the electrophorus. This consists (Fig. 358) of a disc of resin, or some other material easily excited by friction, and of a polished metal disc B with an insulating handle CD. The resin disc is electrified by striking or rubbing it with catskin or flannel, and the metal plate is then laid upon it. In these circumstances, the upper plate does not receive a direct charge from the lower, but, if touched with the finger (to connect it with the earth), receives an opposite charge by induction. On lifting it away by its insulating handle, it is found to be charged, and will give a spark. It may then be replaced on the lower plate (touching it at the same time with the finger), and the process repeated an indefinite number of times, without any fresh excitation, if the weather is favourable. Fig 358.-Electrophorus. The resinous plate has usually a base or sole of metal, which is in connection with the earth while the electrophorus is being worked. This sole, by the mutual induction which takes place between it and the upper plate or cover, increases the capacity of the latter (see Chap. xl.), and thus increases the charge acquired. When the cover receives its positive charge on being connected with the earth, the sole at the same time receives from the earth a negative charge, and as the cover is gradually lifted this negative charge gradually returns to the earth. The most convenient form of the electrophorus is that of Professor Phillips, in which the cover, when placed upon the resinous plate, comes into metallic connection with the metal plate below. That this arrangement is allowable is evident, when we reflect that, when the upper plate is touched with the finger, it is in fact connected with the lower plate, since both are connected with the earth; and it effects a great saving of time when many spark are required in quick succession, for the cover may be raised and lowered as fast as we please, coming alternately into contact with the resinous plate and the body which we wish to charge. 434. Bertsch's Electrical Machine.-A machine which has been called a rotatory electrophorus has recently been invented by Bertsch, and is represented in Fig. 360. A circular plate of ebonite D can be made to revolve rapidly. A sector of the same material, previously BERTSCH'S ELECTRICAL MACHINE. 545 excited by friction, is fixed opposite the lower portion of the plate; and on the other side, immediately opposite to this, is a metallic comb N forming the extremity of a conductor connected with the earth. At the upper part is another comb M connected with the conductor A. Under the influence of the electrified sector, the conductor C discharges positive electricity on the plate through the comb N. In passing the comb M, a portion of this electricity is collected by the points, and charges the conductor A. The effect is increased by connecting A with another conductor E of very large dimensions. Fig. 359.-Electrified Sector. This machine differs from that of Holtz in furnishing no means for increasing, or even sustaining, the charge of the armature. respect it resembles the ordinary electrophorus. In this CHAPTER XXXIX. VARIOUS EXPERIMENTS WITH THE ELECTRICAL MACHINE, 435. Electric Spark.-The spark furnished by an electrical machineof small dimensions is short, and usually straight. Powerful machines. sometimes give sparks of the length of a foot. Such sparks have usually a zig-zag form, like flashes of lightning. One of the readiest means of obtaining long sparks consists in placing, opposite to one of the small knobs of the conductor of the machine, a large conductor, having good earth connection, and presenting a polished and slightly convex surface towards the knob. A more powerful effect will be obtained by connecting this conductor with the rubber or the negative conductor of the machine, instead of with the earth. Very frequently, when the spark is a foot or more in length, finer ramifications proceed from its main track, as shown in Fig. 362. Fig. 361. Electric Spark. 436. Brush.-When a powerful machine is working in a very dry atmosphere, the rubbers being in good order, and the machine being turned rapidly, a characteristic sound is heard, which is an indication of continuous discharge into the air. In the dark, luminous appearances called brushes are seen on the projecting parts of the conductors. They may be rendered very conspicuous by presenting a large conducting surface at a distance a little too great for a spark to pass. It will then be observed that the brush consists (Fig. 363) of a short foot-stalk, with a multitude of rays diverging from it like a fan, and with other smaller ramifications proceeding from these. Positive electricity gives larger and finer brushes than negative. We may add, that, when the the machine is working well, brilliant sparks continually leap across the plate, consisting of discharges between the cushions and the nearest part of the conductor. The conductor itself is also surrounded with luminosity. In the dark, the brilliant spectacle presented by these combined appearances, with the continual crackling which accompanies them, is very impressive, and furnished an inexhaustible subject of curiosity to the electricians of last century. It is probable that the passage of a spark is always preceded by a very high degree of polar tension in all the particles of air in and about its track, and that the spark occurs when this tension anywhere exceeds what the particles are able to bear. The frequent crookedness of the spark is probably Fig. 362.-Spark with Ramifications. due to the presence of conducting particles of dust, which serve as stepping-stones, and render a crooked course the easiest. 437. Duration of the Spark.-We can form no judgment of the duration of the electric spark from what we see with the unaided eye; for impressions made upon the retina remain uneffaced for some 1 thing like of a second, and the duration of the spark is incompar 10 ably less than this. Wheatstone, in a classical experiment, succeeded |