The next time I sauntered down to the mill the boys were working on what they called an electric shower bath. They had fastened upon the wall of the bath room an electric bell (Fig. 149), and placed on a shelf near by a battery of two dry cells, P. The switch which closed this primary circuit was on the wall by the side of the faucet and electric heating switch (Fig. 148). One of the wires, S, for the secondary circuit was carried up and connected to the pan A (Fig. 148). The other wire was fastened to a sheet of zinc about a foot square, which lay upon the floor of the shower bath. The idea was that when one was taking a shower bath, if he chose to vary his sensations he might step upon the sheet of zinc, close the switch in the primary circuit and let the secondary current pass through his body by way of the shower. They said that it was particularly prescribed for slow people.

Speaking of chores, of course the most insistent chore was to keep the storage batteries stored. This process gave rise to many questions, through which the information contained in the next chapter was brought out.

XV

ELECTRIC CURRENTS FROM CHEMICAL ACTION AND CHEMICAL ACTION FROM ELECTRIC CURRENTS

Alessandro Volta (1745–1827) of Como, Italy, took up the discovery of Galvani, interpreted it correctly, and perfected the method of producing electricity by chemical action. What these two men really discovered was that it is possible to produce continuous currents of electricity. Before that electricity was known only by the instantaneous discharge or spark. From the name of Volta is derived the word volt, which designates the unit of electro-motive force. The adjective voltaic is synonymous with galvanic, as voltaic or galvanic cell, voltaic or galvanic current. For a long time it was thought that such an adjective was needed to designate electric currents generated by chemical action as a peculiar kind of electricity. We no longer think of electricity which is generated by chemical action as different from that generated by a dynamo or from any other source.

For about seventy-five years after the discovery of Galvani chemical action was our only method of generating currents of electricity, and it is largely owing to the inadequacy of this method of production that so few uses for electricity were discovered previous to the perfection of the dynamo about a third of a century ago. Two things have conspired to bring about this age of electricity. (1) The dynamo reduced the cost of production from five dollars to ten cents per kilowatt hour. (2) Mankind grew extravagant, greatly increased the number of things which it considered necessary, and at length became both able and willing to spend more for the things which it demanded.

The so-called voltaic cell is of scarcely more than academic interest now. The school which, as a rule, follows half a century behind practical life, has taught and still teaches the philosophy of the galvanic cell with great particularity. It is now being urged to undertake the teaching of the dynamo. Meanwhile the dynamo has almost driven out of existence all electric battery cells except the storage cell and the so-called "dry cell," and each year the dynamo is encroaching more and more upon the territory of the dry cell. In the present day, when a passenger upon a street car pushes a button to stop the car, he uses, not a voltaic cell, but a 500-volt dynamo current to ring a small buzzer, and it costs the company not one-hundredth part as much as it would to furnish him a battery equipment to do the same thing. Small dynamos and magnetos are displacing dry battery cells in the sparking equipment of motor boats and automobiles.

We lifted a dry battery cell out of its pasteboard case and found that it was contained in a metal cup of sheet zinc. The top of this was sealed over airtight with pitch, the purpose of which is to prevent this "dry" cell from drying up. We dug away the hardened pitch and found a black powder which was distinctly moist. In case the pitch becomes cracked or a hole appears in the zinc cup, the moisture passes out and the cell ceases to act as a generator of electric current.

The zinc cup had a lining of pasteboard on the sides and the bottom, similar to the pasteboard which enveloped the outside, only the lining was quite moist. A corrugated rod of carbon about an inch in diameter occupied the middle of the cup, and the space around it was packed full of a mixture of ammonium chloride, manganese dioxide, and other substances like plaster, etc., which differ with different cells. A dry cell which has been long in use is quite apt to show stains upon its pasteboard case. These are caused by holes which appear in the zinc. The production of electric current by the cell is dependent wholly upon a chemical action between the zinc and the ammonium chloride which results in the destruction of both. This chemical action cannot go on without moisture.

