« ÎnapoiContinuați »
LIST OF ILLUSTRATIONS.
1.-APPARATUS FOR SHOWING FRICTIONAL ELECTRICITY 2.-ORDINARY PLATE-GLASS MACHINE 3.-WIMSHURST INFLUENCE MACHINE 4.-ELEMENTARY VOLTAIC CELL 5.—DANIELL'S BATTERY 6.-LECLANCHÉ BATTERY 7.-BICHROMATE OF POTASH BATTERY . 8.-WARREN DE LA RUE's PORTABLE CELL 9.—THE THERMO-PILE 10.—CLAMMOND'S GENERATOR 11.—APPARATUS FOR SHOWING INDUCTION 12.-IMAGINARY FIGURE SHOWING POLARISATION 13.—PRIMARY AND SECONDARY CURRENTS 14.-INDUCTION COIL 15.—MAGNETIC FIELD SHOWING INDUCTION
“ GREAT EASTERN LAYING TELEGRAPHIC CABLE : 16.-TELEGRAPH CIRCUIT 17.-VARLEY'S INSULATOR 18.—MORSE INK-WRITER 19.-MORSE INSTRUMENT 20.-Post OFFICE SOUNDER 21.-SINGLE NEEDLE INSTRUMENT 22.-DOUBLE CURRENT KEY 23.-STATION ARRANGEMENT FOR DUPLEX SYSTEM 24.-COWPER'S SENDING APPARATUS 25.- COWPER'S RECEIVING APPARATUS . 26.-SUBMARINE CIRCUIT 27.-MIRROR GALVANOMETER 28.-SIR WILLIAM THOMSON'S SIPHON RECORDER 29.- DETAILS OF INSTRUMENT 29a.-TELEGRAPHIC WRITING 30.-BELL'S TELEPHONE 31.-SECTION OF EDISON'S CARBON TRANSMITTER . 32.-DIAGRAM OF TELEPHONE CIRCUIT. 33.-EDISON'S RECEIVER 34.
MICROPHONE OF FRENCH NAILS
2 5 6 10 13 14 14 15 18 20 23 24 26 27 30 33 34 36 39 40 41 43 44 47 52 53 56 57 59 60 61 62 64 65 67 68
HARVESTING BY THE ELECTRIC LIGHT
69 71 72 75 76 77 80 81 85 87 89 91 93 91 96 98 99 108 110 111 111 113 114 117 118 120 121 123 124 126 127 129 130 131 132 132 134 136 137 138 140 142 152 156 161 165 167 171 174 175
ELECTRICITY AND ITS USES.
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
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 tubo is taken and vigorously rubbed with a silk handkerchief, and then brought near to a small pith
ball suspended by a silk thread 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 FIG. 1.
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
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.