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Magnet Windings

One of the perplexing things about telegraph apparatus is the necessity for so many different windings of the magnets. One wonders why they are not all made alike, or at least wound with the minimum amount of wire.

We have been told that resistance represents the obstruction to the flow of the current, and we can not understand why we deliberately wind 150 ohms of wire around one class of magnets, while four ohms will suffice for another.

Neither can we see the use of permitting the strength of the current in one wire to be very weak, while that flowing in another to be very strong, thus necessitating the varied construction of apparatus to meet the in- equality. A uniform current would seem more reasonable. Have we not been told that, by increasing or decreasing electromotive force, we may regulate the current to any desired degree of strength?

Now, theoretically, this is true. In actual practice, however, it is necessary to add "but sometimes we mustn't."

There are two very good reasons why we should not always equalise the current in circuits by that means. One is that it would not be economical. The other, and more serious objection, is that to do so would be dangerous, not only to the conductors and instruments, but even to life itself. Just imagine the result of trying to force a sounder current of 250 milliamperes through a long wire of, say, 5,000 ohms resistance. According to the Ohms Law rule E = I X R, we find that 1,250 volts would be required-enough to kill human beings under ordinary conditions, should they accidentally come in contact with it.

Then, again, there is the "heat capacity" of wires to be considered. The reader must know that wherever work is done heat will appear in greater or lesser degree, according to the energy expended. There is a rule under the heading of electrical measurements, which tells us that for every second of time one ampere of current flows through a conductor, 1/24 of a heat unit is evolved. If the 5,000 ohm circuit mentioned should happen to become grounded near the office, the 1,250 volts feeding it would dam up, as it were, within a few hundred feet, the entire current intended to supply as many hundred miles of wire. This enormous increase of current would produce more heat units per second than the wire could dispose of by radiation or otherwise; hence the temperature would rise to a dangerous degree. The small wire in the instruments would be the first to melt.


The foregoing statement boiled down means that we use weak currents in long circuits of high resistance, simply because it cannot be helped. Relays of high resistance are used in conjunction with these weak currents for the same good reason. To make the latter point clear it will be necessary to recur to magnetism and learn the process by which the attractive energy in the iron of a magnet is built up.

It will be remembered that it was stated that every conductor carrying an electric current is surrounded by little "magnetic rings" (their number being in proportion to the strength of the current flowing), which jump off, as it were, and enter the iron core of the relay as the current passes through the coils of the magnet.

Fortunately, the strength of the current in any circuit is equal in every portion, regardless of the size of the conductor at different points. The magnetic strength of our relays, therefore, depends upon how much of the circuit we wind around the core. The uniformity of the current holds good, even though the circuit be composed of a hundred different wires, varying in size from the diameter of a silk thread to that of a clothesline. An inch of the threadlike conductor will contain as many little "rings" of magnetic energy as the same length of the largest wire.

Bearing this fact in mind, it is quite evident that by merely winding the spools of a relay with very fine wire, you place more inches of wire, and consequently a greater number of magnetic "rings" in proximity to the iron, than if wound with large, coarse copper wire, because the latter would fill the spool with a much shorter length. Now, the smaller wire will certainly have a higher resistance than the large size, but we are compelled to use it, because the latter will not fill the bill. Hence, the statement that some magnets are wound with 150 ohms or more, instead of four ohms, is true simply because we have no choice in the matter.

The reader may possibly wonder why we could not obtain equal efficiency with the large wire, by continuing to wind until the core was encircled by the same number of convolutions that the fine wire gave. The explanation is that, with the large wire, the outer convolutions would be too far removed from the iron to add their quota of energy. The fact is that an electromagnet should never be wound much beyond the point where the thickness of the convolutions, one above another, is equal to the diameter of the iron core itself. To wind beyond that point entails a waste of energy, as the returns are very meagre in proportion to the expenditure.

The relation of small wire to small currents and vice versa may be shown in another way. If one turn of wire around the iron core of a relay will give it a certain magnetic strength, two convolutions will double the strength, while 100 or 1,000 turns will increase it that many fold. This is equivalent to saying that the magnet of a relay will be equally magnetised by a current of, say, one ampere flowing once around the core, or a current of one-hundredth of an ampere en- circling it 100 times. Figure it out, and it will be found that the same number of magnetic rings will have had a chance to enter the iron in either case.

Figs. 10, 11 and 12 illustrate the idea in a very clear manner. If the reader will imagine the different sizes of the windings around the respective magnets to represent corresponding variations in the strength of the current instead of that many different gauges of wire, he will at once see that, with a flow of magnetic lines as dense as that shown in Fig. 10, one layer of convolutions around the core would fill the spool to the outer limit. With the density of Fig. 11, two layers would be required, while with the very weak flow in Fig. 12, 50 or 100 turns must be wound one above another.

So it seems after all that we must distribute currents very much as we do the money in our purses. If we want something that costs a dollar, the seller will be satisfied with any denomination we happen to have, provided he receives full value. One dollar, two halves, four quarters of 100 cent pieces is all the same to him.

from "Diagrams and Complete Information for Telegraph Engineers and Students" by Willis H. Jones, 1915 Edition.

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