John Locke's Electro-Chronograph
The diagram of the recording apparatus part of John Locke's Electro-Chronograph shown to the left comes from a plate in his
Report
of Professor John Locke, of Cincinnati, Ohio, on the Invention and
Construction of His Electro-Chronograph for the National Observatory,
in Pursuance of the Act of Congress, Approved March 3, 1849. (source)
The Great Astronomical Clock (1873)
The
Great Astronomical Clock, or "Electro-Chronograph" is placed in the
same room with the Transit-Instrument, and is used in connection with
it to denote sidereal time. It was invented by Professor John Locke, of
Cincinnati, and is one of the most remarkable instruments in the world.
By means of an electrical battery in the building, the movements of
this clock can be repeated by telegraph in any city or town in the land
to which the wires extend. With the wires connected with it, its ticks
may be heard in any part of the country, and it will record the time so
accurately than an astronomer in Portland or New Orleans can tell with
exactness the time of day by this clock. It also regulates the time for
the city. There is a flag-staff on top of the dome, upon which a black
ball is hoisted at ten minutes before noon, every day. This is to warn
persons desiring to know the exact time to examine their watches and
clocks. Just as the clock records the hour of twelve, the ball drops,
and thus informs the city that it is high noon.
From:
Ten Years in Washington: Life
and Scenes in the National Capital, as a Woman Sees Them, by Mary Clemmer, publ. A.
D. Worthington & Co. (1873), page 516
Practical Astronomy (1876)
[How the electro-chronograph was used in astronomical and longitude measurements]
The
automatic registration of time observations by means of
electro-magnetism is an improvement in practical astronomy due to
American ingenuity. The merit of its first suggestion has been somewhat
in dispute, but the earliest experimental demonstration of its
feasibility was certainly made by Professor John Locke, of Cincinnati
and Pittsburgh. The distance is four hundred miles, and the experiment
was continued for two hours, during which the beats were regularly
registered at every station throughout the whole line. The application
to astronomical observation immediately followed. In recognition of the
value of this invention, Congress awarded to Dr. Locke the sum of ten
thousand dollars, and ordered a clock of the same description to be
constructed for the Naval Observatory. As a recording instrument, the
ordinary telegraphic register of Professor Morse was at first employed.
More convenient forms of apparatus were subsequently devised by
Professor Mitchell, Mr. Joseph Saxton, of the Coast Survey, and Messrs.
W.C. and George P. Bond, who introduced the regulator which has since
been so almost universally employed in these instruments, known as
Bond's spring governor. More recently (1871) a printing chronograph has
been invented by Professor George W. Hough, of the Dudley Observatory,
which records to the hundredths of a second, and saves to the observer
who employs it the labor and time required for deciphering and
recording in figures the indications of the register in common use. The
electro-magnetic method of recording transits was adopted without delay
in the observatories of the United States and soon after found its way
into those of Great Britain and the continent of Europe, where it was
known as the American method. Of its great value in promoting accuracy
it is not necessary to speak; but only those who can adequately
appreciate the degree to which it has lightened the labor of the
observer. Previously to its introduction the clock divided with the
object viewed the observer's attention, and the necessity for unceasing
vigilance was exhausting in the extreme. If nothing else had been
gained by it but this, the benefit would be incalculable.
The introduction of the electric chronograph into observatories
furnished a very simple means of determining differences of longitude
between any two places connected by a telegraph wire. These
determinations are made by comparing the exact times of transit of a
given celestial object over the meridians of both places, a single
clock giving the times for both, or by transmitting time signals
alternately in opposite directions compared with clocks at both ends.
The earliest observations of this kind were made in January, 1849,
between Washington and Cambridge, Massachusetts. The method has since
been brought into very extensive use throughout the world. In 1867 and
again in 1871 and in 1872, it was employed to determine the difference
of longitude between Greenwich and Washington, by means, in the first
instance, of the Anglo-American cable, and, in the second and third, of
the French, from Brest to St. Pierre, and Danbury , Massachusetts. It
may be interesting to compare those results with those of the great
chronometric expeditions of 1849 and 1855 between Cambridge and
Liverpool—expeditions which, in the words of Mr. W.C. Bond,
"for the
magnitude and completeness of their equipments have not been equaled by
any of the similar undertakings of European governments. Even the
'Expedition Chronometrique' of Struve was on a scale much less
extensive." In 1855 fifty-two nautical chronometers were transported
six times between Cambridge and Liverpool, giving nearly three hundred
individual longitude determinations. The difference of longitude
obtained was 4
h.
44
m. 30.6
s., showing a
difference between the two of 1.2
s.
The cable results (omitting hours and minutes) were: 1867, 31.00
s.;
1871, 30.96
s; 1872, 30.99
s., and the largest discrepancy being only
four one-hundredths of a second.
In observing for longitude,
the velocity of propagation of electric impulses in the wires of the
circuit becomes a matter requiring attention, and thus the telegraph
has become the means of throwing light upon this interesting question
in physics. The results obtained have differed very widely, being
dependent on differences of material of the conductor, differences of
cross-section, and largely upon differences of surrounding conditions.
In the ordinary iron wires of the American telegraphic lines the
velocity seems not to exceed fifteen or sixteen thousand miles per
second.
From
The
First Century of the Republic: A Review of American Progress,
publ. Harper & Bros., 1876. pages 300-301.
