We took a small induction coil (Fig. 164) c and fastened one end of the primary circuit to a battery, B. The wire at the other end of the primary circuit was bent into a hook h. This hook was adjusted about a quarter of an inch from the end of the iron core of the coil. The other wire from the battery was attached to the steel strings of a piano, P. When the coil c was brought over a string and the hook h was allowed to pass beneath the string and touch it very gently, the primary circuit was closed through the string, which served as an interrupter of the current and vibrated according to its tone. The secondary coil, not represented in the figure, was connected to a distant telephone receiver, which reproduced the tones of the piano strings.
Producing a tone is merely a matter of making something vibrate with the required frequency. It may be a piano string, or a tuning fork, or a reed of an electric buzzer, or the diaphragm of a telephone receiver. If it vibrates 256 times a second, it will produce the same tone as middle C on a piano; if it vibrates 512 times a second it will produce the C which is an octave above, and if 128 times a second an octave below middle C. The human voice is produced by vocal cords in the throat, which vibrate with the proper frequency to give any required tone. But how can we make the human voice act as an interrupter of the primary circuit? An examination of the telephone transmitter will supply the answer to this question.
The boys after taking the transmitter (Fig. 165) apart proceeded to make one which should answer the purpose as follows: A block of wood about one inch thick and three inches square (Fig. 166), A, was hollowed out, making a cone-shaped cavity about one half inch deep and one inch broad. This cavity was filled with small pieces of graphite, G, made by cutting up a lead pencil. An old tin-type, D, was laid over this as a diaphragm and tacked around the edges. A binding post, E, passed through the block, its head being buried in the graphite at the bottom of the cavity. The binding post F furnished contact with the tin-type. One dry cell was placed at B and the sensitive ammeter was connected at C. The needle showed that although a small current was passing it was constantly varying in strength. Tapping upon the table, walking across the floor of the room, shouting, and particularly whistling, caused variations in the conducting power of the graphite and consequently variations in the current strength. This is precisely the condition we wished to produce in the primary circuit.
We next substitute for the ammeter at C the primary and secondary coil of the telephone. In Fig. 167 T is the transmitter, B is a battery of two dry cells, P is the primary winding of the coils, and S is the secondary winding. To this a telephone receiver, R is connected by wires long enough to reach into another room. A person holding the receiver at his ear could hear everything said or done in the room where the transmitter was almost as plainly as though he were present in the room.
Two such transmitters were made and the second one was placed in the room where the receiver had been, while a second receiver was installed near the first transmitter. The arrangement is shown in Fig. 168. T is the transmitter at one end of the line and T' the transmitter at the other end. B and B' are the batteries at each end, P and P' the primary coils, S and S' the secondary coils and R and R' the receivers. With this arrangement two persons carried on a conversation with perfect ease, holding the receivers to their ears, presenting their mouths to the transmitters and speaking in moderate tones. H and H' are hooks upon which the receivers are to be hung when not in use. These hooks act as switches to open and close the primary circuit. A spring normally pushes the hook upward and closes the circuit, but while the receiver is hanging upon it the circuit is open at this point. Thus the battery is saved from running down when the telephone is not in use.
The wires were finally extended from the mill to the cottage and this equipment was installed at each end.
It will be noticed that the secondary circuit includes two receivers and two secondary coils besides the wire of the lines to offer resistance.
The receivers offer 75 ohms of resistance each. The secondary coils offer 250 ohms each and the line wires between the mill and the cottage offer 100 ohms. This makes a total of 750 ohms for the secondary circuit. But the rapid alternations which are induced in the secondary circuit impede the electric current ten times as much as the resistance already mentioned.
When considering alternating currents passing through coils of wire we are obliged to take into account two kinds of resistance:
1. Ohmic resistance.
2. Impedance.
"You boys understand the resistance to the flow of the electric current, which we have so often measured in ohms. But I want to show you that there is another kind of resistance which alternating current meets. Here is a coil containing 1000 feet of No. 20 copper wire. I throw on to it, for only an instant, the 110-volt direct current, and the ammeter reads 11 amperes, showing that it offers a resistance of 10 ohms to the direct current. I now throw on the alternating current, and the ammeter shows only a small fraction of an ampere. The surging of the current back and forth induces a counter electro-motive force, in the successive layers of the coil, which we call impedance. In the experiment which we have just performed impedance is fifty times as important a factor as ohmic resistance. Impedance depends chiefly upon the frequency of alternation. The impedance in telephone circuits is particularly large because of the extremely high frequency of the alternations produced by the tones of the human voice, these being usually not far from ten times as rapid as those of alternating currents in common use.
