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2015
A Modern Bréguet-Type Galvanometer
Tom Greenslade
Kenyon College, [email protected]
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Recommended Citation
“A Modern Bréguet-Type Galvanometer”, The Physics Teacher, 53, 289-290, plus additional material in website (with John Daffron)
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A Modern Bréguet-Type Galvanometer
John A. Daffron and Thomas B. Greenslade Jr.
Citation: The Physics Teacher 53, 289 (2015); doi: 10.1119/1.4917436
View online: http://dx.doi.org/10.1119/1.4917436
View Table of Contents: http://scitation.aip.org/content/aapt/journal/tpt/53/5?ver=pdfcov
Published by the American Association of Physics Teachers
Articles you may be interested in
Edward Weston and the “Modern” Galvanometer Movement
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Hidden Amplifier for a Galvanometer
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Galvanometers
Phys. Teach. 35, 423 (1997); 10.1119/1.2344745
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A Modern Bréguet-Type Galvanometer
John A. Daffron, Scientific Instrument Maker, Memphis, TN
Thomas B. Greenslade Jr., Kenyon College, Gambier, OH
R
ecently the handsome little galNew York. Jesse Bunnell (1843-1899)
vanometer (only 10 cm high) in
was a famous Civil War telegrapher
Fig. 1 came into the Greenslade
who set up a company making teleCollection. This is a design without a
graph and associated instruments in
moving coil and, consequently, is very
1878. This instrument dates from ca.
rugged. At the same time, John Daffron
1900 and once had a glass dome. This
independently sent Tom Greenslade the
form of galvanometer seems to have
picture of a similar galvanometer that
been developed by Louis-Françoishe had made some years back and has
Clement Bréguet (1804-1883), probbeen using for experimental work since
ably as an adjunct to his work with
then. These galvanometers are easy to
telegraphy. He was part of the watchbuild and can still be used in the lecture
making firm of the same name that
hall.
survives to this day.
At its heart, a galvanometer is a
John Daffron has designed several
device that converts a (small) electric
modern versions of the basic Bréguet
current into some sort of movement.
Fig. 1. A Bréguet-type galvanometer in the design. Figure 3 shows his designs,
Greenslade Collection.
If the device is calibrated, it is a galvastarting with a classic shape (but this
nometer, else it is a galvanoscope. The
time with a glass dome) on the leftcurrent passes through a coil producing
hand side. The two instruments in the
a magnetic field that interacts with a
middle have the same meter movenearby permanent magnet to produce
ment, but differ in their housings. One
the motion. The familiar D’Arsonval
is placed in two 3-L plastic soda bottles
galvanometer movement, developed
placed end to end, while the other one
by Jacques D’Arsonval (1850-1940) and
has a case made of balsa wood. The one
refined by Edward Weston (1850-1936),
on the right has an inverted movement,
has a fixed U-shaped permanent magwith the pointer facing downward,
net between which the current-carrying
and uses an empty ketchup bottle as a
coil pivots.1,2 On the other hand, the
housing. Here the scale, drawn on foam
board, can be shifted from side to side
galvanometer in Fig. 1 has a fixed coil
to adjust the zero reading.
and a movable bar magnet.
These instruments exhibit greater
In Fig. 2 the interior mechanism has
sensitivity than the original apparatus
been removed so that it may be examined. A magnetic disk is supported on
Fig. 2. The galvanometer disassembled to show due to the availability of modern magnets. Several magnets may be added to
small projections with knife edges on
the pivoting magnet.
either side of the pivot, increasing the
their lower sides. The disk is magnemagnetic field and magnetic moment of the armature. Typitized with its north and south poles along the curved sides.
cal full-scale deflection for these galvanometers is  300 μA
You can see that the left-hand side of the magnet was painted
for the tall model and  1 mA for the ketchup bottle model.
red at one time, probably indicating that this was the northThe sensitivity may be changed at will by adjusting the posiseeking pole. A counterweight on the bottom of the disk lowtion of the counterweight.
ers the center of mass of the system below the support axis, so
Apart from the ketchup bottle instrument, the coils are
that the pivoting system is in stable equilibrium.
either wound on copper tubing fittings or are placed around
Passing a current through the coil sets up an axial B field,
these fittings; eddy currents induced in the copper serves to
and the pivoted magnet turns to try to align itself with this
damp out unwanted oscillations in the moving system. The
field. At some point the gravitational torque and the magnetic
knife edges on which the system pivots are literally that: segtorque on the pivoting system balance, and the needle comes
ments of utility-knife blades that rest on cylindrical glass
to rest. An analysis shows that the response of the needle is
beads. The three galvanometers on the right-hand side of Fig.
linearly proportional to the current for small displacements
3 can easily be constructed by high school students. Detailed
of the needle, thus making it possible to calibrate the meter,
instructions may be found at TPT Online.3
although not from first principles.
The instrument in Fig. 1 was made by J.H. Bunnell & Co. of
You might ask if there is a place in the modern physics
DOI: 10.1119/1.4917436
The Physics Teacher ◆ Vol. 53, May 2015
289
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4. Albert A. Bartlett, “Torsional oscillations and waves projected
onto the wall,” Phys. Teach. 46, 338–340 (Sept. 2008).
Thomas B. Greenslade Jr., Kenyon College, Gambier, OH
43022; [email protected]
in The Physics Store
Fig. 3. Four modern-day Bréguet galvanometers designed and
built by John Daffron.
course for a revival of a galvanometer designed 160 years ago.
After all, one can buy digital multimeters that give readings
to three figures. You have to take the numbers on trust, for
they are produced by a sealed box, and there is no way to tell
if they are accurate.
The Daffron design can be remarkably sensitive. The instrument shown on the left side of Fig. 3, designed to measure
the output of thermo-junctions, goes full scale at 1 mA or less
depending upon the position of the counterweight. This zerocenter design is useful for demonstrations such as thrusting
a bar magnet into a coil of wire and then withdrawing it;
the needle goes first to one side and then to the other. These
instruments may be used for any demonstration requiring a
lecture table galvanometer.
More important, these designs have their basic physical
principles in full view; the magnet and the coil can be clearly
seen. The late Al Bartlett, writing in The Physics Teacher
about a demonstration of Lissajous figures involving a laser
beam reflected from mirrors on two tuning forks vibrating at
right angles to each other, wrote, “This is the sort of demonstration that will bring students down to the lecture table at
the end of the class…. Students don’t bother to come down to
see how a computer produces Lissajous figures because there
is nothing to see.”4 These instruments should evoke the same
response. Further on in the article, Bartlett wrote, “It would
be wonderful if Tom Greenslade would prepare articles on
the ingenious ways of the ancient physicists so that today’s
teachers could use these old techniques to enhance the contemporary presentation of physics.” Daffron and Greenslade
are following his suggestion.
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References
1. 2. 3.
290
Thomas B. Greenslade Jr., “Galvanometers,” Phys. Teach. 35,
423–426 (Oct. 1997).
Thomas B. Greenslade Jr., “Edward Weston and the ‘modern’
galvanometer movement,” Phys. Teach. 46, 162–164 (March
2008).
See supplementary material at TPT Online, http://dx.doi.org/
10.1119/1.4917436.
The Physics Teacher ◆ Vol. 53, May 2015
Order yours now at
www.aapt.org/store
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