The zinc cup of the particular cell which we were examining appeared to be intact, and we proceeded to dig out the black powder. Its black colour is due to the manganese dioxide. Ammonium chloride is white. We lifted out the carbon rod and scraped the zinc cup clean. The binding posts attached to both the zinc cup and the carbon rod were left intact. Into the zinc cup we now poured a tumblerful of water and added about a quarter of its volume of hydrochloric acid, setting the whole into a large bowl to guard against disaster. Bubbles of gas were formed rapidly, causing the liquid to effervesce as a tumbler of soda water would do. We inverted an empty tumbler over the cup so as to collect this gas. In about two minutes we lifted the tumbler, still holding its mouth downward, and brought a lighted match to it. There was a flash and the contents burned with a pale-blue flame. Some of the zinc had united with some of the hydrochloric acid and set free hydrogen gas, which is one of the constituents of the acid. This is typical of chemical actions. Something similar takes place between the ammonium chloride and the zinc. Three interesting things occur in this experiment:

1. Chemical action, just described, is produced.

2. Heat is produced. This was very evident when we took the zinc cup up in our hands. It was as hot as though boiling water had been put into it.

3. An electro-motive force is produced. This we showed by connecting one end of a piece of copper wire to the binding post of the zinc cup and the other end of the wire to an electric bell. Another wire ran from the bell to the carbon rod. When the carbon rod was lowered into the acid the bell rang.

Within ten minutes holes began to appear in the side of the zinc cup. The acid contents began to flow out into the bowl, and not long after the zinc fell to pieces. After fifteen or twenty minutes the action began to grow less. The acid was being used up as well as the zinc. If enough acid is added the zinc will wholly disappear.

We have chosen substances which would produce striking results in this experiment, but the same sort of thing is going on about us continually.

One summer by the seashore I fastened a brass plate upon my boat with two screws—one of brass and one of galvanized iron. The plate was attached below the water line so that it might be acted upon by the salt water. Within three weeks the head of the galvanized iron screw had entirely dissolved, while the brass screw was as good as ever. A galvanized iron screw near by but not in contact with the brass was still in as good order as ever. I had simply made an electric battery cell out of the ocean by dipping into it zinc and brass in contact.

A most interesting relationship exists between the three kinds of activity in the cell, which have been mentioned, viz.: (1) chemical action; (2) production of heat; (3) production of electric current.

As has been already noted, chemical action produces heat. Conversely, if we apply heat to the cell we greatly increase its chemical action. We have also noted that chemical action produces an electric current, but unless the current is allowed to flow through some external channel like a closed circuit of wire the chemical action is greatly restrained or entirely checked.

In a glass tumbler I put a rod of pure zinc (Fig. 150, Zn), and an electric light carbon, C. A short wire, a, was arranged for connecting the two externally. In the tumbler was put some water with about one tenth its volume of sulphuric acid. No chemical action was evident until the wire was touched to the zinc, closing the circuit. Then bubbles of hydrogen gas gathered upon the surface of the carbon rod, and clung to it very tenaciously. We lifted out the carbon rod and rinsed off the bubbles in another tumbler of water, and then returned the carbon to its place in the cell. The experiment was repeated many times, and each time no bubbles of hydrogen, which is in this case the sign of the chemical action, appeared until the circuit was closed for the flow of the electric current. Incidentally it should be said that the amount of hydrogen produced by the chemical action is a measure of the amount of electric current produced. Incidentally also it should be said that the bubbles of hydrogen clinging to the carbon rod check and almost stop both the chemical action and the production of electric current when the circuit is closed. If now we put in sodium bichromate to use up the hydrogen as fast as it is produced we may have a continuous current whenever the circuit is closed. Chemical action does not entirely cease in this cell when the circuit is opened. But if two cells are prepared, and one is left with its circuit closed while the other remains with its circuit open, it will be found that the zinc disappears and the acid is used up in the closed cell in a short time, while these remain not greatly changed for a long time in the cell on which the circuit is open. No cell will remain forever without chemical action, yet a dry cell which might use up its zinc and ammonium chloride in a few hours if the circuit is closed may be kept idle three or four years, and still be able to furnish electricity enough to ring a bell. Some persons feel defrauded if the author of a book fails to give them all the new words and conventional terms which belong to any subject. For such here is a page or so.

