a like excitement. When differently excited, the one set of plates furnished positive electricity to the comb, while the next set of plates gave negative electricity; as a consequence, no electricity passed the terminal.

To overcome this objection, to dispense with the dangerously cut plates, and also to better neutralize the revolving plate, throughout its whole diameter, I made a large machine having twelve disks 2 ft. 7 in. in diameter, and in it I inserted plain rectangular slips of glass between the disks, which might readily be removed; these slips carried the paper inductors. To keep all the paper inductors on one side of the machine to a like excitement, I connected them together by a metal wire. The machine so made worked splendidly, and your late president, Mr. Spottiswoode, sent on two occasions to take note of my successful modifications. The machine is now ten years old, but still works perfectly. I will show you a smaller sized one at work.

The next machine for observations is the Carre (Fig. 8). It consists essentially or a disk of glass which is free to revolve without touch or friction. At one end of a diameter it moves near to the excited plate of a frictional machine, while at the opposite end of the diameter is a strip of insulting material, opposite which, and also opposite the excited amalgam plate, are combs for conducting the induced charges, and to which the terminals are metallically connected; the machine works well in ordinary atmosphere, and certainly is in many ways to be preferred to the simple frictional machine. In my experiments with it I found that the quantity of electricity might be more than doubled by adding a segment of glass between the amalgam cushions and the revolving plate. The current in this type of machine is constant.

The Voss machine has one fixed plate and one revolving plate. Upon the fixed plate are two inductors, while on the revolving plate are six circular carriers. Two brushes receive the first portions of the induced charges from the carriers, which portions are conveyed to the inductors. The combs collect the remaining portion of the induced charge for use as an outer circuit, while the metal rod with its two brushes neutralizes the plate surface in a line of its diagonal diameter. When at work it supplies a considerable amount of electricity. It is self-exciting in ordinary dry atmosphere. It freely parts with its electricity from either terminal, but when so used the current frequently changes its direction, hence there is no certainty that a full charge has been obtained, nor whether the charge is of positive or negative electricity.

I next come to the type of machine with which I am more closely associated, and I may preface my remarks by adding that the invention sprang solely from my experience gained by constantly using and experimenting with the many electrical machines which I possessed. It was from these I formed a working hypothesis which led me to make my first small machine. It excited itself when new with the first revolution. It so fully satisfied me with its performance that I had four others made, the first of which I presented to this Institution. Its construction is of a simple character. The two disks of glass revolve near to each other and in opposite directions. Each disk carries metallic sectors; each disk has its two brushes supported by metal rods, the rods to the two plates forming an angle of 90 deg. with each other. The external circuit is independent of the brushes, and is formed by the combs and terminals.

Fig. 10.

The machine is self-exciting under all conditions of atmosphere, owing probably to each plate being influenced by and influencing in turn its neighbor, hence there is the minimum surface for leakage. When excited, the direction of the current never changes; this circumstance is due, probably, to the circuit of the metallic sectors and the make and break contacts always being closed, while the combs and the external circuit are supplemental, and for external use only. The quantity of electricity is very large and the potential high. When suitably arranged, the length of spark produced is equal to nearly the radius of the disk. I have made them from 2 in. to 7 ft. in diameter, with equally satisfactory results. The diagram, Fig. 9, shows the distribution of the electricity upon the plate surfaces when the machine is fully excited. The inner circle of signs corresponds with the electricity upon the front surface of the disk. The two circles of signs between the two black rings refer to the electricity between the disks, while the outer circle of signs corresponds with the electricity upon the outer surface of the back disk. The diagram is the result of experiments which I cannot very well repeat here this evening, but in support of the distribution shown on the diagram, I will show you two disks at work made of a flexible material, which when driven in one direction close together at the top and the bottom, while in the horizontal diameter they are repelled. When driven in the reverse direction, the opposite action takes place.

I have also experimented with the cylindrical form of the machine (see Fig. 10). The first of these I made in 1882, and it is before you. The cylinder gives inferior results to the simple disks, and is more complicated to adjust. You notice I neither use nor recommend vulcanite, and it is perhaps well to caution my hearers against the use of that material for the purpose, for it warps with age, and when left in the daylight it changes and becomes useless.

Fig. 13.

Fig. 14.

I have now only to speak of the larger machines. They are in all respects made up with the same plates, sectors, and brushes as were used by me in the first experimental machines, but for convenience sake they are fitted in numbers within a glass case. One machine has eight plates of 2 ft. 4 in. diameter; it has been in the possession of the Institution for about three years. A second, which has been made for this lecture, has twelve disks, each 2 ft. 6 in. in diameter. The length of spark from it is 13⁵/₈ in. (see Fig. 12). During the construction of the machine every care was taken to avoid electrical excitement in any of its parts, and after its completion several friends were present to witness the fitting of the brushes and the first start. When all was ready the terminals were connected to an electroscope, and the handle was moved so slowly that it occupied thirty seconds in moving one-half revolution, and at that point violent excitement appeared.

The machine has now been standing with its handle secured for about eight hours. No excitement is apparent, but still it may not be absolutely inert. Of this each one present must judge, but I will connect it with this electroscope (Figs. 13 and 14), and then move the handle slowly, so that you may see when the excitement commences and judge of its absolutely reliable behavior as an instrument for public demonstration. I may say that I have never, under any condition, found this type of machine to fail in its performance.

I now propose to show you the beautiful appearances of the discharge, and then, in order that you may judge of the relative capabilities of