LIST OF ILLUSTRATIONS.

FIG.

1.-APPARATUS FOR SHOWING FRICTIONAL ELECTRICITY 2.-ORDINARY PLATE-GLASS MACHINE

3.-WIMSHURST INFLUENCE MACHINE

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2

5

6

53.-GRAMME DYNAMO-ELECTRIC GENERATOR 54.-SIEMENS' GENERATOR.

47.-DIAGRAM SHOWING PRINCIPLE OF TELEPHOTOGRAPH

48.-DIAMOND PATTERN TRANSMITTED BY TELEPHOTOGRAPH

49. THE SONOMETER

50.-INDUCTION BALANCE

51.-INDUCTION BALANCE COILS

52. SKELETON DIAGRAM OF GENERATOR

55.-BRUSH GENERATOR

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94

96

98

99

108

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ELECTRICITY AND ITS USES.

CHAPTER I.

INTRODUCTORY.

THE advances made in the application of electricity during the past ten years have been truly marvellous. No doubt the electric era was ushered in some forty years ago by the introduction of the telegraph, but it is only within the last decade that the telephone, the electric light, and the transmission of motive power by electricity have become matters of every day use. Moreover, we have arrived now at a point from which we can foresee a still more remarkable progress in the future. The recent electrical exhibitions at Paris and at the Crystal Palace have not only shown how much has already been done in fitting electricity for the service of mankind, but also how much remains to do. The present, therefore, is, we think, a good time for giving a popular account of electrical appliances such as they are and may yet be, especially as public interest in the subject was never higher than it is just now. In the following pages it shall be our endeavour to describe all the most important developments of electricity in plain language, as free from technical

B

terms as possible. For the sake of greater simplicity, we begin by recalling the elementary facts.

Electricity, as every schoolboy is taught, takes its name from the Greek word elektron, signifying "amber," because the famous Greek philosopher, Thales of Miletus, found out that when a piece of amber was rubbed it acquired the power of attracting very light bodies, such as grains of dust or bits of straw. Thales lived six hundred years before Christ, and for more than two thousand years little else was known about the mysterious agency which works so many wonders in our own day. After the revival of learning, however, it was discovered by Dr. Gilbert, of Colchester, in the year 1600, that glass, sulphur, resin, and many other substances behaved like amber when rubbed, and the true science of electricity then began.

The kind of electricity generated in this way is called "frictional electricity," and its existence is generally demonstrated in the following way. A dry glass tube is taken and vigorously rubbed with a silk handkerchief, and then brought near to a small pith ball suspended by a silk thread

гуда

G

from the arm of a bracket which has a glass stem. The electricity excited on the rod by the friction of the silk will attract the pith ball (as shown in Fig. 1, where G is the rod and p the ball). Almost as soon, however, as the ball touches the rod it will fly off again and take up the position P' with respect to the rod G'. The fact is, that in touching the rod the ball pilfers

FIG. 1.

some of its electricity, and is repelled. If, however, a rod of sealing-wax or resin is now rubbed with the silk and put in the place of the glass rod, the electrified ball will be attracted to the wax. The explanation of this preference is that the rubbing has charged the sealing-wax with an opposite kind of electricity to that on the glass and also on the ball. Hence we have the law that like kinds of electricity repel each other, and unlike kinds attract.

This experiment and many others of the same sort led the celebrated M. Dufay to imagine that there were two opposite kinds of electric fluid pervading all bodies. Having an attraction for each other, he held that they tended to mix and neutralise each other, thereby producing a state of electric quiet in the body. Rubbing separated the two fluids from each other and rendered them appreciable to our tests. It could be shown that when a body is electrified by rubbing, an equal quantity of the opposite kind of electricity was always excited on the rubber, just as if the act of rubbing merely divorced the two different fluids from each other. The electricity produced on glass by rubbing with silk he called the positive, or vitreous, fluid, and that produced on resin by the same kind of rubber he called negative, or resinous, fluid. The nature of the rubber has to be taken into account, for if glass be rubbed with cat's skin, for example, it will become charged with negative instead of positive electricity, and similarly, if resin be rubbed with cat's skin it will become positively electrified. The kind of electricity called forth depends, in fact, on some mysterious relation between the two surfaces which come into contact with one another.