"We may estimate the total resistance of our telephone circuit as equivalent to 7500 ohms.
"Our secondary coils have forty times as many turns as the primary coils, and by means of them the voltage is stepped up to somewhere near one hundred on open circuit. When closed through the line, however, the voltage drops down to about ten. The result is that the actual current which passes between the cottage and the mill when we telephone is not far from .001 ampere. We may, however, hear a whisper transmitted by .000001 ampere or less.
"The tone E´ which is produced by the tenth key above middle C on the piano, is the one most readily heard over the telephone. It is produced by anything which vibrates 640 times per second."
We used No. 12 galvanized iron wire for our telephone lines. Two miles of No. 12 copper wire would offer 16 ohms of resistance. The iron wire offers about 100 ohms. But this is a trifle when compared with the total resistance. We used a double metallic circuit so as to avoid the effects of inductance from our electric lighting circuit.
The next thing that we were obliged to consider was some arrangement for calling persons to the telephone for conversation. We decided to use magnetos and alternating current bells. Fig. 169 shows the essential mechanism of the bells. The bell at each end of the line consists of two gongs a, b and a´ b´, with a hammer c, c´ between them. This hammer is attached to an iron armature h, h´, pivoted over the electro-magnets, m, m´, in such a way that it rocks back and forth when an alternating current passes through the lines d e, f g. The bells at both ends of the line always ring together, since they are connected in series. A magneto (Fig. 170) is situated at each end of the line. This, as has been previously explained, is a generator of electricity, in which the field is furnished by steel magnet, M. The armature A is a coil of wire whose ends are in contact with the leading out wires d and c by means of brushes which slide upon rings. The armature is revolved by hand. The crank and cog wheels employed to produce high speed are not shown in the figure. By turning the armature rapidly this magneto will develop 60 volts e. m. f. on open circuit. The magnets of the bells are wound with a very large number of turns of very fine wire, so that .025 ampere is sufficient to ring them.
Figure 171 shows how the magneto at either end of the line is introduced into the circuit for the purpose of ringing the bells. B and B' represent the bells, m and m' the magnetos, and P and P' represent switches. Springs push them upward so that they normally close the circuit through the bells. When a person at P wishes to call another at P' he pushes the switch P down so as to bring his magneto m into series with the bells. When now he turns the crank and generates the electric current, both bells ring. His own bell serves the purpose of telling him that the line is operating all right. The other bell calls the party desired for conversation. As soon as the operator removes his finger from the switch P the spring throws it upward again, leaving his bell in circuit, so that he may be called at any time, but cutting out of the circuit his magneto, which would introduce unnecessary resistance.
The same wires which carried the current for ringing the telephone bells carried also the current for operating the telephone receiver. When the receiver is removed from the hook it releases a twofold switch. This serves the double purpose of closing the primary circuit through the local battery and substituting the telephone receiver circuit for the bell-ringing circuit upon the line.
We used fifty chestnut poles to carry our line between the mill and the cottage. Each pole had a cross bar, on one end of which the electric light and power wires were carried and on the other end the telephone wires. Glass insulators prevented the wires from coming in contact with the wood of the cross bars. The necessity for this was impressed upon the boys by something which happened while they were stringing the wires. The telephone apparatus at the mill had been installed and the two leading out wires had been connected to it. One of these was coiled up on the floor, while the other had been strung along upon the poles for half a mile, but had not yet been attached to the insulators on the poles. While the boys were lunching at the mill, one of them gave the crank of the magneto a turn, when, to the astonishment of all, the bell rang. The circuit had been completed through the damp wood of the mill, through the damp wood of some of the poles, and through the earth. After lunch the wire, so far as it had been strung, was fastened to the insulators upon the poles. But when some one turned the crank of the magneto the bell still rang. We walked along the line to see where the difficulty was. We found the end of the line about half a mile from the mill dangling free from the ground, but touching a tall spear of grass. When this was moved away from the spear of grass the magneto could no longer ring the bell. The slight current required to ring this bell—.025 ampere—had found its way through the spear of grass, through the woodwork of the mill and through the earth.
We had no sooner got the two telephone wires properly strung and attached to the hundred glass insulators when a thunder storm came up, and drove us back to the mill for shelter. Pretty soon the bell rang and we, supposing that some one at the cottage was trying to call, went to the instrument, but could get no response, nor could we make the bell ring. Lightning had sent an alternating current over the line which rang the bell, but the strength of the current was too great for our coils of fine wire and one of them was burned out, as we say. In other words, the wire had melted at the point where it offered the greatest resistance.
