A ballistic galvanometer is a type of sensitive
galvanometer
A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. Galvanomet ...
; commonly a
mirror galvanometer
A mirror galvanometer is an ammeter that indicates it has sensed an electric Current (electricity), current by deflecting a light beam with a mirror. The beam of light projected on a scale acts as a long massless pointer. In 1826, Johann Chri ...
. Unlike a current-measuring galvanometer, the moving part has a large
moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular/rotational mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is defined relatively to a rotational axis. It is the ratio between ...
, thus giving it a long
oscillation
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
period. It is really an
integrator
An integrator in measurement and control applications is an element whose output signal is the time integral of its input signal. It accumulates the input quantity over a defined time to produce a representative output.
Integration is an importan ...
measuring the quantity of
charge
Charge or charged may refer to:
Arts, entertainment, and media Films
* ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary
Music
* ''Charge'' (David Ford album)
* ''Charge'' (Machel Montano album)
* '' Charge!!'', an album by The Aqu ...
discharged through it. It can be either of the moving coil or moving magnet type.
Before first use the ballistic constant of the galvanometer must be determined. This is usually done by connecting to the galvanometer a known
capacitor
In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
, charged to a known
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
, and recording the deflection. The constant K is calculated from the
capacitance
Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...
C, the voltage V and the deflection d:
where K is expressed in
coulomb
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI).
It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...
s per centimeter.
In operation the unknown quantity of charge Q (in coulombs) is simply:
.
Grassot Fluxmeter
The Grassot Fluxmeter solves a particular problem encountered with regular galvanometers.
For a regular galvanometer, the discharge time must be shorter than the natural period of oscillation of the mechanism. In some applications, particularly those involving inductors, this condition cannot be met. The Grassot fluxmeter resolves this problem, by operating without any restoring force, making the oscillation period effectively infinite and thereby longer than any discharge time.
Its construction is similar to that of a ballistic galvanometer, but its coil is suspended without any restoring forces in the suspension thread or in the current leads. The core (
bobbin
A bobbin or spool is a spindle or cylinder, with or without flanges, on which yarn, thread, wire, tape or film is wound. Bobbins are typically found in industrial textile machinery, as well as in sewing machines, fishing reels, tape measures ...
) of the coil is of a non-conductive material. When an electric charge is connected to the instrument, the coil starts moving in the magnetic field of the galvanometer's magnet, generating an opposing
electromotive force
In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transducer ...
and coming to a stop regardless of the time of the current flow. The change in the coil position is proportional only to the quantity of charge. The coil is returned to the zero position manually or by reversing the direction of the current.
References
{{Commons category, Ballistic galvanometers
* Earle Terry, ''Advanced Laboratory Practice in Electricity and Magnetism.'' McGraw-Hill, New York 1929 Page 24-34
* ''Electrical Instruments, "Tylor-Cambridge"'', Cambridge Scientific Instrument Company, Trade catalog, 1908 Page 34
* Bakshi, U.A. and Bakshi, L.A.V., ''Electronic Measurements and Instrumentation'', Technical Publications, 2020, Chapter 2
* Bakshi, U.A. and Bakshi, L.A.V., ''Electric Measurements and Instrumentation'', Technical Publications, 2020, Chapter 2
Galvanometers
Historical scientific instruments