It is conventional to speak of the electric current as flowing from the carbon through the wire to the zinc, although every one has suspicions that it may flow in the other direction or even that it may not flow at all. It is conventional to designate any part of the circuit from which the current comes as positive (+) to any other part toward which it flows, this latter being considered negative to the former and designated (-). The current is conceived of as making a complete circuit, from carbon to zinc through the wire and from zinc to carbon through the liquid. Hence, the binding post of the carbon rod is called the + pole and that of the zinc is called the-pole, while the zinc rod or plate beneath the surface of the fluid is called the + plate and the carbon is called the-plate. The liquid is termed the electrolyte. The sodium bichromate, introduced to cause the hydrogen to unite with oxygen, is called an oxidizing agent or even a depolarizing agent, and hydrogen collecting upon the negative plate is said to polarize the cell.

Hydrogen may be made to collect upon the carbon or negative plate until the electric current reverses its direction. The hydrogen is said to be more - than the zinc. If we connect the zinc and carbon rods with the wires bringing an electric current from the dynamo we may make either one positive as we choose, according to which is connected with the positive wire. Hydrogen bubbles will collect upon whichever plate we make the negative one.

When we send an electric current from the dynamo into this cell it is called an electrolytic cell, and when it is used to generate an electric current it is called a battery cell. In either case the electrolyte is decomposed and put through a chemical change, though the chemical action in one case is the reverse of that in the other, and the direction of the electric current in one case is the reverse of that in the other. For example let us consider the case of a zinc rod and a carbon rod immersed in sulphuric acid and the external circuit closed. The current passes as indicated by the arrows in Fig. 151, and the chemical actions result in hydrogen leaving the sulphuric acid and zinc taking its place, forming zinc sulphate. This is a white salt and for purposes of this experiment must remain dissolved in water. So far we have been considering a generator of electricity—a battery cell. We may introduce something at m, say a motor, which will indicate that an electric current is flowing. At length the cell ceases to generate current and is, as we say, "run down." Suppose now we substitute a dynamo in place of the motor in this circuit, connecting it so that the carbon rod shall be its positive pole and the zinc its negative pole. We now call this an electrolytic cell, (Fig. 152). The current will decompose the zinc sulphate. The zinc will be coated upon the zinc rod and hydrogen will be procured from the water present, of which it is a constituent, to form again sulphuric acid as originally.

We shall thus restore the conditions which prevailed in the first case as represented in Fig. 151. H₂SO₄ is the chemist's designation of sulphuric acid and ZnSO₄ is his expression for zinc sulphate.

The experiment illustrates a storage battery so called. It might better be called a chemical transformer.

It is wholly unnecessary that one rod be composed of zinc. If we begin with both rods of carbon immersed in a solution of ZnSO₄, and send into this cell the dynamo current, the carbon which acts as the negative pole will be coated with zinc in a short time, and we shall have in effect a rod of zinc and one of carbon as before. After a minute or two we may disconnect the generator and substitute in its place a bell as indicator, and it will ring, showing that we have transformed electrical energy into chemical energy which is now being retransformed into electrical energy. We say that we store electricity by this means, which is, however, no more true than that a farmer stores his farm in the bank when he sells it and deposits the money until he shall need it to buy another farm.

Here is a very beautiful blue salt. I will drop a few crystals of it into a tumbler of water and dip in two carbon pencils connected to the dynamo current, using between fifty and sixty ohms of resistance in the circuit so as to have two amperes flowing. After a minute or two I lift out the negative carbon and you see that it is well plated with copper. The blue salt is copper sulphate. If we weigh the negative carbon, both before and after the experiment, we shall find that copper has been depositing at the rate of one ounce in twelve hours. If we reduce the current one half, making it one ampere, it will deposit copper at the rate of one ounce in twenty-four hours. One ampere will separate three ounces of lead in a day from a solution of any lead salt; it will separate .9 ounce of iron in a day from a solution of any iron salt, and it will liberate from water, which is a compound of hydrogen, one gallon of the gas in ten hours. The amount of chemical action is a measure of the amount of electrical energy expended. Before the present form of commercial wattmeter was devised electrolytic cells were used to determine what the consumer's bill for electricity should be each month. These chemical meters contained a solution of zinc sulphate for the electrolyte and both the positive and the negative plates were of zinc. While the current is passing, zinc from the solution is coated upon the negative plate and zinc from the positive plate takes its place in the solution, thus maintaining a constant strength of solution.

