Page:The American Cyclopædia (1879) Volume VI.djvu/514

 506 ELECTKICITY nected with insulated conductors which ter- minate in adjustable knobs, m and n. An ori- fice in the centre of the fixed plate gives pas- sage to the axis of the movable one, which can be rapidly rotated by a system of band wheels. The machine is started by bringing the knobs m and n of the conductors together and elec- FIG. 7. Holtz's Electrical Machine. trifying one of the armatures, say/, by hold- ing against it a plate of ebonite which has been negatively excited. After a few turns of the plate both armatures become highly charged with opposite kinds of electricity, /' becoming positive, and if the knobs are separated a stream of sparks will pass from one to the other. By increasing, within a certain limit, the distance between the knobs, the sparks become larger and less frequent ; but beyond this limit, which depends upon the insulation and working or- der of the machine, the sparks will no longer pass, and unless the knobs are quickly brought together the machine will cease to act. The following is a brief explanation of its action. The negative electricity of the first armature tends to repel the same fluid in its vicinity, and to attract the opposite ; consequently negative electricity flows from the face of the movable plate to the points of the comb, while positive electricity is discharged by the comb upon the plate. This will cause the comb of the second conductor, which at the commencement, as has been said, is in connection with the first, to become negative, while each portion of the glass plate will pass from the first to the second comb positively electrified. When successive portions of the plate thus charged arrive op- posite the second armature, the latter, through its point, discharges negative electricity upon the plate and receives positive in return, thus becoming positively charged. Positive elec- tricity from the face of the plate also passes to the second comb, and the latter discharges negative electricity upon the plate, which then passes on to the first armature and comb nega- tively electrified. Therefore the comb will dis- charge positive electricity on and receive nega- tive from the plate, and the armature will also receive a higher negative charge from the other side of the plate, because the latter is charged with higher tension. The effect is to cause the plate to leave the first comb more highly positive than it was the first time, and on again coming opposite the sec- ond armature to increase its positive charge. Both armatures become thus in a short time highly charged with opposite electricities, whose tension is only limited by the degree of insulation. A strong current of positive elec- tricity, it will therefore be seen, is constantly passing through the conductors from the second to the first comb, and a corresponding current of negative electricity from the first to the second comb ; and when the knobs of the con- ductors are separated the electricity will leap from one to the other. It is moreover evident that the action of the machine requires that each part of the movable plate be charged with electricity of an opposite kind to that of the armature it leaves, and of the same kind to that of the one it approaches. This condi- tion, however, cannot continue if the conduc- tors are so far separated a*fe to prevent com- munication, or are beyond striking distance, be- cause there would then simply be two armatures in opposite electrical conditions, with a moving plate passing from one to the other and gradu- ally equalizing the charge. The inventor has recently modified this machine by placing the plates horizontally and turning them in opposite directions, as represented in fig. 8. Neither plate has openings, but two combs are placed above the upper plate opposite each other, and two others below the lower plate, at right angles to the upper ones. Each of the upper FIG. 8. Holtz's Machine with Horizontal Plates. conductors is connected with one of the lower, so that there are only two conductors. The machine is started by holding for a short time an excited plate of ebonite opposite one of the combs. Sometimes, as in the figure, a third