Similar experiments to that described above have been carried out by
Prof. Fernando Sanford. He placed a coin on a dry-plate and connected
it with the terminal of a small induction coil, capable of giving a
spark of three or four millimeters, while a piece of tin foil upon the
opposite side of the plate was connected with the other terminal of
the coil.
Several negatives were made in this way, the accompanying photograph,
Fig. 58, being from one of them. With one exception, they all show a
fringe around them, due to the escape of the charge from the edge of
the coin, which accounts for the formation of the dark ring observed
around the breath figures made upon glass.
Later on he undertook to photograph in the same way objects insulated
from the photographic plate, and has since made negatives of coins
separated from the plate by paraffine, shellac, mica, and gutta
percha. The accompanying photograph, Fig. 59, was made with the coin
insulated from the photographic plate by a sheet of mica about 0.04
mm. thick. The mica was laid directly upon the film side of the plate,
and the coin was placed upon it and connected to one terminal of the
small induction coil already mentioned. A circular piece of tin foil
of the circumference of the coin was placed upon the glass side of the
plate directly opposite the coin, and was connected to the other
terminal of the induction coil. The little condenser thus made was
clamped between two boards, and was covered up in a dark room. Two
small discharging knobs were also attached to the terminals of the
induction coil, and were separated by a space of less than a
millimeter, so that, when a single cell was connected with the primary
coil, the spark between the knobs seemed continuous.
[Illustration: FIG. 58.]
[Illustration: FIG. 59.]
The plate was exposed to the action of the waves set up in this
condenser for one hour, when it was taken out and the negative image
developed upon it by the usual process.