The burned-out coil was replaced, and then we installed lightning arresters which were of two kinds. The first were simply fuses which were introduced into the line to protect it against any current too large for the apparatus to carry, and the second was a plate, c (Fig. 172). These are to be found upon the top of the magneto cases. A wire is connected with c, and its other end is grounded by being connected with a piece of iron pipe which is driven deep into moist earth.
The plate a b is inserted in the line, and the gap between this and the plate c offers sufficient resistance so that the telephone circuit suffers no leakage at this point, but lightning has such extremely high tension that it readily passes across this gap and finds its way to the earth without damaging the instruments.
We have already noticed that our alternating current dynamo, which produces 60 vibrations per second in the telephone receiver, causes it to give a tone very nearly like the C, which is two octaves below middle C upon the piano. C requires 64 vibrations per second. We may speed up our dynamo so as to make it yield a tone exactly like C or even above it.
Dr. Cahill of Holyoke, Mass., has devised an organ in which alternating current dynamos produce the necessary number of vibrations for each tone. The name telharmonium has been proposed for this organ. It has a separate dynamo for each tone, each dynamo having a frequency corresponding to the tone required of it. The dynamo, for instance, which produces middle C makes the electric currents surge back and forth 256 times a second, and this causes the diaphragm of a telephone receiver to vibrate 256 times a second, and this sends forth 256 air waves per second, and when these reach our ears we recognize the tone we call middle C. The frequency of alternation in a dynamo may be increased by either increasing its speed of revolution or by increasing the number of coils upon its armature.
Mr. Cahill's great organ looks like a large machine shop with many counter shafts geared so as to run at different speeds. On each shaft are a large number of little dynamos whose armatures have various numbers of coils. The organist, who may be far removed from this "machine shop," fingers an ordinary keyboard. Each key opens and closes a switch, thus bringing into action its own dynamo.
If the key which is known as C, one octave below middle C, is pressed down, a switch closes the circuit between the telephone and a dynamo which gives 128 double alternations of current.
The tone which is produced by 128 vibrations per second is the one most often heard from a man's voice in ordinary conversation.
Another key brings into action upon the same telephone receiver—and at the same time if desired—a dynamo which gives twice as many alternations per second and produces the tone most often heard in female conversation. It is middle C.
Another key might bring into action a dynamo which gives 64 vibrations per second to the diaphragm of the telephone receiver. This would send forth a tone very nearly like the base note of our 60-cycle alternating current dynamo.
The following table shows a series of ten tones which might be produced by the same little piece of sheet iron in a telephone receiver played upon by ten dynamos at the same time. The whole list of ten tones would sound well when produced simultaneously. The great mystery is that the iron disc can vibrate in such a complex manner. It is important to note, however, that the number of vibrations in each of the upper tones is a multiple of that of the lowest tone:
| 2nd octave above Middle C | C´´—1024 | (= 16 × 64) |
| G´ — 768 | (= 12 × 64) | |
| E´ — 640 | (= 10 × 64) [A] | |
| 1st octave above Middle C | C´ — 512 | (= 8 × 64) |
| G — 384 | (= 6 × 64) | |
| E — 320 | (= 5 × 64) | |
| Middle C | C — 256 | (= 4 × 64) [B] |
| G — 196 | (= 3 × 64) | |
| 1st octave below Middle C | C, — 128 | (= 2 × 64) [C] |
| 2nd octave below Middle C | C,,— 64 | (= 1 × 64) |
| [C] The tone most easily reproduced by the vocal cords of a man. | ||
| [B] The tone most easily reproduced by the vocal cords of a woman. | ||
| [A] The tone which the telephone receiver responds to most readily. | ||
| The table covers the range of the human voice, male and female. | ||
All the intermediate tones, with their sharps and their flats, are produced each by its own separate dynamo.
The insignificant amount of current required to operate a telephone receiver makes it possible to furnish the music of these dynamos to many and far distant telephones. This naturally suggests the idea of having a great musician perform upon the keyboard and have many auditors scattered about the city in their private homes or even in many public halls, for the telephone receiver can readily be made audible to a good-sized audience.
