MAGNETISM AND ELECTRICITY. 151 
the arrangement of the rubbers. Here also the plate is partly covered by 
pieces of oiled silk to prevent the escape of electricity. The most powerful 
plate machines now constructed consist of two disks on the same axis, each 
with its set of rubbers. 
The great plate machine of Van Marum (pi. 20, fig. 31) is distinguished 
from the preceding by admitting the collection of either positive or negative 
electricity. The two rubbers are placed at the extremity of the horizontal 
diameter of the disk, and are attached to two wooden globes, sustained on 
glass posts. AB and CD are two movable metal arcs, their planes at 
right angles to each other. 
To determine the degree to which the prime conductor is charged, we 
make use of the quadrant electrometer of Henley (pl. 22, fig. 59). Its 
construction will be evident from an inspection of the figure. The greater 
the charge of the conductor, the greater will be the ascent of the cork ball, 
this being repelled from the electrified foot of the electrometer. A 
graduated semicircle indicates the angle of divergence. 
The electrophorus ( pl. 20, fig. 27) in many instances may advantageously 
replace the electric machine. It consists of a cake of resin, sealing-wax, 
or mixture of shellac and Venetian turpentine, poured into a shallow disk, 
or upon a metal plate. The surface must be as smooth and polished as 
possible. The latter is electrified negatively by rubbing with a fox’s tail or 
cat's skin. A plate of metal, somewhat less in diameter than the cake of 
resin, and provided with a glass handle, is now to be laid upon it. The 
negative electricity of the lower cake decomposes the combined electricity 
of the upper, attracting its positive and repelling its negative fluid. On 
touching the upper surface of the metal plate with the finger, this negative 
electricity passes off, and on lifting the plate by its glass handle, it will be 
found to be charged positively. It will now give off a spark, a succession 
of which may readily be obtained, without further excitation, by touching 
the plate with the finger, replacing it as before, and thus continuing the 
operation. ig. 28 is an enlarged representation of the edges of the 
plate. 
It has been ascertained quite recently that a jet of steam, escaping from 
a narrow aperture, is electrified positively, and upon this fact has been 
founded the construction of the hydro-electric machine (pl. 22, figs. 73-75). 
This consists, in the arrangement of Eisenlohr, of a boiler supported on four 
glass legs. F#ig.'75 is a section of the boiler, showing the method of heating 
the water. Upon the middle of the boiler is placed a cap, in which is 
screwed a short brass tube, which can be closed by a cock. Upon the 
latter the escape apertures are screwed, as shown in fig. 74. There 
are six horizontal tubes passing through a tin box, filled with cold water, 
which serves to condense a part of the escaping steam. When the steam 
is of sufficient tension it is made to escape with great violence by turning 
the cock (fig. 73) a quarter round. On account of the friction of the 
steam against the sides of the escape pipes, the two become oppositely 
electrified. To obtain the most intense action of the machine, it is 
necessary to draw off the electricity of the steam, which is done by 
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