Toward the end of 18th century, Volta discovered
that two dissimilar metals discs stacked alternating on electrolyte impregnated
pads produce an "electric medium" flow. How Volta measured electric
flow and potentials to be able to come up with his electrochemical series?
I have no idea. Probably he stuck up enough metallic disks to produce
a measurable electric shock or the intensity of twitching of a dead frog
leg in series with a Volta electrolytic pile depended on the type of metals
used. Stronger the shock or the twitching, larger the electric medium
flow was, I presume. At the time of his discovery Volta was experimenting
with electrophorus and probably built understanding using a hydraulic
model. Twenty years later,Oersted observed accidentally that a compass
needle is deflected whenever the electric current passes nearby. Everything
else was easy from now on and hundred of designs were invented.
The tangent galvanometer was first described in an 1837
paper by Mathias Pouillet. The design must be similar to our set up or
the other tangent galvanometers in our collection. The magnetic field
produced by the bundle of wires combines with the magnetic field of the
earth. A magnetized needle will show the direction of this combination
and can be taken as a measure of the current passing through the wires.
A serious defect of the galvanoscope and other simple galvanometers is
that the needle responds to the vector sum of the horizontal component
of the magnetic field of the earth and the magnetic field produced by
the current being measured. As long as the galvanometer is being used
for qualitative measurements, it is sufficient to turn the apparatus so
that the compass needle points toward magnetic north when no current is
applied to the coil. The development of the astatic magnetic needle by
Leopoldo Nobili, reported in 1825, eliminated the effect of the
earth's magnetic field on the needle. A pair of needles with poles reversed
is used. One magnetic needle is inside the coil and one outside. This
combination is not sensitive to earth's magnetic field but is subject
to torque due to current through coil. A supporting silk wire is providing
the opposing momentum. D'Arsonval Galvanometer was invented,
in the early 1880s, by Jacques D'Arsonval and Marcel Deprez. In the D'Arsonval-Deprez
design, the coil has many turns of fine wire, and is suspended by a conductive
wire. The connection to the lower end of the coil is provided by a light,
helical spring. I am not sure what provide the opposing tork: the lead
in wire or the spring below. I always thought the lead wire balance the
electromagnetic torque but I read other ways. The electro-magnetic torque
is greatest when the magnetic field lines are perpendicular to the plane
of the coil; this condition is met for a wide range of coil positions
by placing the cylindrical core of soft iron in the middle of the magnetic
gap, and giving the magnet pole faces a concave contour. Since the electro-magnetic
torque is proportional to the current in the coil and the restoring toque
is proportional to the angle of twist of the suspension fiber, at equilibrium
the current through the coil is linearly proportional to its angular deflection.
This means that the galvanometer scales can always be linear.
Oerstead Demonstration Apparatus
Our setup to demonstrate that current flow produce magnetic field.
The second picture was taken when 2.6 A circulate through the heavy
wire around the compass. The needle was deflected almost 45 degree.
In the 1916 catalogue of the L.E. Knott Apparatus Company of Boston
this astatic galvanometer is described as designed according to
the requirements of the National Physics Course, built on the close-coil
type, with astatic system in which one needle moves between the
coils and one above. Windings of No. 18 copper wire, resistance
approximately one ohm. The instrument is provided with three leveling
screws and binding posts. A convenient and sensitive instrument
where low voltage is to be used
Gambrell Bros 5612 Galvanometer. I bought this one in Montreal
for $ 20. The seller excused himself: "this one does not
show to much"
Griffin & Tatlock D'Arsonval galvanometer with superb red
gold finish. Built like a tank although have very fragile core
and no provision for "park" the fragile movement.
Cast iron D'Arsonval galvanometer. It is not marked although I
am sure I seen and identified it before.
Cast iron D'Arsonval galvanometer by Knot, Boston Apparently has
a mercury bath for the low contact (probably I am wrong).
A top table galvanometer good for class room. Everybody will witness
"the presence of electric current.
A famous Top Table galvanometer by Central Scientific. patented
in in 1901(?) displays dramatically milliamperes in the circuit.
This small instrument is listed in Leeds and Northrup of Philadelphia
catalog in 1907 as a portable d'Arsonval galvanometer. Ours has
serial No. 7405. Probably, was manufactured by Thompson-Levering
Co., Makers of Scientific Instruments, Philadelphia, Pa.
The museum will be opened in September and October
2014 by appointment only. Give Nick a call at 716 282 2975 to schedule
a visit (one day notice, please). Nick will be happy to show you our
Admission is free! A $10 donation/family is suggested.