Galvanometer

History

The apparatus owes its name with Luigi Galvani. Even if William Thomson (Lord Kelvin) had already had the idea to use similar apparatuses to measure and record currents, the first galvanometer was built by Johann Schweigger of Nuremberg to the Université of Market on September 16th 1820. Andre-Marie Ampère contributed then to the development of the galvanometer. Leopoldo Nobili improved the instrument by withdrawing it from the influence of the terrestrial magnetic field (astatic galvanometer). Arsène d' Arsonval invented a model adapted to the measurement of very weak currents in electrophysiology (ballistic galvanometer).

Tally mobile

A moving-coil galvanometer, also called electric magneto, or movement of Arsonval, consists of a winds assembled on pivot, bathing in the Magnetic field of a Aimant fixes, on this reel is fixed the needle of visualization and a Ressort charged to point out the moving element in the position indicating the zero.

the reel of weak impedance is connected in series in the circuit where circulates the current to measure.
the current, while crossing the reel, armature in this one a electromagnetic Champ, which causes a swivelling by repulsion of the magnetic fields. More the current is intense plus the reel rocks.

This system is most precise, but also most fragile. It functions only in D.C. current. The electric apparatuses tape recorder measure median values.

Ferromagnetic

The ferromagnetic galvanometer uses two soft iron pallets inside a reel.

One of the pallets is fixed, the other interdependent one of the needle is assembled on pivot.
When the power is on in the reel, the two pallets magnetize and are pushed back, whatever the direction of the current.
As for the mobile framework a Ressort is charged to point out the pallet in the position indicating the zero.

This assembly is often used for the bill-posters of tables. It is of average precision, but functions as well into alternate as uninterrupted. The moving-iron instruments measure effective values.

Thermics

A thermal galvanometer is composed of a wire in which the current circulates. This wire warms up and lengthens, which causes the movement of a needle.

This assembly is most robust of the three, but they is also the least precise and he also suffers from very slow reactions.

General characteristics

A galvanometer is characterized primarily by:

its sensitivity " s" : it gives the number of divisions of the scale of reading corresponding to a current fluctuation of a unit (generally the microampere). For the apparatuses with spot of light, the sensitivity is often expressed in millimetres per microampere.
its resistance interns " ri"
its class of precision: it gives, expressed as a percentage gauge, absolute uncertainty on the measured value.

Advantages and disadvantages

These assemblies allow more or less precise measurements.

Associated with a rectifying assembly, a variable current and tension divider, it makes it possible to measure: as well tensions as of the continuous and alternating currents.
While adding to a system with mobile framework a pile for the food of a resistant circuit one obtains a Ohmmètre to measure the resistances.
small instantaneous displacements of the needle make it possible to visualize certain variations of the measured signal, difficult to translate on a digital display panel.

This assembly is based on a Mécanique of precision, therefore rather fragile, sensitive to the vibrations.

the precision of this apparatus became insufficient for many applications of point of the electronic .
the ferromagnetic galvanometer can take valid measurements only on sinusoidal alternative courses.
the electronic systems of measurement became largely simpler of use, more ergonomic and definitely more precise.

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