Here are three iron nails. I propose that you plate one with zinc and another with copper and then expose all three to the weather and see which will rust. I propose that you replate all the spoons at the cottage and the metal tops of the salt cellars with silver. Electro-plating results better if done slowly. Ten volts and .1 ampere will be sufficient current.

In the storage battery we generally use lead for both positive and negative plates and dilute sulphuric acid for the electrolyte. Hydrogen is liberated at the positive plate and oxygen unites with the negative plate. When the charging current is cut off the chemical action reverses, and an electric current is produced by the cell.

In all other batteries there is a destruction of one plate and of the electrolyte, which cannot be fully restored by a charging current, although in the case of the lead and sulphuric acid combination the charging and discharging of the cell may go on alternately for a very long period without permanent change or loss of any substance except water. There is, however, plenty of loss of energy in this as in other transformers. One hundred ampere hours of current expended to charge a storage battery will yield from seventy-five to eighty-five ampere hours while the battery is discharging.

The lead storage battery is, however, full of disappointments for those who do not properly care for it. It is irretrievably ruined if neglected and allowed to charge too far, or discharge too far, or evaporate too much water, etc. The voltage of a lead cell must not rise above 2.2 nor fall below 1.8. It must not be allowed to furnish at any one time a greater number of amperes than it is rated for. It must not stand idle too much. It must not be cleaned up and put away for a period. In fact, the lead-sulphuric acid battery is so poorly adapted to our need that I feel disposed to try Mr. Edison's new storage battery. This has nickel hydrate packed in tubes of metallic nickel for the positive plates and iron oxide pressed into pockets in a sheet of metallic iron for the negative plate. A solution of potassium hydrate in water is used for the electrolyte. This is said to be uninjured by being emptied out and left idle, as our batteries must be for a large part of the year. The e. m. f. of this battery is less than that of the lead battery, being only 1.2 volts. We shall therefore need ninety-six cells (type B-4) for the machine shop and ninety-one cells of the same kind for the cottage. Our dynamo will be unable to charge at one time more than sixty of these cells connected in series.

The particular chore which you boys must perform is to see that the voltage of these batteries is maintained at about 1.2. It should be charged up to 1.8 volt at least once a week and never allowed to discharge to a lower pressure than one volt. The level of the electrolyte must be maintained one half inch above the plate by adding distilled water occasionally.

A few years ago every student of chemistry was more or less agitated by the thought that more than half of every clay bank was composed of metal nearly as valuable, or at least as costly, as gold. This is aluminum. By all the methods then known it was a very difficult and expensive process to extract the metal from the clay. At length, by the perfecting of the dynamo, the chemist had under his control great and powerful electric currents which enabled him to unlock any chemical compound however refractory and isolate its elements. As a result aluminum became common enough and cheap enough for even kitchen utensils.

The metal calcium which a short time ago was an exceedingly rare substance worth $40 an ounce is now fairly abundant and cheap for chemical experiments, although it has no qualities which will give it an extended use.

Powerful electric currents, such as are obtained at Niagara, enable us to combine elements into hitherto unknown chemical compounds. Carbon and silicon are made to unite to form carborundum, which vies with the diamond for hardness. Carbon and calcium unite to form calcium carbide, used with water to form acetylene gas.

In such processes the intense heat of the electric arc—perhaps 6000 degrees—is employed, together with the electrolytic action of the current, to separate and combine substances. Enormous currents are used in the electric furnaces for producing chemical reactions—from 1000 to 30,000 amperes at a time.

Electric currents passing through the human body expend their energy partly in heat and partly in electrolysis. So simple and harmless a thing as common salt would become a virulent poison if it could be electrolized in the body into its elements sodium and chlorine.

