The instrument at the left below is marked Queen of Philadelphia. The Hampden-Sydney instrument in the second row is by A.P. Gage, while the one from Denison is by E.S. Ritchie, both of Boston.
The three instruments below use astatic
needles to cancel the effects of the earth's magnetic field. The picture in
the middle shows a closeup of the interior of an instrument which is just
like the one at the left. Two parallel groups of magnetic needles are attached
to the torsion fiber, with the upper set pointing in the opposite magnetic
direction to the lower set. The net magnetic moment of the needles is thus
zero, and they do not respond to the earth's magnetic field. Unlike the simple
form of astatic
galvanometer, both sets of needles are in the middle of a pair of coils.
On one side, the upper and lower coils are connected in series, with the current
passing through them in opposite directions. Thus, there is twice the torque
on the needles. The coils on the other side can be connected to a separate
circuit, thus making the instrument a differential galvanometer. Or, they
can be connected in series with the other coils, thus doubling the sensitivity
of the instrument once more.
The Duke instrument is marked "Willyoung", but is clearly the same as the Leeds and Northrup instrument in the center picture. Elmer Willyoung sold his business to Morris E. Leeds in 1893, and in 1903 Leeds joined forces with Edwin Northup to form Leeds & Northrup.The cost in the 1903 L & N catalogue was $65. The Denison instrument at the right is by Ritchie. All three instruments have a magnetic compensating bar (below the base in the Leeds and Northup galvanometers and on top in the Ritchie galvanometer), allowing any residual effects of the earth's magnetic field to be offset.
The small motions of the needles are amplified by means of
a beam of light reflecting from the small mirror between the two sets of
| At the left is a beautiful differential galvanometer from
the University of Vermont in Burlington.
It was made by Latimer, Clark and Muirland & Co, Ltd of Westminster [London], and is the only apparatus by this company I have ever seen in the United States.
| When I spent a day in March 2001 photographing portions
of the Garland Collection of Classical Physics Apparatus at Vanderbilt University,
I photographed quite a number of galvanometers. I think that this
differential galvanometer is the one described in the 1983 catalogue to the
collection, written by Prof. Robert T. Lagemann as:
"Galvanometer: Standing 21 cm high and supported on three adjustable brass legs, this galvanometer is marked "Th[?] A. DUBOSCQ / A PARIS / QUEEN & CO. SOLE AGENTS / in PHILADELPHIA". A Central column supports the end of the thread or wire suspending the needle."
|| This differential galvanometer by Morris E. Leeds of Philadelphia
I photographed at the Unversity of Mississippi in November 2002. It is part
of the Millington/Baranrd Collection in the University Museum.
The apparatus was made between 1898 and 1903.
The unmarked differential galvanometer at the right is in the apparatus collection at Case Western Reserve University in Cleveland, Ohio.