IMPROVEMENTS IN ELECTROMETERS, AND IN APPARATUS FOR APPLYING ELECTRICITY TO GIVE TELEGRAPHIC SIGNALS AND WORK TELEGRAPHIC RELAYS, AS ALSO IN STANDARDS OF ELECTRO-MOTIVE FORCE TO BE EMPLOYED IN THE GRADUATION OF ELECTROMETERS, AND FOR OTHER USES."
Year: 1876

page_text: PROVISIONAL SPECIFICATION left by James Dewar at the Office of the
Commissioners of Patents on the 14th July 1876.
JAMES DEWAR, of Brook Side, Cambridge, Jacksonian Professor at the University
of Cambridge. "IMPROVEMENTS IN ELECTROMETERS, AND IN APPARATUS FOR
APPLYING ELECTRICITY TO GIVE TELEGRAPHIC SIGNALS AND WORK TELEGRAPHIC
RELAYS, AS ALSO IN STANDARDS OF ELECTRO-MOTIVE FORCE TO BE EMPLOYED IN THE
GRADUATION OF ELECTROMETERS, AND FOR OTHER USES."
Heretofore in electrometers and in apparatus for applying electricity to give tele-
graphic signals and work telegraphic relays it has been proposed to employ a hori-
zontal capillary tube of about one-sixtieth of an inch in connection at one or both
ends with a cup or reservoir, the capillary tube having a globule of electrolytic
liquid between two columns of mercury in the cups or reservoirs, or vice versa;
but such arrangement was uncertain in action, whilst in electrometers and ppa-
ratus for giving telegraphic signals it necessitated the use of microscopes or other
enlarging apparatus to enable the motions of the globule of electrolytic liquid' or
the globule of mercury to be readily seen.
Now according to the first part of my Invention I avoid these difficulties by dis-
pensing with the use of a capillary tube and by employing a horizontal tube of
larger calibre, say about one-twelfth of an inch, which by offering little or no
capillary resistance to the motions of the globule of electrolytic liquid or of mercury
will render the apparatus much more sensitive to the action of electricity, and
thereby cause the globule of electrolytic liquid or of mercury to traverse a greater
distance for a given electric force than when a capillary tube is employed in the
manner heretofore adopted.
This arrangement also dispenses with the necessity for using a microscope or
other enlarging apparatus, and enables the motions of the globule of electrolytic
liquid or of mercury to be readily seen by the naked eye. The area of the bore of
the cups or reservoirs in relation to that of the horizontal tube will be proportioned
to the delicacy required in the instrument.
This arrangement of apparatus also enables the exact measurement of electro-
motive forces to be obtained in comparison with a given standard as the distance
traversed by the globule of electrolytic liquid or of mercury in the horizontal tube
is proportional to the electro-motive forces acting thereon. The motions of the
globule of mercury or of electrolytic liquid in the horizontal tube may be recorded
on a cylinder by means of a marking instrument set in motion by a fine wire
passing out through a small hole in the horizontal tube, such wire being connected
[Price 20.1

page_text: to a moveable float worked by the globule of electrolytic liquid or of mercury. The
electro-motive force instead of being measured by the distances traversed by the
globule of electrolytic liquid or of mercury in the horizontal tube may be measured
by additional weights of mercury added in either of the cups or reservoirs required
to retain such globule in the normal position when charged electrically.
The second part of my Invention relates to standards of electro-motive force by
which apparatus constructed according to the first part of my Invention, and other
electrical apparatus, may be graduated and compared.
For this purpose I employ a cup or reservoir of mercury having at the lower end
thereof a capillary orifice dipping into an electrolytic liquid. The height of mercury
in the cup or reservoir must be maintained at a constant level above the capillary
orifice or of the level of the electrolytic liquid. Thus by the dropping of mercury
from the capillary orifice into the electrolytic liquid a definite electric potential is
obtained, depending in amount on the diameter and length of the capillary tube
and the height of mercury above the orifice, which must be kept constant in order
to produce an invariable potential..
The electro-motive standard conveniently obtained and suitable for the purposes
of my Invention, and for other purposes, is equal to about one-third of a Daniell's
cell, but may vary as above described. In every case the amount must be found
by comparison with a standard battery or other means of comparison.