ONE principle underlies almost the entire field of electrical development. And yet, strange to say, very few people not actually in the electrical business know of even the existence of this law or principle. Perhaps, however, this is not strange, after all, for, though we see our electric lights and ride in electric cars, or use our telephone every day of our lives, these wonderful inventions are so common and work so well and are so seldom out of order, that we never see more than the outside of them, and it never occurs to us to wonder what makes them work as they do.

The incandescent lamp in your house, the arc lamp in the street, the motors under the car, all must have current supplied to them from a wire, and the current in the wire must come from some electrical generator. In almost every case this generator is a "dynamo," though, of course, there are other ways to supply it on a small scale. But, in commercial work, the dynamo is the source of electrical supply for all our lights and motors, telephone and telegraph systems, and the myriad other devices we depend upon for our daily comforts and necessities. It is interesting, therefore, to examine into this machine a little more

(1) If you send a current through a coil of insulated wire wrapped around a piece of iron, the iron immediately becomes a magnet-an "electromagnet." (See Fig. 1.)

(2) The region around the magnet, especially at its ends, is influenced by the magnetism: that is, a "field of magnetism" is set up about the iron as soon as it becomes a magnet. (See Fig. 2.)

(3) If you pass a piece of wire sidewise across this magnetic field, an electric pressure will be set up in the wire somewhat as water pressure is set up in a water-pipe by a pump.

This pressure tends to force electricity from one end of the wire to the other; so if you were to continue the wire around in a loop and join the ends so as to form a complete ring, a current would flow right around the ring, forced by the pressure caused by passing a part of it through the magnetic field.

These three items, then, form the basis for the design of all our dynamos, from the little toy machines run by hand to the huge generators driven by steam turbines of ten thousand horsepower.

Now, if you should take in one hand a small straight magnet and in the other a loop of wire, and should move the loop past the end of the magnet, as a part of the loop cuts the magnetic field you would have generated an electric current in that wire. It will be a very small current, to be sure, but a true electric current will, nevertheless, flow around the loop. The stronger the magnet or the more magnets you have and the faster you move the wire, the greater will be the pressure, and, consequently, the current set up.

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closely than is possible on a casual visit to a power-house.

To understand how it operates one has to keep firmly in mind these three important facts:

But, as we said, the best you can get by hand is a feeble current,-too feeble to be measured by any but the most sensitive of instruments. Sup

pose, now, instead of having only one turn of wire you had not joined the ends to form a closed loop, but had brought them around again to form a double loop, and then, joining the ends, had performed the experiment. Two parts of the same wire would cut the magnetic field at once, and, the same pressure being set up in each part, twice the former current would flow. With three turns to the loop, or coil, three times the current would flow-and so on. The writer has performed this experiment in the field of a strong magnet with a coil of very fine wire of a great many turns, and, by jerking the coil very quickly through the field so as to cut it with one side of the coil, has been able to light a small battery lamp for an instant.

So, if you can get enough wire to cut the field of magnetism and do it fast enough and have the field strong enough, you can get very strong currents. But it would be of no use if the current continued but an instant, as it does in these experiments. So we arrange the turns of our coils on a cylinder or wheel, and fix a number

FIG. 3.

DIAGRAM OF SIX-POLE GENERATOR, END VIEW.

A, commutator segments; B, brushes; C, coils of wire collecting pressure impulses; D, wire connecting negative brushes; E, wire connecting positive brushes; N-S, the field magnets - N, north poles; S, south poles; +, the positive, or outflowing, current; -, the negative, or incoming, current.

huge wheel thirty-two feet in diameter, and they get from these machines a pressure of eleven thousand volts, and it takes an engine of eight thousand horse-power to run each of them. Al

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of electro-magnets all around this and pointing toward it, and, by keeping the cylinder or wheel turning rapidly and continuously, we keep parts of our coils cutting the fields of magnetism all the time. And the pressures in all the small sections of wire that cut the fields add together and produce a continuous pressure around the cylinder in all the coils. (See Fig. 3.)

