XXI Electric Waves
Much has been said about bringing young people up to do what they don’t like to do so as to make them strong and virtuous. My own life has always been guided by a different principle. It is: Find something worth while which you will enjoy doing, and do it with your might. I am bringing up my boy on the same principle. In September we have a real desire to get back to our work in the city, and in June we have an eager longing for the occupations of Millville. I am not aware that there is any part of my work which I would like to be relieved from, and Harold and his mother said that they were now ready to return to the city apartment with real pleasure for a winter.
One evening we were seated about the dinner table when Harold asked me how electricity could travel without wires. I replied, “It travels as light does. But I am very much puzzled to know why it ever follows a wire when light does not.” This did not settle the question and left us both unsatisfied, so I told him to invite two or three of his best friends in to-morrow evening, and I would perform some experiments for them that would at least help them to think further upon this subject.
When the evening came I showed the boys an automobile spark coil to which I had attached two knobs, a and b (Fig. 179), and with which I had connected two dry battery cells. When I touch the wire c to the binding post d a spark passes between the knobs a and b. When this spark occurs at least four kinds of waves pass out in all directions from the spark gap between the knobs.
First, sound waves go through the air. Our ears detect these. If the air is removed from around the apparatus no sound wave can go forth. A careful examination of the internal ear shows us that it is constructed so as to respond to such air waves.
Second, light waves go forth. These affect our eyes. We are blind to the first kind of waves and deaf to the second. The light waves travel without air—somewhat better without air than with air. A microscopic examination of the eye indicates that it is constructed so as to respond to waves. We believe there are waves in the ether which fills all space. Sound waves travel in air at the rate of one mile in five seconds. We had this nicely illustrated at the sea shore one summer. The steamer touched each morning at a wharf which we could plainly see two miles distant. We could see the steam arise when she blew the warning whistle, and with our watches we found that it always required ten seconds for the sound to reach us after we saw the steam of the whistle. This at least showed us that it takes five seconds longer for sound waves to travel a mile than it does for light waves to travel the same distance. For light had to travel the same distance before we could see the steam arise from the whistle. Although the time it takes for light to travel a mile is inconceivably small, we have a simple method of finding out that it requires eight minutes for light waves to come to us from the sun.
The satellites of the planet Jupiter, in revolving about that body, disappear and reappear at regular intervals, acting as flash lights to mark time.
The earth, being 92,000,000 miles distant from the sun, is 184,000,000 miles farther from Jupiter when at B than it is when at A. (See Fig. 180.) It is found by observation that sixteen minutes more are required for the light waves from a reappearing satellite to reach us at B than when we are at A. Hence eight minutes would be required for light waves to travel the distance from the sun to the earth. Although light travels at the inconceivable velocity of 186,000 miles per second, the nearest star is so far distant that it takes light three and a half years to come from it to us. The North star requires forty-two years to send its light to us, and Arcturus is so far away that waves of light sent out from it one hundred and sixty years ago are only just reaching us now, and if it should cease to send forth light now men would continue to see it for five generations yet to come.
A third kind of wave which goes forth in the ether from the spark gap of our coil is a heat wave. This affects neither our eyes nor our ears, but I will undertake to make you conscious of it by another method.
Before a mixture of gasolene vapour and air can be ignited its temperature must be raised to about 2000 degrees Fahrenheit. I will show that heat waves pass out from this spark gap by placing my watch crystal filled with gasolene underneath the knobs of the spark coil, (Fig. 181). When now I close the electric circuit at the battery the mixture of gasolene vapour and air just above the watch crystal is ignited. If I increase the distance between the knobs you still hear the crackle of the sound waves and see the light waves, but the mixture of gasolene vapour and air does not ignite, because there are not heat waves enough. The automobilist expresses this fact by saying a “fat” spark or a “warm” spark is needed. A battery which has ceased to give a sufficiently hot spark to explode the mixture of gasolene and air in the cylinder of a gasolene engine may serve all other purposes quite as well as ever. It may ring bells almost as long as it ever would.
