Galvanometer
The galvanometer, a device used to measure extremely small electrical
currents, traces its origin back to 1820. In that year Hans Christian
Oersted (1777-1851) discovered that an electric current flowing in a
wire created a magnetic field around it, deflecting a magnetized needle.
This effect became the basic principle behind the galvanometer. In the
same year André Ampère (1775-1836) used the effect to invent a
device to measure electric current. He suggested it be called the
galvanometer, in honor of Luigi Galvani (1737-1798), a pioneer in the
investigation of electricity.
The first practical use of the galvanometer was made by Karl Friedrich
Gauss in 1832. Gauss built a telegraph that sent signals by deflecting a
magnetic needle. This style is known as a moving-magnet
galvanometer. More commonly used today is the moving-coil or
moving-mirror galvanometer, sometimes called a D'Arsonval
galvanometer.
The invention of the moving-coil galvanometer is credited to Johann
Schweigger in 1825, three years later Italian physicist C. L. Nobilli
designed an astatic type. It consists of a coil that has been wound with
very fine wire mounted between the poles of a permanent magnet.
Attached to the coil is a pointer. When electric current is turned on, the
coil turns and the deflection angle is measured as the pointer moves
along a graduated scale.
In the case of a moving-mirror galvanometer, a mirror is attached to the
coil, and illuminated with light. When the coil moves the deflection of the
light is measured along a scale. The mirror galvanometer was of major
use in laying the transatlantic telegraph cable between the United
States and Europe in 1866. William Thomson, later known as Lord
Kelvin, used it to keep track of how much electric current was coursing
through the cable. Thomson also invented a "siphon recorder," which
was a more sensitive galvanometer. Ink was siphoned through a thin
glass tube that was attached to the coil of wire which was mounted
between the poles of a horseshoe magnet. The moving tube carried the
ink onto a paper tape where it traced a line.
Galvanometers come in a variety of types. Ultraviolet recorders use
light-sensitive paper and ultraviolet light in place of ink. A photoelectric
galvanometer amplifies the signal using a photocell. The ballistic
galvanometer is used to measure an electric pulse or burst. A cousin of
the galvanometer is the direct current ammeter, which is a calibrated
galvanometer that measures larger currents. Another cousin still is the
direct-current voltmeter, which uses Ohm's Law to measure voltage.
Digital display galvanometers, the best of which can measure a current
as small as one hundredth billionth (10-11) of an amp, have almost
entirely replaced the early analog galvanometers of yore.
RECORDER DEFINITIONS
http://www.omega.com/temperature/z/selectrecorder.html
Hybrid recorder: A recorder that combines analog trend
representation and digital measured value printing on the
same chart paper, without disruption of trend printing.
Servo balancing: A means of positioning the pen of a drag
pen recorder. Null-balance operation has no current flow at
balance, nullifying the effect of lead resistance.
Conventional servo balancing recorders use contact
mechanisms in the feedback loop and brushes in the servo
motor. New technology allows the use of a noncontact pen
positioning transducer and a brushless dc servo motor.
Scanning recorder: A multi-point recorder that scans all of
its inputs to obtain new measured data every set time
period (usually 2 to 6 seconds). Printing for all points is
often performed during each cycle of the printing
mechanism.
Multi-color printing: A recorder that records trend traces
in more than one color to make traces easier to
differentiate. Drag pen recorders use a different color for
each pen (usually four pens maximum). Mulit-point
recorders typically record in six colors.
Linear scaling: Recording of a voltage input in terms of the engineering variable, such as
temperature, that the voltage represents. Transformation is Y (variable to be recorded) = mX
(slope x input signal) + b (Y intercept).
Pen offset compensation: In traditional multiple input drag pen recorders, each pen can travel
the full width of the recording chart. In order to do so, the pens must be physically offset from one
another. This puts the different pen traces on different time lines of the chart. By placing the
measured data of the front-most pen(s) into a buffer and delaying their printing, the traces can be
synchronized to the same time line, thereby compensating for their offset.
Accuracy: The closeness to the actual signal that the measured value or trend position takes,
stated as either a percentage of full scale or percent of reading. Separate accuracy statements
are typically provided for measuring and recording functions.
Tag ID: A means of designating a trace or digital measured value by an alphanumeric identifier
instead of a numeric identifier. Typically available with up to seven characters.
Digital printing: Printing of the precise measured numerical values for the various channels,
along with their channel identifiers. Digital printing usually occurs in a margin of the chart so as
not to interrupt trend recording.
Log report: A printout of precise measured numerical values for the various channels, along with
their channel identifiers. Typically prints in full character height per print cycle. During trending,
prints on demand, resuming trending automatically. When trending is not being used, prints at a
preselected time interval. May also include alarm status indication.