In the first machines made years ago by Thomas Edison there were only two magnets, and there was such a small coil of wire that he had to revolve the cylinder, upon which it was mounted, at a tremendously high rate of speed to make up for it. And even then, at over two thousand revolutions per minute, he got up an electrical pressure of only fifty "volts," or about one tenth of the pressure on our present trolley lines. In

FIG. 4.
DIAGRAM SHOWING SIDE VIEW OF ARMATURE
WINDING, THE COILS, C (CORRESPONDING TO CIN
FIGURE 3), AND HOW THEY ARE CONNECTED
TO THE COMMUTATOR SEGMENTS, A.
N-S, FIELD MAGNETS.

though the big wheel only turns over seventyfive times a minute, it is so big that the wires

cut the magnetic fields at the rate of a mile and a quarter a minute. So quite a pressure is set up in each part, and there is room for a great many sections of wire around so large a wheel.

But in order to use the pressure and current generated by thus passing these wires through the magnetic fields, it is necessary to take it from the coils and send it out over wire to where it is to be used. This is done in the commonest type of machine by arranging at one end of the cylinder or one side of the wheel a ring of small copper bars, each one connected by a copper wire to some part of the revolving coils. This ring of copper bars is called a commutator. Small blocks of "carbon," which is like compressed charcoal powder, are made to bear lightly on this revolving ring of bars, and are so placed that before one bar has passed out from under the block another is coming under it, so that at least one bar is always touching each block, or "brush," as it is called. (See Fig. 4.) By properly placing these "brushes" and connecting one set to the supply wire and the other set to the return wire, the current from the coils passes out to the copper bars and is "picked off," or is allowed to pass out, by one of these sets of brushes; goes out over the wire to the lights or motors and, coming

back on the return wire, goes into the coils again through the other set of brushes and the copper bars-all forced by the pressure set up in the innumerable small sections of wire cutting the magnetic fields. So, really, the wires outside the machine are like that part of the loop which you held in your hand in the first experiment, while the part of the loop which cut the magnetic field is mounted on the wheel and is continuously generating current as the wheel, or "armature," revolves.

So the work of a dynamo is simply to create an "electrical pressure" which will force a current to flow if a path is provided for it to flow through. This path is usually of copper wires leading to lights or motors or heaters which in themselves form part of the path the current follows. These wires are often very long and carry great amounts of power. In this country electric power is sometimes carried over two hundred miles from some mountain waterfall to a big city where there is a demand for it in all its many phases. In such cases the pressures are very high, fifty or sixty thousand volts, or over a thousand times as much as Edison obtained from his first machines. So much have we progressed in thirty years.

NOTHING disgusts an old sailor quite so much as a landsman's clumsy attempts to tie a knot. Every one can tie a knot of some sort. But any one who thinks he or she knows how to tie a knot properly, and does n't do it as a sailor does it, has a lot to learn about the art of knot-tying. Passing over in dignified silence the temptation to make a humorous point of the last words in the preceding sentence, the humor being entirely too obvious to deserve especial attention, let us return to the old sailor and his knotty point of order.

I. THE SLIP-KNOT THAT SLIPS WHEN IT SHOULD N'T.

He says landsmen only know how to tie "granny knots." The point is well taken. A landsman can tie what is called by him a "slip"knot. A sailor will tell you the landsman's slipknot is rightly named. It is a knot that slips; that is, one that slips when it ought to hold. The sailor ties his slip-knot so that it cannot slip in the way it is not wanted to. In fact, all the sailor's knots are tied to stay tied. For centuries the sailor has known that his very life may at times depend upon the firmness of a knot. So generations of sailors have had to study the art of knottying, seeking to improve on methods of fastening. together two ends of rope or of joining the end or ends to a stationary object, so that nothing short of the breaking of the rope will cause a separation.

Through all these centuries, landsmen have gone contentedly on tying their "granny" or "slip"-knots, indifferent to the fund of information that sailors could give them on the subject. Men have n't time to learn to tie new knots. But boys have, or, at least, school-boys have.

This thought prompted one of the instructors

in tying a parcel, 2. THE EXTRA LOOP THAT KEEPS

binding bundles,

or any of the

THE SLIP-KNOT FROM SLIPPING WHEN IT SHOULD N'T.

score of emergencies when men and women have to tie a knot in a hurry.

It is remarkable to one who studies these knots what a difference one little simple twist of the cord will make in the holding power of a knot. The "granny knot" is changed, by this little sim