I proved that the temperature for igniting a mixture of gasolene vapour and air was nearly as high as melting iron, by heating an iron rod to a dull red heat and bringing it to the watch crystal containing gasolene. It did not take fire. I showed that it could not be ignited by a lighted cigar, nor even by a glowing coal taken from the fire.
It was necessary to heat the iron rod to a very bright red heat—nearly white heat, or nearly to its melting point, before it would ignite the mixture.
These heat waves are ether waves, differing from light only in having greater wave length. They travel at the speed of light, they travel better without air than with air. They come from the sun and all other light-giving bodies. Indeed, an ordinary incandescent electric lamp gives out about twenty-four times as much energy in heat as in light. Heat waves are being thrown off from all bodies which are around us. The steam radiators are placed in this room for the express purpose of sending out heat waves through the ether in this room. This is the chief method of distributing heat, and it is hindered rather than helped by the presence of the air. The walls, ceiling, floor, furniture, people—everything here is sending out heat waves.
The fourth kinds of waves, which go out from the spark gap of our coil, are also waves in the ether. They are still longer than heat or light. We have ears for sound, eyes for light, and temperature sensation for heat, but as yet we have not evolved a delicate sense organ for detecting electric waves. At least few of us claim to have such a sense. I will, however, undertake to make you feel electricity. I then adjusted the coil so that each boy might take a mild electric shock from it by touching the two knobs. That is by placing himself in the spark gap. They agreed that although they could not hear, see, taste, or smell electricity they were a little more familiar with it now, having felt it.
Sound waves in air, as given out by the piano, vary in length from, say, four inches to forty feet, those having the shorter wave length being the higher pitched tones.
Light waves in the ether, as given out by the sun, vary in length from, say, 1⁄60000 to 1⁄80000 of an inch, those having the shorter wave length being the violet-coloured light, which may be seen in the rainbow, and those having the longer wave length being the red-coloured light of the rainbow or the sunset.
Heat waves, which are also waves in the ether, vary in length from above 1⁄80000 to, say, 1⁄5000 of an inch. Roentgen or X waves are ether waves, shorter than light; while Hertzian, or wireless telegraph waves are very long ether waves, varying from a few feet to many rods in length. Those used by Marconi in sending despatches across the Atlantic Ocean are as long as 1000 feet, four or five of them cover a mile, and 12,000 of them cover the whole distance from Cape Cod to Poldhu.
Electric waves are easily broken up into the shorter heat waves, or the still shorter light waves. On the other hand Roentgen waves are readily transformed into the longer light waves, and are thus brought within our powers of vision.
Sound waves of various lengths (of high and low pitch) all travel at the same speed (one mile in five seconds), else how would the piccolo and the bass horn of the distant band sound together. So ether waves of various lengths (light, heat, electricity, etc.) all travel at the same speed, i. e., 186,000 miles per second.
For detecting the electric waves which may be sent out from the spark gap of our automobile spark coil I shall ask you to help me prepare a special piece of apparatus. One boy may file this silver ten-cent piece and another may file this nickel five-cent piece, each gathering the filings upon a piece of paper. A third boy may select a piece of glass tubing about one eighth of an inch in the inside diameter, and with a three-cornered file cut off a short piece, about one and a half inches long, and smooth the ends with a wet file. A fourth boy may select a piece of stout copper wire nearly as large as the bore of the glass tubing, and cut from it two pieces, each about two inches long. Wind one end of each of these with thread to make them fit snugly in the glass tubing.
We thrust one of the wires into the tube, then mixed equal parts of the silver and nickel filings and put as much of the mixture into the tube as we could hold upon the tip of a penknife blade, and then thrust in the other copper wire. (See Fig. 182.) The ends of the wire were about one eighth of an inch apart and the gap was loosely filled with the metal filings. This was connected by short pieces of copper wire, as shown in Fig. 183, to a dry battery cell, B, and a sensitive ammeter. When all connections were made the needle of the ammeter remained at zero, showing that no electric current was passing, that is, the battery cell was unable to send any electricity through the metal filings.
