Ammeter.
The commercial name for an ampere-meter, an instrument designed
to show by direct reading the number of amperes of current which are
passing through a circuit.
A great variety of ammeters have been invented, based on different
principles. The definitions following this one give some idea of the
lines of construction followed.
Synonym--Ampere meter.
Ammeter, Ayrton's.
A direct reading instrument for measuring current intensity.
A solenoid receives the current. In the axis of the solenoid an iron
tube is suspended by a long spiral spring that passes down within it,
and the upper end of which spring is fastened to the glass top of the
instrument. The tube is provided with proper guides so as to maintain a
vertical position, and is free to rotate. Its upper end carries an
index.
The whole operates as a magnifying device. A slight longitudinal
displacement of the tube causes it to rotate through a considerable
angle by the action of the spring. By properly proportioning the parts,
the angle of displacement of the index is directly proportional to the
current between 15º and 270º angular displacement.
The same instrument is wound for use as a volt-meter.
Its principal fault is its restricted range.
Ammeter, Commutator.
A commutator ammeter is one whose windings consist of separate strands,
each of any desired number of turns, and provided with a commutating
attachment for throwing them into series or into parallel as desired.
The essential condition is that all the wires shall be of equal
resistance and of equal number of turns. Such an instrument can be used
for heavy or light currents. Two sets of graduations are marked on its
scale if it is a calibrated instrument. (See Calibration.) Commutator
volt-meters are constructed on the same principle.
Ammeter, Cunynghame's.
A modification of the Siemens' electro-dynamometer. (See
Electro-dynamometer, Siemens'.) An electro-magnet with very massive core
is excited by the current. As the core is of small reluctance the
strength of the magnet is nearly proportional to the current strength.
Between the poles of the magnet a soft iron armature or induced magnet
is pivoted. It carries a pointer so adjusted that when the axis of the
soft iron magnet is at an angle of about 30º with the line joining the
poles of the electro-magnet the pointer will indicate zero.
The soft iron armature is so massive that the magnetism induced in it is
proportional to the strength of the electro-magnet. Hence the couple
exerted by the electro-magnet on the pivoted armature will be
proportional to the square of the current.
The armature is retained in place by a spiral spring lying in line with
its axis of rotation. The instrument is operated as a zero reading
instrument. The current is passed through it. The needle is deflected;
it is brought back to zero by turning a milled head which twists the
spring. The current will be proportional to the square root of the angle
of displacement of the milled head. A scale with index is provided,
giving directly the square roots of the angle over which the pointer is
moved.
The same instrument is wound for use as a volt-meter.