[1] NOTE.--In 1749, Benjamin Franklin, observing lightning to possess almost all the properties observable in electric sparks, suggested that the electric action of points, which was discovered by him, might be tried on thunderclouds, and so draw from them a charge of electricity. He proposed, therefore, to fix a pointed iron rod to a high tower, but shortly after succeeded in another way. He sent up a kite during the pa.s.sing of a storm, and found the wetted string to conduct the electricity to the earth, and to yield abundance of sparks. These he drew from a key tied to the string, a silk ribbon being interposed between his hand and the key for safety. Leyden jars could be charged, and all other electrical effects produced, by the sparks furnished from the clouds. The proof of the ident.i.ty was complete. The kite experiment was repeated by Romas, who drew from a metallic string sparks 9 feet long. In 1753, Richmann, of St.

Petersburg, who was experimenting with a similar apparatus, was struck by a sudden discharge and killed.

[2] NOTE.--Suppose that the conditions are as in the fig. 34, that is, the segment A_1 is positive and the segment B_1 negative. Now, as A_1 moves to the left and B_1 to the right, their potentials will rise on account of the work done in separating them against attraction. When A_1 comes opposite the segment B_2 of the B plate, which is now in contact with the brush Y, it will be at a high positive potential, and will therefore cause a displacement of electricity along the conductor between Y and Y_1, bringing a large negative charge on B_2 and sending a positive charge to the segment touching Y_1.

As A_1 moves on, it pa.s.ses near the brush Z and is partially discharged into the external circuit. It then pa.s.ses on until, on touching the brush X, it is put in connection with X, and has a new charge, this time negative, driven into it by induction from B_2. Positive electricity, then, being carried by the conducting patches from right to left on the upper half of the A plate, and negative from left to right on its lower half.

A similar process is taking place on the B plate, but in this case the negative electricity is going from left to right above, and the positive from right to left below. On the whole, therefore, positive electricity is being supplied to the left hand main conductor Z by both upper and lower plates, and negative to Z_1.

[3] NOTE.--The discovery of this property of matter is due to Stephen Gray, who, in 1729, found that a cork, inserted into the end of a rubbed gla.s.s tube, and even a rod of wood stuck into the cork, possessed the power of attracting light bodies. He found, similarly, that metallic wire and pack thread conducted electricity, while silk did not.

Gray even succeeded in transmitting a charge of electricity through a hempen thread over 700 feet long, suspended on silken loops. A little later, Du Fay succeeded in sending electricity to no less a distance than 1,256 feet through a moistened thread, thus proving the conducting power of moisture. From that time the cla.s.sification of bodies into conductors and insulators has been observed.

[4] NOTE.--Copper is pre-eminently the metal used for electric conduction, being among the best conductors, it is excelled by one or more of the other metals, but no other approaches it in the average of all qualities.

[5] NOTE.--A current of electricity always flows in a conducting circuit when its ends are kept at different potentials, in the same way that a current of water flows in a pipe when a certain pressure is supplied. The same electrical pressure does not, however, always produce a current of electricity of the same strength, nor does a certain pressure of water always produce a current of water of the same volume or quant.i.ty. In both cases the strength or volume of the currents is dependent not only upon the pressure applied, but also upon the _resistance_ which the conducting circuit offers to the flow in the case of electricity, and on the friction (which may be expressed as resistance) which the pipe offers to the flow in the case of water.

[6] NOTE.--The prefixes "meg" and "micro" denote million and millionth.

For example, a megohm equals 1,000,000 ohms, a microhm equals 1/1,000,000 of an ohm.

[7] NOTE.--The reciprocal of a number is equal to 1 the number; for instance, the reciprocal of 3/20 = 1 3/20 = 20/3 = 6-2/3

[8] NOTE.--A writer in the _New Science Review_ undertakes to answer the question: "What is electricity?" In order to lead the reader up to the main question, he first considers the natural forces, gravitation and heat. Examples are given of how these forces are manifested and how energy is changed from one form to another. Every form of force, the author says, should be regarded as a different method in which energy makes itself known to the senses. He calls particular attention to the important fact that the "resistance of one kind or another is always the agent that acts to alter energy from one form to another," and suggests that electricity is simply a form or manifestation that energy may a.s.sume under given conditions, and generally is a mere transitory stage between the mechanical form and the heat form. "In most operations," he continues, "mechanical force pa.s.ses to the heat form without pa.s.sing through the electric form; but whenever magnetism is brought into play as a resistance that must be overcome, then mechanical power applied to overcome this resistance always becomes electricity, if only momentarily in its pa.s.sage from the mechanical to the heat form." In conclusion, he asks if the question: "What is electricity?" cannot be answered in a fairly satisfactory way by saying that it is simply a form that energy may a.s.sume while undergoing transformation from the mechanical or the chemical form to the heat form or the reverse.

[9] NOTE.--The cathode is the conductor by which current flows away as distinguished from the _anode_ or conductor through which the current enters. The terms usually apply to conductors leading the current through a liquid or gas, as an electrolytic cell, or vacuum tube.

[10] NOTE.--The name _voltameter_ was given by Faraday to an electrolytic cell employed as a means of measuring an electric current by the amount of chemical decomposition the current effects in pa.s.sing through the cell.

[11] NOTE.--Faraday's own description of his discovery is as follows: "Two hundred and three feet of copper wire in one length were coiled round a large block of wood; another two hundred and three feet of similar wire were introposed as a spiral between the turns of the first coil, and metallic contact everywhere prevented by twine. One of these helices was connected with a galvanometer, and the other with a battery of one hundred pairs of plates, four inches square, with double coppers, and well charged. When the contact was made there was a sudden and very slight effect at the galvanometer, and there was also a similar slight effect when the contact with the battery was broken."

[12] NOTE.--In reality it would be impossible to have a magnetic field exactly like fig. 129, for in the less dense part, the magnetic lines would be of curved complex form.

[13] NOTE.--These values are correct for effective sinusoidal voltages.

[14] NOTE.--It should be understood that a dynamo does not generate electricity, for if it were only the quant.i.ty of electricity that is desired, it would be of no use, as the earth may be regarded as a vast reservoir of electricity. However, electricity without pressure is incapable of doing work, hence a dynamo, or so-called "generator," is necessary to create an electromotive force by electromagnetic induction in order to cause the current to flow against the resistance of the circuit and do useful work.