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.

[Illustration: Figs. 11 & 12.]

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 13-5/8 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 com