Part 17

The kind of coil we have made (only larger) is very much used in houses as a gas-lighting coil (to be described later). It is very much used also for exploding gasolene engines. It generally passes under the name of the "make and break" coil. The revolving shaft of the engine is made to push together the ends of the wire and separate them at the right instant to make the spark for explosion. Of course this is done inside of the engine cylinder.

That type of coil does not offer resistance enough to protect the battery, and dry cells soon run down if used with it. The coils that we have in this boat are somewhat different from that, the details of which we cannot now entirely explain.

They offer enough resistance to cut the current required of the battery down to one third what the "make and break" coil would take and at the same time they raise the voltage so much higher that the spark will jump across an air gap without being led across as an arc. Hence they are called "jump spark" coils.

It will be remembered that when we were studying the dynamo we produced an electric current by moving a magnet. We may now add that an electric current may be produced by simply changing the strength of a magnetic field. The coil that we have just made creates a magnetic field in the region about itself whenever a current is passing through it. The tongue at T (Fig. 117) detects an extra current while the magnetic field is being produced, or while it is dying away, or it will detect any slight variations in the strength of the current which produces the magnetic field. It is customary to distinguish between these two currents. The battery current which produced the magnetic field is called the primary current and the current which is detected by the tongue is called the secondary current. The primary current in our experiments had only a few volts of pressure, from one to seven. The secondary current had many volts, as indicated by the spark. If we rub the end of the wire c across the binding post under b (Fig. 117) no spark occurs. The current does not in this case go through the coil, and no secondary current is produced. Whenever we touch the wire b to that post we have, in addition to the primary current which has not voltage enough to produce a spark, a secondary current flowing in the same wire at the same time and having voltage enough to produce a spark. The primary current is continuous while the contact is closed; the secondary current is momentary, as the tongue detects, and is produced only while changes are being made in the strength of the magnetic field. We will now take another piece of wire and wind upon the coil about two hundred more turns, leaving this outer coil wholly disconnected from the inner one, (Fig. 118). I connect c and d, the terminals of what we may call the secondary coil, with my measuring instrument and I connect a, one of the terminals of the primary coil, with the battery. I then rub b, the other primary terminal across the free binding post of the battery. At the instant of closing the primary circuit--that is, of building up the magnetic field--a secondary current is induced in the secondary coil, which lasts for only an instant, too brief a time for the needle to measure it, although its motion indicates both the presence and the direction of the induced current. While the primary circuit remains closed--that is, while no change is occurring in the strength of the magnetic field--the needle returns to zero, indicating no secondary current. But when now the primary circuit is broken and the magnetic field loses its strength, the needle indicates a momentary current in the secondary coil and in the opposite direction from what it had been at first.

If, therefore, I rapidly make and break the current at b I produce an alternating current in the secondary coil. I will connect c and d with a miniature lamp and, resting a coarse file upon the free binding post, I will rake the end of the wire b up and down upon this file so that, as it dances along upon the file, it will rapidly make and break the primary circuit, and therefore rapidly change the strength of the magnetic field. You notice that the lamp lights up moderately well. It is being lighted by an alternating current. I move the wire a little more slowly and you see the flicker of the alternations. According to the label upon the lamp it requires ten volts, and our battery could not give that. We have therefore "stepped up" the voltage as we say and we have a veritable step-up transformer.

In this case the primary and secondary circuits are entirely separate. It is a familiar fact that different electric currents may pass through the same wire at the same time without apparent conflict. We send numerous telegraph despatches through the same wire at the same time. It is quite as easy for several pairs of persons to telephone over the same wire at the same time as it is for those same several pairs to carry on separate conversations in the same room at the same time, at, say, an "afternoon tea." We may use the same wire at the same time to carry direct and alternating currents. This fact was first discovered in 1902 by Bedell of Cornell University.

Primary and secondary currents do not require separate primary and secondary coils to convey them. They may or may not be connected into one continuous coil. It is quite immaterial whether they are connected or not so long as they are in the same magnetic field. Indeed, it seems that the field outside of the wire may be quite as important as the wire itself.

We have now 100 turns in the primary and 200 turns in the secondary coils. Let us connect b with c so as to make one continuous circuit of 300 turns. Let us then put a branch upon b to connect with the battery, thus having 100 turns for the primary circuit, and put a branch upon a to connect with the lamp, thus having 300 turns upon the lamp, (Fig. 119). When now we rub b upon the file, as before, the lamp lights up more brightly than before, indicating that we have stepped up the voltage still higher. Varying the strength of the magnetic field induces a secondary current and the voltage of the induced current is determined, in part, by the number of turns in the secondary circuit. If what we have been saying is true we ought to be able to get these same results from an electric bell. To test this we connected wires with a and c, (Fig. 120), and since I knew that the secondary current at S would be too severe for the tongue we decided to feel it with the hands. For this purpose we want a larger surface than the wires themselves offer for contact with the hands, and so I twisted the bare end of each wire around an iron spike. The four boys then arranged themselves in line, joining hands, and the boy at each end of the line held a spike in his free hand. Thus we had put the enormous resistance of four human bodies joined in series in the secondary circuit. When now I connected two dry cells with a and b (P, Fig. 120) the hammer of the bell acted, like the file in the former case, as interrupter of the primary circuit. As it rapidly made and broke the primary circuit, it produced rapid changes in the strength of the magnetic field and thus induced a secondary current which the boys all felt. The fact that it forced its way through four bodies shows that its voltage was high. The high voltage was also indicated by the spark which always occurred in the bell. The primary circuit in this case has not more than three volts while the secondary has more than a hundred. We have it in our power to give the secondary current almost any voltage we choose, with this limitation each increase in voltage necessitates a proportional sacrifice of quantity. The watt power induced in the secondary circuit cannot exceed that contributed to the primary circuit--indeed cannot quite equal it since there is some loss in heat.