Let us make use of an electric current to decompose water into its elements, hydrogen and oxygen. I have a three-ounce wide-mouthed bottle (Fig. 153) and through its cork I pass two short pieces of No. 24 platinum wire by pushing a stout needle through first. I fill this bottle with pure water and cut a slight furrow in the side of the cork, where water may drip out when the gas is produced in the bottle. We crowd the cork firmly into the mouth of the bottle and invert it. No water drops out. We bend the ends of the platinum wires into hooks and hang upon them the wires bringing the dynamo direct current. There is no evidence of chemical action. Pure water is an exceedingly poor conductor of electricity. Let us now put about fifty-five ohms of resistance into the dynamo circuit, so that it will pass about two amperes, and put a very small pinch of salt into the water, which makes it so good a conductor that its resistance may be ignored. When now we close the circuit, as before, a brisk effervescence takes place. Bubbles of gas rapidly form on the platinum wires and break away, rising through the liquid. Twice as many form on the negative wire as on the positive one. As these gases rise to the top of the bottle an equal volume of the water drips out through the small hole in the cork.

Two amperes of electricity will liberate two fluid ounces of hydrogen at the negative pole and one fluid ounce of oxygen at the positive pole, in five minutes. Hence in five minutes the bottle should be full of a mixture of two gases, two thirds of which, by volume, is hydrogen and one third oxygen. We will catch the water which drips out so that we may measure it. The bottle being now full of gas I shut off the current, and removing the cork I bring a flame to its mouth. A very loud and startling explosion takes place. We pour the water back into the bottle, and it seems to fill it as well as before. We have decomposed a few drops of water—not enough to measure—into two gases, one of which, the hydrogen, occupied two thirds of the bottle, and the other, oxygen, occupied the remaining third. At ordinary temperatures they would not reunite, but when raised to their kindling temperature they united, producing light, heat, a loud noise, and the few drops of water which had been originally decomposed by the current.

This is the electrolysis of water. I wonder if any such chemical action took place in Ernest's body when he received that severe shock on the motor boat the other day.

It is significant that the "dry" battery cell must be moist in order that chemical action may go on in it. Compare with that fact several others that we may learn from observation, for example: Baking powders must be kept dry to retain their strength. That is, if they get moist chemical action will begin in them, and the gas which is one of the products of this chemical action will pass off. Now it is the sole function of baking powders to produce gas within the dough, and if the gas has wholly or partially escaped they will fail to make the bread stuff "light." The same reasons obtain for keeping seidlitz powders and other effervescing salts, such as vichy and kissingen, dry. It is to prevent the chemical action which is provoked by the presence of water. The same thing is true of the rusting of iron, and the various kinds of corrosion of metals. We may prevent such action indefinitely by keeping them dry. Berries, fruits, meats, milk, eggs, grain—all kinds of foods—are preserved from spoiling—from chemical changes—by drying them and keeping them dry. The same thing is true of wood, paper, cloth, etc. A wooden fence post may last from five to ten years. A fence rail, being less exposed to moisture, may last two or three times as long. The interior wood of a house may last a century or two, while the exterior wood, being exposed to the weather, may require repairs very frequently. Shingles on the roof do not last as long as shingles on the side of the house. Those on a steep roof last longer than those on a flatter one. A pitch of at least forty-five degrees in a roof is desirable to keep it dry. The north and west sides of a house being least exposed to storm in this climate last the longer. Precious books, records, deeds, wills, etc., on paper must be preserved in dry air. A sail will keep strong and white if kept dry.

But it is impressed upon us by our experiences that sunlight is even more potent than moisture to produce chemical change. Photographic processes are dependent upon the power of light to produce chemical changes. The fading of our tapestries and our garments, the tanning of our skins, the development of green material in the leaves of plants, all are evidently the direct result of sunlight. A picture hung on the wall prevents the wall paper behind it from being faded by the light, or it prevents the wood behind it from being turned yellow by the light. Folds in our garments prevent them from being faded all alike. Very many substances to be found in a chemical laboratory, in a drug store, or in a kitchen must be kept in the dark if they are to be guarded against chemical change. No experienced housewife would let a barrel of flour or potatoes sit in the sun, and every housewife knows that the sun is the best agent for bringing about those chemical changes which she desires. Hence she puts her bedding, her milk pans, her bread box, her butter jar, etc., "out to sun." She has open plumbing, that the sun may enter those dark and dirty corners.

