102 E L E G T ft I C I T Y [ELECTROMOTIVE FORCE. let the capacities of A and C in presence of B be each equal to p, and the coefficient of induction between A and B, or C and B, be q. Let us also suppose that the plates A and C are so distant from each other that there is no mutual influence, and that p is the capacity of one of the discs when it stands alone. A small charge Q is communicated to A, and A is insulated, and B, uninsulated, is brought up to it ; the charge on B will be - -Q. B is now unin sulated and brought to face C, which is uninsulated ; the charge on <? 2 C will be -, 2 Q. C is now insulated and connected with A, which is always insulated. B is then brought to face A and uninsulated, so that the charge on A becomes rQ, where Darwin. Cavallo. Nichol son. Typical convecto- inductive machine, Water- dropping machine. A is now disconnected from C, and here the first operation ends. It is obvious that at the end of n such operations the charge on A will be r"Q, so that the charge goes on increasing in geometrical progression. If the distance between the discs could be made in finitely small each time, then the multiplier r would be 2, and the charge would be doubled each time. Hence the name of the apparatus. Darwin, Cavallo, and Nicholson 1 devised mechanism for effect ing the movements which in Bennet s instrument were made by hand. Cavallo s was a reciprocating movement, but in the machines of Darwin and Nicholson the motion was continuous and rotatory. Nicholson s doubler is a very elegant instrument. A drawing of it is given by Mascart (t. ii. 845)]; the specimen there represented is very like one which was found among the late Professor Willis s apparatus, and is now in the Cavendish Laboratory at Cambridge. A still more elegant machine is " Nicholson s spinning condenser," which bears a remarkable resemblance to the induction machine of Topler. 2 A description, with a figure, will be found in the Encyclopaedia, Afetropofiiana, art. " Electricity," 112. It is obvious that if any conductor be connected with the part of any of these machines corresponding to the conductor A in the above description, and the potential of A be raised to any small positive or negative value, 3 we can by means of the machine in crease the charge, and therefore the potential, up to any required amount. We have, in fact, an electric machine which may be used for all the ordinary purposes. It was not with this view, however, that these pieces of apparatus were first invented, but rather for the purpose of demonstrating small electric differences. In this they were but too successful, for it was found that it was impossible to prevent them from indicating electric differences unavoidably arising within the apparatus itself. It was this difficulty no doubt that led to their being ultimately abandoned, and for a time forgotten, although they were once in high favour. Of late, however, they have been taken up as electromotors with great success. The type of all these machines is an arrangement of the following description. A conductor or carrier C, or a series of carriers, is fastened upon the circumference of an insulating disc. At the ends of a diameter are two hollow conductors, A and B, embracing the disc on both sides, so that twice in the course of a revolution the carrier is virtually in the interior of a hollow conductor. Inside each conductor are two springs : one of these is in metallic connec tion with the conductor, and may be called the receiving spring ; the other, called the inductor spring, is insulated from the con ductor, and is connected either to earth or with the corresponding spring belonging to the other conductor. Suppose A to be at a small positive potential, and B at zero potential ; starting with C in Connection with the inductor spring inside A, it becomes negatively electrified and carries away its charge; it next comes in contact with the receiving spring in B, and, being now part of the interior of a hollow conductor, it parts with the whole of its charge to B; then it passes on and is charged positively at B s inductor spring ; then discharges to A at A s receiving spring ; nnd so on. The positive and negative charges are each a little increased every revolution, and the difference of potentials accord ingly augmented. This is the principle of Varley s machine 4 (I860), and of Thomson s mouse mill and replenisher 5 (1867); it is virtually that of Bennet s doubler. Closely allied to these machines is Thomson s water-dropping potential equalizer. This consists, of an insulated reservoir of water, with a long pipe, from the nozzle of which water is allowed to break in drops. It is obvious that if the potential of the reser voir be above that of the air surrounding the spot where the water breaks into drops, each drop will carry away with it a positive charge, and this will go on till the potentials are equalized. This device was introduced by Thomson in observations on atmospheric
- Phil. Trans., 1788. p off y. Ann., 1865.
By connecting the conductor with the positive or negative pole of a small galvanic battery, for instance. ^ Jenkin, Ekct. and Jfag., cap. xix. Described in the art. electricity. The burning match which he uses in conjunction with the portable electrometer acts in the same way. He has also constructed a water-dropping electric machine on a similar prin ciple. Two streams of water break into drops inside two inductors connected with the internal armatures of two Leyden jars, A and B ; the drops from each inductor fall into a receiver connected with the other inductor. A very small difference of potential between the jars starts or reverses the action of the apparatus ; in fact, it will in general start of itself, and very soon sparks are seen passing between the different parts, and the drops are scattered in all directions by the strong electrical forces developed. The most remarkable, as well as the most useful, of all these Holt? machines is that of Holtz. 6 Here the convection is effected by m ach means of a disc of glass, which is mounted on a horizontal axis F (fig. 58), and can be made to rotate with considerable angular velocity by means of a multiplying gear, part of which is seen at X. Close behind this glass disc is fixed another vertical disc of glass, in which are cut two windows, B, B. On the side of the fixed disc next the rotating disc arc pasted two sectors of paper, A, A, with short blunt points attached to them, which run out into the windows towards the rotating disc, without quite touching it. Two metal combs C are placed on the other side of the rotating FIG. 58. Holtz s machine. disc (that nearest the reader), the teeth being put opposite the parts of A, A which lie towards the windows. The combs are fixed to metal shanks, which pass through a stout horizontal bar of ebonite. One of these shanks terminates in a couple of balls at E, and the other carries a sliding electrode D with a long ebonite handle. The framework which carries the horizontal ebonite bar and supports the fixed plates, &c., will be understood from the figure. The machine, as originally constructed by Holtz, contained only the parts we have described. Poggendorff doubled all the parts (except, of course, the electrodes D and E). The figure represents Ruhmkorffs modification of this construction. Behind the fixed disc there is another fixed disc, with windows and armatures like the first, and, beyond that, another movable disc mounted on the axis F. The combs are double, as will be seen from the figure. To start the machine, D and E are brought together, and one of the armatures (or one pair), say the right hand one, is electrified in any manner, let us say positively, and the disc set in rotation. After a little time a hissing noise is heard, and the machine becomes sensibly harder to turn, as if the disc were moving through a resisting medium. If the room be dark, long curved pencils of blue light will now be seen issuing from, the points of the left hand comb, and running along the surface of the disc in a direction opposite to its motion, while little stars shine upon the points of the right-hand comb. After this state has been reached, the balls D, E may be separated, and a continuous series of brush discharges will take place between them, even when the distance is very con siderable. If two Leyden jars, L, L, be hung upon the conductors which support the combs, the outer coatings being connected by a conductor M, then a succession of brilliant and sonorous sparks will take the place of the brushes. Instead of using the two jars L, L, we may connect D and K with the internal and external arma tures of a condenser; it will then be found that, as we augment the capacity of the condenser (the angular velocity of the disc being constant), the frequency of the sparks diminishes, while their brilliancy increases. If we insert a high resistance galvano meter between D and E, it will indicate a current flowing from D
6 Pogy. Ann., 1S65-