Galvanometers
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.
Astatic Galvanometer
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.
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Our setup to demonstrate that current flow produce magnetic field.
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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 |
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