Tamidine.
Reduced nitro-cellulose. Nitro-cellulose is dissolved in a proper
solvent and is obtained by evaporation as a translucent solid mass. By
ammonium sulphide or other reagent it is reduced so as to be virtually
cellulose. It is cut into shape for filaments of incandescent lamps,
which shapes are carbonized and flashed.
Tangent Law.
In a galvanometer the tangents of the angles of deflection of the needle
are proportional to the deflecting force--
I. When the controlling force is unaltered in absolute magnitude and
direction by the motion of the needle.
II. When the deflecting force acts at right angles always to the
controlling force.
These conditions are usually secured by having the actuating coil
through which the current passes flat and of large diameter compared to
the length of the needle; by using the uniform field of the earth as the
control; by having a short needle; by placing the coil with its plane in
the magnetic meridian.
For best proportions of tangent galvanometer coils see Bobbins.
Fig. 310. GRAPHIC CONSTRUCTION OF TANGENT SCALE.
Tangent Scale.
An arc of a circle in which the number of graduations in any arc
starting from zero are proportional to the tangent of the angle
subtended by such arc. The system is for use with tangent galvanometers.
Thus if for 45° a value of 100 is taken and marked on the scale then for
the arc 26° 33' + a value of 50 should be marked on the scale because
such are the relative values of the tangents.
Thus the scale instead of being divided into degrees is divided into
arcs of varying length, growing shorter as they are more distant from
the zero point, of such length that the first division being subtended
by a tangent of length 1, the first and second divisions added or taken
together as one arc are subtended by a tangent of length 2, and so on.
In the cut a simple method of graphically laying out a tangent scale is
shown. In it C is the centre of the arc, and H the radius running to the
zero of the instrument. From C a circle is described and on H a vertical
line tangent to the arc is erected. Taking any part of the tangent, as
the length shown ending at D, it is divided into any number of equal
parts. Radii of the circle are now drawn whose prolongations pass
through the divisions on the tangent. These radii, where they intersect
the arc of the circle, determine equal divisions of the tangent scale,
which, as is evident from the construction, are unequal angular
divisions of the arc.