showing that for V-V' to increase there must be an initial difference of potentials V,, and that ne must be greater than 7. 75. Holtz's Machine. The action of Holtz's machine is analogous, except that the increase of the charge is continuous instead of intermittent. It consists of two glass plates (Fig. 58) placed parallel at a small distance from each other; one is fixed by means of wooden knobs supported by glass rods, the other is movable about a horizontal axis. Besides the central aperture, through which passes the axle of the movable plate, the fixed plate has two apertures or windows at the ends of the same diameter. Along one edge of each of these windows is an armature of paper provided with a tongue, also of paper, terminating in a point in the centre of the opening. Opposite the armatures, on the other side of the free movable plate, are two combs connected with two small insulated conductors, P and N. By an insulated handle it is possible to vary at will the distance of the two knobs, P and N, which will be called the poles of the machine. Two sliding knobs, P' and N', may be brought into contact with the corresponding poles, so that either one or the other may be put to earth. To get the machine to work, it must first be primed. The two poles, P and N, are brought into contact, and the movable plate being put in rotation by the handle, so as to move towards the points of the tongues, an ebonite plate or stick of sealing-wax charged by friction with negative electricity is brought near one of the armatures. When the machine is in action a peculiar hissing sound is heard; from the comb near which the ebonite was held, a luminous sheet spreads in a direction opposite to the motion of the plate, this is the positive comb (19); every point of the negative comb is tipped by a small star of light. The electrified ebonite may then be taken away, and the two poles can be separated; a brush discharge then passes between the two poles as long as the rotation continues. It is readily shown that the electricity of each of the poles is of the opposite kind to that which escapes from the corresponding comb. The action stops if the knobs P and N are drawn too far apart. The brush discharge depends upon the small capacity of the conductors. In order to increase this capacity, the two conductors, P and N, may be connected with two Leyden jars, K and H, the outer coatings of which are connected with each other. The two jars form thus a cascade between the two poles, and each of them has half the difference of potentials of the two poles (57). If the outer coatings are connected to earth, the two conductors are at equal and opposite potentials. When jars are used, the sparks are not continuous, as with the brush, but they follow each other at regular intervals, and are much more luminous, denser, and produce a louder crack. With a machine the plate of which is 60 cm. in diameter they may attain a length of 20 cm. Most of these machines, as now constructed, are double; they have two fixed plates close to each other. The two movable plates, which are mounted on the same axle, rotate on the outside. The armatures are placed opposite each other, those which are similarly electrified being together; the same U-shaped comb surrounds the two movable plates. The action of the machine is in no way altered, except that the yield is double. It is sometimes difficult to start the action of the machine, especially in damp weather; it then will not begin unless the ebonite plate is strongly electrified. The glass plate is usually made to rotate with a velocity of eight to ten turns a second. In order to explain the action of the machine and to make the figures clearer, we will suppose the glass plates replaced by concentric cylinders (Fig. 59). The circumference represents the movable cylinder; A' and B' the two paper armatures; the fixed cylinder is not shown, as it has scarcely any other function than that of supporting the armatures; Aa and B are the two conductors represented as in actual contact. C Suppose the plate to be at rest. The ebonite plate c charged with negative electricity is brought near the armature A'. It acts by influence on the conductor AB; positive electricity is attracted towards A,. and negative repelled towards B; but owing to the points the positive electricity escapes on to the glass at A and negative at B until equilibrium is established. FIG. 59. When the plate is turned, the positively electrified glass is carried away from A, and the negatively electrified glass from B, so that the conductor AB is again left subject to the action of the ebonite, which, acting as before, causes a stream of positive electricity to escape from the comb A, and a stream of negative from B. Each part of the plate, as it passes A, thus receives a positive charge, and, as it passes B, a negative charge, until, after half a turn, the whole plate is electrified as indicated by Fig. 60. + + a b + -B' + Henceforward the armatures come into play the paper tongues that project from them through the windows in the fixed plate graze the surface of the approaching parts of the revolving plate, and thus the armature A' acquires a negative charge, and B' a positive charge. The electric force due to these charges acts in the same way as that due to the electrified ebonite, so that the electric influence on the conductor AB is increased, and a greater discharge of electricity-positive at A, negative at B-takes place on to the surface of the revolving plate. The consequence is that the armatures, in their turn, become more strongly charged, or acquire a greater difference of potentials, and that the action. already described goes on still more actively, so that, after a few FIG. 60. H turns of the plate, the electrified ebonite may be removed and the machine continues to act. As described, the upper half of the plate carries positive electricity from left to right, and the lower half carries negative electricity from right to left, which is electrically the same thing. This transfer, due to the motion of the plate, is compensated by a flow of positive electricity from B to A through the conductor, and of negative electricity from A to B. When the difference of potentials between the armatures has attained a sufficiently high value, the electric flow through the conductor continues, even if the knobs P and N are separated, and it then becomes evident as a stream of sparks. If the knobs are separated too far, the action of the conductor ceases, and there being now no electrical interchange between the combs and the revolving glass-plate as it passes them, negative electricity is carried round to the positive armature B', and positive electricity to the negative armature a'. The electrification of the armatures is thus lessened, and may be destroyed altogether, or even inverted. When the knobs are quickly put into contact again, if the electrification has only been diminished, or if it has been inverted in sign, the action of the machine recommences, either in the same way as before or in the opposite, as the case may be. If the jars are connected with the conductors, the flow, instead of passing continuously across the gap ab, charges the jars until the potential reaches the value corresponding to the striking distance; a spark passes and the same succession of phenomena is repeated. When the jars are used, if the distance of the two poles is too great for the spark to pass, it sometimes happens that the action stops, then starts again, the signs of the poles being reversed, and the same series of alternations are reproduced. Each jar is periodically charged and discharged by the corresponding comb. This is specially liable to occur with batteries of large capacity. Holtz's machine affords a means of making a curious experiment on reversibility. If the two combs of a machine in the ordinary state are connected with the poles of a second one, from which the driving band has been removed so as to allow the plate to turn freely, and if the first machine is then set in action, and a slight impulse is given to the plate of the second machine, it continues to rotate; the electricity thus transmits the motion of the first machine to the second. 76. Diametral Conductor.-Various attempts have been made to prevent the machine from ceasing to act or reversing its poles. A useful contrivance for this purpose is the diametral conductor (Fig. 61). Each armature occupies almost the whole of a quadrant, and an insulated conductor, A'B', with combs at the ends, can be placed between them along any diameter. So long as the two poles B' + B FIG. 61. the principal conductor ceases to act, the diametral conductor replaces it, and the machine continues to work. 77. Voss's Machine.-Holtz's machine has been modified in many ways. Voss's machine (Fig. 62), which is now widely used, is a machine with a diametral conductor, and has some of the features of the Holtz machine and of the replenisher. The movable plate is provided with metal studs, which act like the carriers of the replenisher. The influencing conductors are represented by two strips of tinfoil fixed to the centre of the paper armatures. They are connected with two small brushes which touch the studs just before their passing under the combs; these brushes play the same part as the two springs a and b (Figs. 55, |