The boys asked me what arrangement of electric bells we needed at the cottage and so I gave them this problem to work out by themselves:
1. We want a bell in the kitchen to be rung by a push button at the front door. But there are times when no one is in the kitchen and hence,
2. We want a bell upstairs to make a single stroke whenever the kitchen bell is rung from the front door.
3. We want a floor push under the dining-room table which will cause the kitchen bell to ring a single stroke.
4. We want a push button in the dining-room which will cause both bells to clatter and call people from their beds, from the piazza, the lawn, etc., to their meals.
This equipment needs only one battery of two dry cells, two bells, three push buttons and about two hundred feet of wire. It should cost less than five dollars.
The boys drew many plans and tried many schemes and at last determined upon the plan shown in Fig. 173.
P is the floor push under the dining-room table. When the circuit is closed at this point the current leaves the battery from the carbon pole c, passes up and around the magnets of the kitchen bell and back to the zinc pole of the battery z by way of the push button P. All other circuits are open.
P´ is the push button at the front door. When the circuit is closed at this point the current leaves the battery at c, passes up to the right-hand binding post of the kitchen bell and divides, part going through each bell. The portion of the current which goes through the kitchen bell passes around the magnets and through the armature to the left-hand binding post before it can find a path back to the battery. Hence, the kitchen bell clatters. The portion of the current which passes to the upper bell goes around its magnets and finds a path back from the middle binding post to the battery by way of P´. Hence the bell upstairs rings with a single stroke.
P´´ is a push button situated upon the wall by the side of the door which leads from the dining-room to the kitchen. When the circuit is closed at this point, the current leaves the battery at c, passes up to the right-hand binding post of the kitchen bell and divides, part of it going through each bell. The portion which goes through the kitchen bell passes around its magnets and through its armature to the left-hand binding post, then up to the middle binding post of the upper bell, through its armature to its left-hand binding post and back to the battery by way of the push button P´´. The other portion of the current passes directly up to the right-hand binding post of the upper bell, around its magnets, and through its armature to its left-hand binding post, thence back to the battery by way of the push button P´´. Hence, both bells clatter and keep time with each other. The upper bell will ring independently of the lower bell, but the lower bell is dependent upon the upper one to open and close its circuit, somewhat as a relay.
Soon after the cottage had been equipped with electric bells I went to the mill one day and found a push button at the door. Upon going in I was curious to examine the electric bell outfit of that place and found what is illustrated in Fig. 174.
A switch, S, had been attached to the bell. The boys said that when they felt well they kept the switch upon the left-hand point and the bell rang as a clatter bell. When they felt a little sick they put the switch upon the middle point and the bell rang with a single stroke, but when they felt very sick they put the switch upon the dead point and the bell did not ring at all.
For the sparking equipment of the motor boat we use dry cells which have an internal resistance of not more than .06 ohm. They will, when short circuited through the ammeter for only an instant, give 25 amperes.
(1.5 volt)/(.06 ohm) = 25 amperes
When we allow for a slight resistance in the ammeter itself, and for the drop in voltage, we see that the internal resistance of a cell must be even less than .06 ohm.
After being used about two months upon the motor boat these cells develop more internal resistance, and they will then show not more than six to ten amperes when short circuited through an ammeter. They are then not reliable for ignition of the engine, but are quite as good as ever for bell-ringing, and often continue so for more than a year. The result is that we always have more partly run-down dry cells than we can use. Seeing them about has stimulated the boys to devise ways for using them.
The housekeeper is distracted by carrying on so many cooking processes at one time. She forgets the eggs, and lets them boil five minutes instead of three because the coffee must percolate twelve minutes, and she lets the coffee percolate twenty instead of twelve minutes because the biscuit must bake twenty minutes, and the biscuit are forgotten because the pies must come out in thirty minutes, and the cake in forty minutes. All this worries the cook. Harold is a sympathetic boy and enters into the troubles of others. I had at one time shown him how to bore a hole in a glass plate in five or ten minutes by using a round file wet with water. One day he presented the kitchen with a clock, intended to relieve the burdened memory of the cook. This is represented in Fig. 175.
An ordinary kitchen clock had a hole bored through the glass which covers its face. This glass is easily moved around in its metal rim, bringing the hole over any desired minute upon the face. One wire of the battery is attached to a leg of the clock, the other goes to a bell, and then the wire from the bell is poked through this hole. When the minute hand reaches that point the electric current is closed through the metal of the clock, and the bell rings warning that the eggs, coffee or what not are done.
