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the combining may be unfortunate. In by the use of the vertical line. Some making a wash drawing, no thought is drawings can be made entirely by this necessary concerning the direction of the
means. See Fig. 3, every line of which
wash; but in using lines, the query at is vertical. This illustrates the value of a once arises as to what direction they good individual line. It will be observed shall take. A method is something one that, although vertical, these lines are not must grow into from a small, simple be severely straight and stiff ; they tremble a ginning. The accompanying illustration little, or have a slight suggestion of a (Fig. 2) is an example of rare skill in
curve. In the shadow at the bottom of method. There is
the drawing, each an utter absence of
line is emphasized at anything rigid or
the top by a light mechanical in the
pressure, and made whole. Observe how
thin at the lower end softly the edges of
in order to soften off the drawing merge
the edges of the into the white of the
drawing as a whole. paper. The vigor
Fig. 4 shows anof the drawing is
other method. The gathered in the
vertical line is disdoorway itself. E
carded and the freest The simplest
possible line is used. method is obtained
No one direction is
effect caused by the white spaces between the ends of the lines. This is a very common fault with beginners. Fig. B goes to the opposite extreme. Long, severely straight lines produce a stiff, mechanical effect which is very unpleasant. In Fig. C there is greater freedom, and the feeling in the lines is very good. The lines are too close together, however, producing a fussy, scratchy, labored effect. Fig. D is an example of lines that are too heavy and too widely spaced. The direction of the lines is good, especially on the side roof, but the effect is too coarse to be artistic.
The four figures mentioned illustrate faulty methods that are very common among draftsmen. In Fig. É an effort is made to avoid all the faults shown in the others—the harsh, stiff, coarse line, the spotted effect, and the scratchy, labored combination. The lines are free and natural, and the effect is soft and artistic.
'If several lines are drawn parallel and quite close together, but not touching, a gray color is the result. This is terined
is harsh; the second is pale; the third seems the most satisfactory.
This is a safe rule to follow-Get into every pen drawing, black, gray, and white. C'sually, in early attempts, there is a tendency to omit the black. Look for the place in the drawing where you can locate this black; you are not likely to get too much of it. Let the half-tone or gray be rather light, midway in strength between white and black. A heavy half-tone is a dangerous value. The black may often grade off into the gray, or there may be distinct fields or areas of each value.
HERE are two common methods heat, the greatest heat is in the air imof generating electromotive force mediately surrounding the pipe. Simi-by chemical action in a battery, larly, in the case of the conductor, the
and by moving a conductor in greatest density of the lines of force is. an electric field. The machine which pro near the conductor ; at a slight distance, duces electromotive force by the last the field is comparatively weak. named method is called a Dynamo or Electric Generator. Before considering
The Dynamo the generator, let us consider the subject of magnetism and the magnetic field.
It has already been said that when a current flows in a conductor, a magnetic
field is created around the conductor. The Magnetic Field
Conversely, if a magnetic field is created A permanent magnet is a piece of steel
around a conductor, electromotive force which has the property of attracting iron
will be induced in it. This, then, is the and steel. Experiment shows that the
fundamental principle :-To obtain elecspace immediately surrounding a mag tromotive force, there must be relative net, called its “field,” is subject to the in
movement between a conductor and a fluence of the magnet. This influence is
magnetic field. Either the field or the exerted in definite directions or lines, conductor may be the part in motion. which are called "lines of magnetic
If the dynamo be constructed accordforce," or, simply, "lines of force." The ing to the above—that is, consisting of first figure on the blackboard represents
a simple bar magnet, and a conductor the lines of force of a bar magnet; and
moving in the field—an electromotive the second shows the field of a horseshoe
force will be generated and current will magnet.
flow, if the circuit be closed. Such an The strength of the field is propor
arrangement, however, would produce tional to the number of lines per square
but little electromotive force; and, moreinch of cross-section, or “density,” as it
over, the machine would not be efficient. is called.
In order to increase the power developed,
certain changes are introduced: The Conductor
(1) The magnet is greatly strength
ened by winding wire which carries a When a current flows in a conductor current, about a soft iron core; this, of (a copper wire), the conductor is sur
course, strengthens the field also. rounded by circular lines of force, as (2) The iron core is extended and shown in the third figure. When 'the shaped so that the poles will be close stcam pipes of a radiator are giving off together. This is shown by the fourth (760)
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figure on the blackboard. The curved the pole pieces, at C and C'; the armashape, and the slight distance between ture, at D; and the armature coils, at G. the poles, result in a convenient form The spaces between the pole pieces C and and a strong field.
C' and the surface of the armature, are (3) The conductor, instead of being called the "air gaps,” shown at E and E'. a single wire, is made up of a soft iron These are always made as small as core and coils of wire, so that the num mechanical considerations will permit, so ber of convolutions will be increased. that the distance in air through which the
lines of force must pass, may be as small Parts of the Dynamo
as possible. The brushes H and H' are
for the purpose of collecting the current. The fifth figure on the blackboard The dotted lines show the path of the shows diagrammatically the arrangement magnetic lines. These lines pass through of the parts of the dynamo. The field the yoke, the field cores, and the pole cores are shown at A and A'; the field pieces, then pass across the air gaps and windings, at F and F'; the yoke, at B; through the armature core.
A Telephone Pole foremen of construction of the Bell Derrick
system. Ordinarily the great chest
nut or cedar poles, weighing a couple SOME remarkably quick work has been
or more, are erected by a going on this past summer in the con score of men struggling with unwieldy struction of a new long-distance tele- pike-poles ; but this simple apparatus enphone service between New York and ables four men without difficulty to do Boston, the crew completing a daily aver the work of twenty. age of more than 3,000 feet of line. Prob The new contrivance consists of a ably more telephonic messages are trans heavy wagon, with two 28-foot spars mitted annually between these two cen which stand like an inverted V, one leg ters than between any other two cities over each pair of wheels. When the derof the world; and the American Tele rick has been dragged to the vicinity of phone & Telegraph Company, which a pole hole, it is guyed to nearby trees or planned the
avenue of com rocks—or, by an arrangement of crowmunication has determined to create bars, to the ground-in order that the the best telephone and telegraph line heavy pole may be raised without overin existence. An efficient aid in the un turning it. dertaking has been a newly invented pole Once the apparatus stands ready, the derrick, which was devised by one of the men screw on the pole cross-arms and