Suppose we operate a bell on a primary current having three volts and .25 ampere, that is, .75 watt. Suppose then the voltage of the secondary current is stepped up to fifty times three, or 150 volts. The quantity of secondary current will be found to be somewhat less than one fiftieth of .25 or .005 ampere. The 150-volt alternating current from the bell is more tolerable than that from a 150-volt dynamo, because the quantity is limited in the former case.

Our spark coil has a vibrator which acts precisely like the hammer of the bell to make and break the primary circuit and thus make rapid changes in the magnetic field produced by the primary coil. The primary coil of the spark coil is many times larger than the coil of the bell, that is, it contains many more turns of wire. It has much more iron in the core. We use upon it five cells instead of the two cells upon the bell. The result of all this is that we have a much more powerful magnetic field than that in the bell and many more watts of energy from which to induce a secondary current. Now the number of turns employed in the secondary circuit of our spark coil is very great, stepping its voltage up to thousands where the bell induced hundreds.

Suppose we now repeat our experiment in which we tried to light the gasolene in the watch crystal, using now the spark coil of the boat instead of our small "home-made" coil. In Fig. 121, B is the battery of five dry cells. S is a switch. V is the vibrator, which, like the hammer of an electric bell, makes and breaks the primary circuit. Of course the coil has a core of iron, although that is not here represented, and, of course, the coil has many hundred turns instead of the few here represented, and of course also it is built up of many layers instead of one as here represented. The secondary has very many more turns than the primary, but those in which the primary current passes are common to both circuits. There is also a condenser--not here represented, and not to be described in this book. The result of all this is that the secondary circuit has a voltage of between 5000 and 10,000, and a spark jumps across the gap at c between one sixteenth and one eighth of an inch long. This spark is hot enough to light the gasolene which I have put in the watch crystal at c.

Let us return to the bell for a few minutes. I have here a miniature lamp which requires 10 volts and .1 ampere, that is, 1 watt, which I will connect at S (Fig. 122). When now I close the primary circuit with two cells at P you notice that the lamp lights up, but faintly. It is not receiving .1 ampere. Remember we have only .75 watt at our disposal and this lamp requires 1 watt. Hence it is getting only three quarters enough energy. We connect in a third cell and now it lights up to full brilliancy. The resistance of this lamp must be about 100 ohms.

(10 volts)/(100 ohms) = .1 ampere

The resistance of the four boys might have been 60,000 ohms, and the voltage of the secondary circuit might in that case have been, say, 150.

(150 volts)/(60,000 ohms) = .0052 ampere

How does it happen that the secondary current had a pressure of 150 volts on the boys but cannot supply even the 10 volts required by the lamp?

Perhaps we can be brought to appreciate the answer to that question best by asking ourselves some others quite like it.

Why did not the man who built our mill two generations ago locate it upon the small stream that flowed near his house? The small stream was more conveniently located for him and it has quite as much fall as he got at the foot of this lake. We sometimes express the fact by saying that the "head of water" or the water pressure was quite as much in one of these cases as the other.

One boy said that the stream sometimes gives out. Another one said that it never did have water enough to run that wheel. "Undoubtedly the trouble is with the quantity," said I, "but I want to show you that we cannot maintain the pressure unless there is sufficient quantity back of it."

In Fig. 123, suppose A represents a small, slim tank of water three feet high. The water-wheel W, requires one gallon of water a minute pushed along by a three-foot head of water pressure to run it. The supply pipe S is bringing into the tank not more than one quart of water per minute. A gate at R enables us to regulate the flow of water, as we regulate the flow of electricity, by using more or less resistance. Now it is evident that if we close the gate, or partially close it, and allow the tank to fill with water, we may then open the gate and run the wheel for a short time, but the level of the water in the tank soon begins to fall and the pressure grows less and the wheel stops moving. It is just so with all generators of electric current. If we take from them more than they can supply continuously the voltage falls. This is notoriously true of dry cells. Like the water tank represented in Fig. 123, they "run down" if used continuously to furnish, say, one ampere of current, but they may furnish it for a short time, the voltage rapidly falling meanwhile. Then if given a short rest they "pick up" and will again furnish full pressure. The voltage of a dry cell falls somewhat when it is required to give the very small amount of current required to actuate a volt meter, say .015 ampere. Hence, our volt meter will not quite correctly show what the voltage of a single cell would be on open circuit. Notice that, when I put one cell upon this volt meter the needle shows 1.42 volts; but when I put four cells in series upon it the needle indicates six volts, as nearly as we can read it. That is, the voltage of each cell in this case appears to be 1.5. What has increased the voltage of a cell from 1.42 to 1.50? Simply this: when .015 ampere, the amount required by the volt meter, was taken from one cell it reduced its pressure, but when a multiplier with ten times the resistance was added we secured our reading by using only .006 ampere of current, and this did not appreciably reduce the true pressure of the cells.

