Torque.
A force tending to produce torsion around an axis. An example is the
pulling or turning moment of an armature of an electric motor upon its
shaft. It is often expressed as pounds of pull excited at the end of a
lever arm one foot long.
The expression is due to Prof. James Thompson, then of the University of
Glasgow.
"Just as the Newtonian definition of force is that which produces or
tends to produce motion (along a line), so torque may be defined as that
which produces or tends to produce torsion (around an axis). It is
better to use a term which treats this action as a single definite
entity than to use terms like 'couple' and 'moment,' which suggest more
complex ideas." (S. P. Thompson.)
A force, acting with radius r gives a torque equal to f X r ; f and r
may be expressed in any units. S. P. Thompson gives the following
equivalents :
To reduce
dyne-centimeters to gram centimeters, divide by 981
dyne-centimeters to meter-kilograms divide by 981E5
dyne-centimeter, to pound-feet divide by 13.56E6
pound-feet to meter-kilograms divide by 7.23
In each of these compound units the first unit is the force and the
second unit is the radius or lever arm of the torque.
Synonyms--Turning Moment--Moment of Couple--Axial Couple--Angular
Force--Axial Force.
Torsion Balance, Coulomb's.
Originally an apparatus in which electrostatic attraction or repulsion
is measured against the torsion of a filament, often of silk-worm cocoon
fibre. It consists in one form of a cylindrical glass vessel in which a
light shellac needle is suspended horizontally by a fibre. This needle
carries at one end a gilded disc or sphere and is suspended by a fine
wire, or filament. A proof plane, q. v., is excited by touching it to
the body under trial; it is then inserted in the case. The disc on the
needle is first attracted and then repelled. The position finally taken
by the needle is noted. The force of torsion thus produced is determined
by twisting the filament by the torsion head on the top of the apparatus
so as to move the needle a certain distance towards the proof plane. The
more the torsion-head has to be turned to carry the needle through a
specified arc the greater is the torsion effected or the greater is the
repulsion exerted, The torsional force of a wire is proportional to the
angle of torsion; this gives the basis for the measurement.
With magnetic needle it is used to measure magnetic repulsion and
attraction. The best material for the filament is quartz, but the
instrument is not very much used.
Torsion Galvanometer.
A galvanometer in which the torsion required to bring the index back to
zero, when the current tends to displace it, is made the measure of the
current strength or of the electro-motive force. It involves the use of
a torsion head, q. v., or its equivalent.