an very well understand and fully
appreciate the statement made by Professor Holtz in _Uppenborn's
Journal_ of May, 1881, wherein he writes that "for the purpose of
demonstration I would rather be without such machines."

The first type of Holtz machine has now in many instances been made up
in multiple form, within suitably constructed glass cases, but when so
made up, great difficulty has been found in keeping each of the many
plates to 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 cha