The induced current from our bell when held back by 60,000 ohms of resistance in the four boys was able to push with 150 volts of pressure, and .0025 ampere passed without noticeably reducing this pressure, but when the same current was held back by only 100 ohms in the filament of the lamp nearly forty times as much current passed, and the pressure dropped to something less than ten volts.

"We will try an experiment to show how the voltage will suddenly fall when we reduce the resistance of your four bodies.

"Fill these two empty tin pails in which our lunch was brought with water from the lake and sprinkle in the salt left over from the lunch. Now twist a bare copper wire around the bail of each pail and connect these with the bell so as to get the induced current from its magnet. (See Fig. 124.) Let the two pails of water be the terminals of the two wires at S. Now you four boys wet your hands in the water and then join hands, and those at the two ends of the line put your free hands upon the outside of the pails of water while I close the primary circuit. You of course feel the current just as you did when you held the spikes in your hands in a former experiment. But now you two end boys put your free hands into the salt water, and you instantly get a very smart shock. The resistance is no longer 60,000. It has dropped way down to 2000, and if the voltage had remained at 150 you would have received a terrible shock, but the voltage has dropped down to five. It is as though you had been pushing very hard against a post and it suddenly gave way. You cannot push against a thing which offers no resistance. So the voltage falls when resistance is reduced, and particularly if the source of supply has very little capacity. Here is another experiment you must try when you go back to the city. At a certain water faucet in my laboratory the pressure is disagreeably high. The water flows with great force and spatters badly. We can easily reduce the pressure so that the water will flow in a limpid stream. Fig. 125 shows the situation; f is the faucet, and in the pipe underneath the sink there is a stop-cock c. This may be adjusted permanently so that the faucet f will act pleasantly. The same thing is represented again at the gas stove. Let f in the Fig. 125 represent a gas cock at the stove. Suppose the pressure is so high that the gas flames pass more gas than is readily consumed. It is possible to adjust a stop-cock like c further back in the pipe so as to produce hotter flames, get rid of the poisonous fumes of half burned gas, and cut down the monthly gas bills one half.

"My garden hose will usually throw a stream across the street, which is very desirable when one wishes to sprinkle the street, but this pressure is disastrous when I wish to sprinkle the flowers. Turning down the stop-cock at the nozzle makes it shoot a smaller stream but more spiteful in pressure, knocking the flowers to pieces and washing the soil away from their roots. But if I partially close the stop-cock at the side of the house where the hose is attached I may have the stream of water flow as gently as I choose.

"I should meet precisely the same situation if I tried to ring an ordinary electric bell by a 110-volt current, and I should use the same method of overcoming the difficulty.

"The great virtue of the dynamo is that it can furnish a large supply so that the voltage is kept constant on a great flow of current.

"I have not forgotten the question, but have tried to work toward its answer all this time. The question is, why did Ernest get a shock this morning when he touched only one binding post, and when the battery of five cells is not capable of giving shocks to any one who touches its binding posts directly? We need one more diagram to give the final answer. In Fig. 126 e represents the binding post from which the shock was received. B is the battery of five cells, C is the spark coil, G is the engine cylinder, f is the spark plug. When one wishes to start the engine he closes the switch S. This makes a continuous conductor from the battery to the metal cylinder itself. As the engine rolls over it closes the gap in the conductor at d for an instant. The primary circuit is then completed and the current passes from B to the cylinder, through the metal of the cylinder to d, then to the coil C, where it passes through a portion of the coil and then back to the battery. The vibrator on the coil causes the magnetic field to rapidly vary in strength. This induces a secondary current in the whole coil which, because it passes through a very great number of turns, has a high voltage. This passes from C through B to the base of the engine, then up the walls of the cylinder to the plug f, then jumps across the gap at a, causing the spark which explodes the mixture of gasolene and air in the cylinder. The spark plug f is porcelain--an exceedingly good insulator. Through the centre of this passes a wire from a to e. The current passes up this and back to C. Now the engine rests upon the floor of the boat, and Ernest stood upon the same floor. The wood of this floor when dry and clean is a very good insulator, but when wet, and particularly when wet with water that has ever so slight an amount of any salt in solution, it becomes a conductor for such high tension currents. When therefore Ernest, standing upon the floor of the boat, touched the binding post, e, this induced current of high voltage found it about as easy to pass from the metal of the engine cylinder through the wood to his body and through his body to e as to jump across the short air gap at a. There are two things upon which he may congratulate himself.

"1. While the coil stepped up the voltage so high it reduced the available quantity of the current, so that the shock was a safe one.