, •cktron,amber: emmected with 17X/srwp, Mektar, bright sun. Skt. arka, sun, from arr. to shine). 1n instrument which generates electricity by friction or by the inductive action of one set of conductors on another. It is believed that the earliest known form was that of Otto von Oncricke, which is described and illustrated in his work Experi• aienta Nora 1672). a drawing from which is reproduced in Fig. 1. It consisted of a ball of sulphur mounted on an axle which could be re volved. The dry hand of the VW. riinenter placed in contact with the sulphur sphere, which was electrified by the friction thus developed. In tlravesande's Physices Element(' (1720) is described an electrical machine where the bail of sulphur is replaced by a hollow glass sphere which is rotated by means of cords or belts con nected with a wheel and crank. In this ap paratus threads are provided which are attracted to the charged sphere to show its electrification. In 1744 the electrical machine was improved by Bose of Wittenberg. who introduced a metallic conductor, by means of which the electricity could be conducted from the sphere to the earth and produce a spark on its way. In the same year Winkler of Leipzig devised a machine with a (%tlinder of glass instead of a sphere. and sub stituted for the hand two pieces of wool or leather to rub against the glass. Subsequent developments in the eighteenth century included devices to rotate the cylinder, collecting combs or conductors with points, the use of amalgam on the rubbing surfaces, and, filially. the sub. stitution of a disk of glass for the cylinder and sphere previously employed.
In it- usual form the frictional machine, as these early pieces of apparatus were termed. is shown in Fig. 2. The glass disk is electrified by the friction of the rubber and the silk flaps, and a positive charge on the glass is pro dueed. which csea at the rings or points to the prime conductor, where it can he used to charge a Leyden jar or perform other experi ments. while the negative charge produced im the rubber passe: to the ground. Nearly all forms of frictional machines have been rendered obsolete and their place taken by influence ma chine•, whose action is far more effective, and is less dependent on the dryness of the atmosphere. Of the influence machines which are extensively those of Tiipler, Wint.hurst. and Holtz, 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 ease of the electrophorus. In the 'ropier 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 fixed and has cemented to it two pieces of tinfoil, which are termed lield-plates. A charge is given to these tiebbplates 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 CI 01111n to them a portion of it- and is then discharged by a neu tralizing brush similar to that with whichvon tact was first trade, and connecting with the fiehl-plate, thus increasing its charge. The same
Focess takes place at the opposite side of the plates: only tile kind of electricity is changed, with the result that the process is continuous. The electricity from the collecting combs passes to Leyden jars, where it is collected. In tile Wimshurst machine, which is shown in Fig. 3, 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 Tdpler machine. In the Holtz machine, shown in Fig. 4. there arc 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 ease 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 cliarg 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 .1i 4: is revolved, and as a result we have one set of field-plates and the combs and conductors opposite them charged positively, vihile the others are negatively charged. The knobs of the conductors are at first in contact, but after a few of the disk they may be separated and a series of sparks produced cc hose length can he increased as long as the litierente of potential is sufficient to overcome t he air-gap.
In a number of these electrical machines, plates of rubber or vulcanite are for and the effect is also intensified by increasing the rumber of disks carried on the same axle, together with the combs, and other appliances. In the best of these machines sparks over a foot in length are ob tained. in electrical machines, as in the case of all apparatus it here electricity at high potential is used, the greatest care must he observed in insulating the different parts and conductors. Particularly, all glass parts on which the mois ture of the atmosphere is likely to settle must be covered with shellac. varnish, or paraffin. Electrical machines are available for many (A pt riments in static electricity, but save for ig niting gas-jets for use in eleetro-therapeutics they have few practical applications in ordinary life. They will he found described in a simple manner in S. 1'. Thompson, Eicnontary Lcssons in L'lectricity owl JIngnelism (New York, 1901). An interesting historical discussion is given in Gerland and Tramniiller. Ueschiehte der Physik nliselo Exptrimentierkunst (Leipzig, 1899). The advanced student may consult with profit the various volumes of the .Innnlen (ler Physik, and such treatises as MiilIer-Pouillet-Plaundler, Lcio-buch th r Physik, vol. iii. ( lIrunswiek, 1S88 90 I Wiillner, Lch•buch der Experinuntal Physik (Leipzig*, 18s01; and Gustav Wiedeman, Die Leh•r con vice E/telreitiit ( Brunswick, IS82-85). See articles ELECTRICITY: ELECTROPHORI'S.