wash; but in using lines, the query at once arises as to what direction they shall take. A method is something one must grow into from a small, simple beginning. The accompanying illustration (Fig. 2) is an example of rare skill in method. There is an utter absence of anything rigid or mechanical in the whole. Observe how softly the edges of the drawing merge into the white of the paper. The vigor of the drawing is gathered in the doorway itself.

The simplest method is obtained

E

is vertical. This illustrates the value of a good individual line. It will be observed that, although vertical, these lines are not severely straight and stiff; they tremble a little, or have a slight suggestion of a curve. In the shadow at the bottom of

the drawing, each line is emphasized at the top by a light pressure, and made thin at the lower end in order to soften off the edges of the drawing as a whole.

Fig. 4 shows another method. The vertical line is discarded and the freest possible line is used. 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 termed

FIG. 6. GRAY AND WHITE.

a half-tone value. Lines drawn so close together that the ink of one runs into that of the other, with little or no white space between, give a black value. The white of the paper untouched by the pen gives a white value. Fig. 5 shows only two values-black and white; Fig. 6 also has two-gray and white; Fig. 7 has the three-black, gray, and white. The first

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. Usually, 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.

T

HERE are two common methods of generating electromotive force -by chemical action in a battery, and by moving a conductor in an electric field. The machine which produces electromotive force by the lastnamed method is called a Dynamo or Electric Generator. Before considering the generator, let us consider the subject of magnetism and the magnetic field.

The Magnetic Field

A permanent magnet is a piece of steel which has the property of attracting iron and steel. Experiment shows that the space immediately surrounding a magnet, called its "field," is subject to the influence of the magnet. This influence is exerted in definite directions or lines, which are called "lines of magnetic force," or, simply, "lines of force." The first figure on the blackboard represents the lines of force of a bar magnet; and the second shows the field of a horseshoe magnet.

The strength of the field is proportional to the number of lines per square inch of cross-section, or "density," as it is called.

The Conductor

When a current flows in a conductor (a copper wire), the conductor is surrounded by circular lines of force, as shown in the third figure. When the steam pipes of a radiator are giving off

(760)

heat, the greatest heat is in the air immediately surrounding the pipe. Similarly, in the case of the conductor, the greatest density of the lines of force is near the conductor; at a slight distance, the field is comparatively weak.

The Dynamo

It has already been said that when a current flows in a conductor, a magnetic field is created around the conductor. Conversely, if a magnetic field is created. around a conductor, electromotive force will be induced in it. This, then, is the fundamental principle:-To obtain electromotive force, there must be relative movement between a conductor and a magnetic field. Either the field or the conductor may be the part in motion.

If the dynamo be constructed according to the above-that is, consisting of a simple bar magnet, and a conductor moving in the field-an electromotive force will be generated and current will flow, if the circuit be closed. Such an arrangement, however, would produce but little electromotive force; and, moreover, the machine would not be efficient. In order to increase the power developed, certain changes are introduced:

(1) The magnet is greatly strengthened by winding wire which carries a current, about a soft iron core; this, of course, strengthens the field also.

(2) The iron core is extended and shaped so that the poles will be close together. This is shown by the fourth

figure on the blackboard. The curved shape, and the slight distance between the poles, result in a convenient form and a strong field.

(3) The conductor, instead of being a single wire, is made up of a soft iron core and coils of wire, so that the number of convolutions will be increased.

Parts of the Dynamo

The fifth figure on the blackboard shows diagrammatically the arrangement of the parts of the dynamo. The field. cores are shown at A and A'; the field windings, at F and F'; the yoke, at B;

the pole pieces, at C and C'; the armature, at D; and the armature coils, at G. The spaces between the pole pieces C and C' and the surface of the armature, are called the "air gaps," shown at E and E'.

These are always made as small as mechanical considerations will permit, so that the distance in air through which the lines of force must pass, may be as small as possible. The brushes H and H' are for the purpose of collecting the current.

The dotted lines show the path of the magnetic lines. These lines pass through the yoke, the field cores, and the pole pieces, then pass across the air gaps and through the armature core.

A Telephone Pole-
Derrick

SOME
OME remarkably quick work has been
going on this past summer in the con-
struction of a new long-distance tele-
phone service between New York and
Boston, the crew completing a daily aver-
age of more than 3,000 feet of line. Prob-
ably more telephonic messages are trans-
mitted annually between these two cen-
ters than between any other two cities
of the world; and the American Tele-
phone & Telegraph Company, which
planned the new avenue of com-
munication has determined to create
the best telephone and telegraph line
in existence. An efficient aid in the un-
dertaking has been a newly invented pole-
derrick, which was devised by one of the

foremen of construction of the Bell system. Ordinarily the great chestnut or cedar poles, weighing a couple of tons or more, are erected by a score of men struggling with unwieldy pike-poles; but this simple apparatus enables four men without difficulty to do the work of twenty.

The new contrivance consists of a heavy wagon, with two 28-foot spars which stand like an inverted V, one leg over each pair of wheels. When the derrick has been dragged to the vicinity of a pole hole, it is guyed to nearby trees or rocks-or, by an arrangement of crowbars, to the ground-in order that the heavy pole may be raised without overturning it.

Once the apparatus stands ready, the men screw on the pole cross-arms and