If you would guard a substance against chemical change, keep it in a dry, dark place. We have come to associate the sun and the weather as disintegrating forces. Hence the south and east sides of the building need most frequent repairs. Every one who has made time exposures in photography knows that the sunlight from the east is, as a rule, two or three times as powerful as that from the west. There is less moisture and dust in the air to screen us from the early morning sun than from the late afternoon sun. When there is enough moisture in the air to make the sun look red, those rays from it which would produce chemical action, called actinic rays, are cut off. Photographic processes are then exceedingly slow. It is like exposing a plate in a dark room behind the ruby glass.

But our daily experiences teach us that not only moisture and light but also heat stimulates chemical action. We restrain chemical action by cold when we put things in the ice box. We hasten chemical action by heat when we put things on the stove. Winter restrains all the chemical activities of nature, and summer quickens all the vegetable and mineral kingdoms into chemical activity. If we would preserve a substance from chemical change we must keep it in a cool, dark, dry place. Now those conditions which will favour the chemical activity of a battery cell will enable it to produce electricity, and those conditions which will restrain chemical action will enable us to preserve the cell from running down.

But we have lately learned that other forms of radiation besides light and heat exist and aid in chemical action. We may produce radiographs—pictures on photographic plates—without light but with invisible rays, which are akin to light and to electricity.

XVI

ELECTROCUTION AT MILLVILLE

"They have a regular thoroughfare, a beaten highway, along by the wall, under the mill and up through a hole in the floor of my bedroom," said Dyne.

"Well," said Harold, "I propose an electric trap which shall have two compartments. We will keep cheese in the inner compartment, the walls of which shall be of wires so that the rats may see the cheese. The floor of the outer apartment shall be covered with wire, as shown in Fig. 154. The wires of the secondary circuit from the bell (Fig. 156) shall be fastened to the binding posts b and c (Fig. 154). The partition d shall be a swing door into the apartment A where the cheese is. This is shown in profile in Fig. 155. d must act as a switch to close the primary circuit through the bell P (Fig. 156). We will have three dry cells in the primary circuit. Now this is the way it will work: A rat comes up from under the mill with wet and slimy feet—just suited for making contact for the electric current to enter his body. The smell of the cheese attracts him. He circles around the trap several times, watching the cheese in apartment A through the wire screen. He sees a narrow opening into this apartment under the door d. He puts himself in position upon the floor of the outer apartment B, his feet bridging the gaps between the two systems of wires belonging to the secondary circuit. When he thrusts his head under the door and pushes it, it swings in a little, bringing one metal strip against another, which belongs to the primary circuit. This closes that circuit. He will never hear the bell ring, for the electric current which will shock him to death travels 186,000 miles per second, while his sensations travel only sixty miles an hour. If the involuntary recoil of his muscles does not make him jump back, so that the door will shut and stop the bell from ringing, Dyne will be awakened and he will close the door, since we will put the trap at that hole where the rats enter his bedroom."

XVII

THE TELEPHONE

There were more than eleven billion messages sent by telephone in the United States in 1907. The capital invested in telephone business was $814,616,004. The income for that year was $184,461,747. All of these items had more than doubled during the previous five years. In 1880 there were about eight times as many miles of telegraph wires as of telephone wires. In 1907, there were about eight times as many miles of telephone wires as of telegraph wires. The Bell system had 3,132,063 stations, and independent companies had 2,986,515 stations in 1907.

The first telephone line ran from Salem to Boston, Mass. This was in 1877. The next year the first telephone exchange was established. It was eight years before a telephone line was extended from Boston to New York. On October 18, 1892, the first telephone message was sent from New York to Chicago. Previous to 1895 telephoning, like telegraphing, was done by one wire, using the earth, as we say, to complete the circuit.

But at about that time electric car and electric lighting lines became so common that they interfered with telephoning. These currents running in lines parallel to the telephone wires induced currents in them, and when a person put a receiver to his ear for conversation he heard the hum of electric light dynamos and the buzz of electric cars so loud that conversation was quite impossible. The next step was to introduce a return wire—the double metallic circuit as we call it. Thus outside currents induce equal and opposite currents in the two wires of the circuit, which neutralize each other.