We each urged that our memories should share in the vacation, and applied for one of these outfits. I took one of the clocks and cut back the minute hand so as to make it shorter than the hour hand, and then had the hole in the glass made so that the hour hand should close the electric circuit. This was kept at my study table and reminded me of my appointments. Some used these clocks to alarm themselves in the morning when they slept overtime.
Another reminder is shown in Fig. 176. C is a float which rises and falls with the water in our house tank. A cord running over two pulleys connects this with a weight, d, hanging in front of a scale upon the wall of the kitchen. This indicates how much water there is at any time in the tank, which is situated in the garret. The boys arranged a bell and battery so that when the tank is nearly empty the weight d will pull upward a spring, a, and make it close the circuit through the bell to warn that water must be pumped. When the tank is nearly full the weight d pushes down the spring b and rings the bell again.
Harold said that yeast cakes were the heaviest tax upon our memories. If some one started for the village store, before he got out of hearing, a call would come after him, "I forgot the yeast cake. Please put that on the list." When one returned from the village store with numerous packages, he would generally hear, "My yeast cake was forgotten." We tried all sorts of schemes to get rid of this yeast-cake nuisance, and finally adopted Harold's "curled bread" project.
We had built a brick oven out back of the house for experimental purposes. Harold proposed that the boys bake a month's supply of bread at a time, and, when it was a day or two old, cut it all into thin slices and let it dry. These slices curled up as they dried and were known as "curled bread." A flour barrel was filled with it each month. It kept perfectly any length of time. The family voted it to be better than crackers and better than fresh breadstuff of any kind.
Harold's suggestion regarding yeast cakes worked so well and was such a relief to our memories that I proposed he next attack the problem of the often forgotten salt in cooking.
We had no end of experiments with brick ovens. One of the most interesting was that wherein we used the brick fireplace as an oven and did the family baking in it. On a cold morning we would build up a smart wood fire in the fireplace and enjoy it during breakfast time. Then we shovelled out the coals and the ashes, and shut it up tight with a sheet iron arrangement and utilized the heat stored in the bricks for doing all sorts of cooking.
Our outdoor brick oven and our monthly baking day were such a success that they led to the construction of another oven of smaller dimensions for the kitchen. This one was heated by electric lamps—one in each of the eight corners. It had double glass doors in front so that the cooking process might be watched. The glass of the inner door would be clouded with moisture for a while, when the cooking first began, but this would soon clear up, and then the lamps enabled us to watch the colour changes in baking, etc. The lamps in the upper part of the oven were connected with a different switch from those in the lower part of the oven, so that we were able to control the browning on top or bottom at pleasure.
Harold introduced a device for automatically controlling the temperature of this oven.
Strips of brass and iron, B and I (Fig. 177), were riveted together. These were fastened in the socket A. They are shown edgewise in the diagram. The upper end of this compound strip is free to bend back and forth in the plane of the paper, as here represented. They normally touch the screw C. One of the electric light wires runs from the lamps in the oven to this screw C. One wire of the dynamo circuit G goes to the lamps, and the other connects with A. Thus the compound strip acts as a switch to open and close the circuit upon the lamps.
This thermostat, as it is called, was placed inside of the oven. Heat causes brass to expand more than iron and therefore when the temperature reaches a certain height the thermostat curves, so as to break the contact with C, and the lamps go out. When the temperature falls a little the thermostat straightens until contact is again made with C. C is a screw and can be made to advance or recede in its socket E, so that the temperature of the oven may be maintained at any point desired. The wire of the screw C extends to the outside of the oven, where it carries an index, D, over the face of a dial, as shown in Fig. 178.
The cook may set this index at any desired degree, and the lamps will indicate when that degree has been reached. The thing to be baked is then put inside and the clock, illustrated in Fig. 175, is set so as to warn when the time is up.
The electric spark which occurs when the thermostat breaks contact with C causes the metals to corrode at that point, and corroded metals are poor conductors. This corrosion is due to the oxygen of the air. There is one metal—the expensive platinum—which is not corroded by the electric spark. We drilled small holes in the end of the screw C and in the brass strip and pounded into these holes little pieces of platinum wire. Harold said he felt like a dentist filling a tooth. This furnished good, clean contact at all times.
It takes a long time to heat up the brick oven, but it holds its heat a long time and makes an excellent fireless cooker after the lamps are turned out. It does not allow heat to escape into the kitchen, which makes it a comfort in our summer cottage. We are all becoming daft on slowly cooked food—a sort of ripening process which gives time for the chemical changes to take place and develops the finest flavours of the food.
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.