This is the apparatus which is to help us detect electric waves when they pass about us. Electricity has been called invisible light, that is, invisible to our eyes, and this apparatus has been called an “electric eye” because it will detect electric waves in the ether, just as our eyes may detect light waves passing through the ether.
We placed the automobile spark coil upon the table near to the tube containing the filings of silver and nickel, and as soon as we made a spark pass between the knobs the ammeter needle moved half way across the scale, indicating that the spark had somehow influenced the metal filings in the tube so that now they permitted the battery cell to send some electric current through them and through the ammeter. I asked one of the boys to tap the tube slightly with a lead pencil so as to jar the metal filings, and as soon as he did so the needle of the ammeter went back to zero.
The spark coil sent electric waves out in every direction, and those which hit the metal filings made them cohere together. In that condition they allowed the dry cell to send through them enough current to move the needle of the ammeter. Tapping the tube made the metal filings break apart again, in which condition they do not allow the current of the cell to pass in sufficient quantity to move the needle. This tube is called a coherer, because the filings in it cohere together. The apparatus then serves to indicate when electric waves are passing. As yet, however, it would not respond when the spark coil was more than one foot away. Our next step was to attach extra pieces of wire, each ten or twelve feet long, at either end of the coherer, as indicated in Fig. 184. One of these wires was stretched out upon the floor while the other one was connected with the wire of a picture hanging upon the wall.
We now found that the coherer would respond when the spark coil was operated several feet away. The extra wires which we had attached to the coherer are called antennæ, because they suggest the long “feelers” or antennæ of some insects.
Our next step was to put antennæ upon the spark coil also, as shown in Fig. 185. One of these wires was stretched out upon the floor, while the other one was connected with the wire of a picture hanging upon the wall on the opposite side of the room from where the coherer was. We now found that the coherer would respond when the spark coil was operated in the farthest part of the room. With the wires which were lying upon the floor extending toward each other, but lacking several feet of touching, the coherer responded when the spark coil was operated in various other rooms of the house, although the doors between were shut. When the floor wires were connected to the water pipes the coherer would respond when the spark coil was operated in a neighbouring house. We tried a similar experiment, substituting an ordinary electric bell for the spark coil. The coherer or electric eye detected that ether waves were sent forth from an electric bell every time a spark was produced in the bell. For this purpose connections were made, as shown in Fig. 186. One dry battery cell was used to ring the bell. The floor wire a, or, as it is usually called, the ground wire, was connected to the binding post 1, and the other antenna was connected to the screw 3, and then supported aloft on a picture hung upon the wall. With this transmitter we sent waves across the room which were detected by the coherer.
We constructed a simple spark coil as follows: We bought a pound of No. 24 single cotton covered copper wire, such as is used in the electro-magnets of bells. It was, when we bought it, wound upon a wooden spool. We filled the hole in the centre of this spool with wire nails. One dry cell was connected with this (Fig. 187). When the wires at d were touched together, and then separated, a spark was produced at that point. A ground wire was connected at b, and an antenna at c, as before. Using this apparatus now as a transmitter of ether waves, we found that the coherer detected them.
We next gave our attention to making changes in the receiving apparatus, not to change the coherer, but to provide substitutes for the ammeter. A sensitive relay was procured, which is essentially like a bell or buzzer except that it does not clatter. It will be readily understood, by referring to the accompanying Fig. 188, that R is a coil of insulated wire around an iron core exactly like what we see in the electric bell. (In practice there will be a pair instead of one of them.) Such coils are called electro-magnets, because when electricity flows in the wires they become magnets, and will attract iron. A is an iron spring, B is a dry battery cell and C is the coherer. Whenever an ether wave passes the coherer permits the battery cell to send a current around the magnet of the relay, and it attracts the iron spring a, so that it hits against the metal post d with a click. Whenever we used this to respond to ether waves the click of the relay suggested the telegraph sounder. How it served in wireless telegraphy will appear in the following pages.