It was this same year, 1895, that the "central battery" system was introduced into telephone equipment. This is not usually a battery at all, but a dynamo.

The price of all electrical supplies in 1895 was about one tenth what it had been in 1885, and at the same time the goods were of far better quality.

Important telephone patents expired in this year, and immediately private and independent lines began to be established. It was also in 1895 that the telephone company began to use an automatic registering device which enabled it to charge telephone rates according to the number of calls.

The boys unscrewed the end of a telephone receiver (Fig. 157) and found inside a permanent magnet made of several steel bars bolted together (Fig. 158). This was shown to be a magnet by presenting a small pocket compass to either end. The left-hand end of this magnet proved to be its north pole by repelling the blue end of the compass needle.

On the left-hand end of the magnet was a small spool of No. 36 copper wire, silk covered. It offered 75 ohms of resistance, and since it takes 2½ feet of this wire to furnish 1 ohm of resistance the spool contains 187½ feet. A thin disc of soft iron .01 inch in thickness is held by the hard rubber case very near to but not quite touching this end of the magnet. We drew this disc to one side, as shown in Fig. 159, and connected the receiver by wires to a magneto. We turned the crank of the magneto slowly and the iron disk danced up and down, keeping time with the revolutions of the armature. The magneto furnished an alternating current, which, when it flowed around the coil in one direction, strengthened the pole of the magnet, and in the reverse direction weakened the pole. When the crank was turned so as to produce twenty to thirty revolutions of the armature per second the dancing of the disc sounded like the low hum produced by the wing of a humming bird. When a large, wide-mouthed bottle was brought near to this the sound was greatly reinforced, as the sound of a bee becomes louder when he appears at your open window. We next replaced the iron disc and put on the cap again. We then connected the receiver at S (Fig. 160) and connected two dry cells at p. When the primary circuit was closed the disc vibrated in time with the hammer of the bell making the same tone. We substituted for the bell a series of buzzers. The smallest had an armature about one inch long, while that of the largest was about two inches long. The shorter the armature the faster it vibrated, and the higher was the pitch of its tone. We arranged these as shown in Fig. 161. A, C, D, E and F are the buzzers. B is a battery of two cells and G, H, I, J and K are springs of sheet brass which act as push buttons. By operating upon these springs with one's fingers, as upon the keys of an organ, it was possible to represent the tones of a reed organ after a fashion. The armatures are reeds and they are made to vibrate by electro-magnets. We called it an electric organ. The telephone receiver was connected at T, and the wires which led to it were lengthened so that the receiver might be a long distance away. The disc in the receiver kept time with the armature of each buzzer when it sounded and faithfully reproduced its sound. But the strangest thing was that when any two buzzers sounded together, or, indeed, if all five buzzers sounded together, the receiver responded to them all at the same time, so that a person in another room or in another house, with the receiver at his ear, might hear exactly what those did who were in the same room with the buzzers. The wires from the receiver were connected with the coil in each buzzer so as to get the induced current, as shown in detail in Fig. 160.

We took a telephone induction coil (Fig. 162) and fastened it to a board as represented in Fig. 163, I. One wire of the primary circuit was fastened to the binding post a. The other wire from the primary coil passed to the switch S and then to the battery. From the battery the wire ran to the binding post b. C is a steel tuning fork. The secondary circuit is closed through a telephone receiver. These wires are extended so that the receiver is too far distant for the tuning fork to be heard through the air. When the switch S is closed the tuning fork acts as the interrupter for the primary circuit, and it interrupts according to its time of vibration. If, for instance, the fork gives the tone of middle C on the piano it vibrates 256 times a second. It interrupts the primary circuit 256 times a second. It induces an alternating current of the same frequency in the secondary circuit. The diaphragm of the telephone receiver vibrates in perfect time with the tuning fork and produces the same tone as the tuning fork. We had a series of tuning forks giving a variety of tones, which we could substitute one after another in place of this one. The receiver reproduced accurately the tone of each one of them.