Reaction of Dynamo, Field and Armature.
A principle of the dynamo current generator, discovered by Soren Hjorth
of Denmark.
When the armature is first rotated it moves in a field due to the
residual magnetism of the field magnet core. This field is very weak,
and a slight current only is produced. This passing in part or in whole
through the field magnet cores slightly strengthens the field, whose
increased strength reacts on the armature increasing its current, which
again strengthens the field. In this way the current very soon reaches
its full strength as due to its speed of rotation.
The operation is sometimes termed building up.
Sometimes, when there is but a trace of residual magnetism, it is very
hard to start a dynamo.
Reading Telescope.
A telescope for reading the deflections of a reflecting galvanometer.
A long horizontal scale is mounted at a distance from the galvanometer
and directly below or above the centre of the scale a telescope is
mounted. The telescope is so directed that the mirror of the
galvanometer is in its field of view, and the relative positions of
mirror, scale and telescope are such that the image of the scale in the
galvanometer mirror is seen by the observer looking through the
telescope.
Under these conditions it is obvious that the graduation of the scale
reflected by the mirror corresponds to the deflection of the
galvanometer needle.
The scale may be straight or curved, with the galvanometer in the latter
case, at its centre of curvature.
Reamur Scale.
A thermometer scale in use in some countries of Continental Europe. The
temperature of melting ice is 0°; the temperature of condensing steam
is 80°; the degrees are all equal in length. For conversion to
centigrade degrees multiply degrees Reamur by 5/4. For conversion to
Fahrenheit degrees multiply by 9/4 and add 32 if above 0° R., and if
below subtract 32. Its symbol is R., as 10° R.