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Between the poles of the galvanometer was stretched a slender thread of fused quartz plated with silver. It was the finest thread I could imagine, only a thousandth of a millimeter in diameter, far too tenuous to be seen. Three feet further away was a camera with a moving plate holder which carried a sensitized photographic plate. Its movement was regulated by a big fly-wheel at the extreme right.
He may be said to have carried the electro-magnet, in its lifting powers, to its greatest perfection. Reflecting upon the principle of Professor Schweigger's galvanometer, he constructed magnets in which great power could be developed by a very small galvanic element.
This was a very prettily arranged electric balance, and by placing plates of different substances between the inductor and one of the inductometers Faraday expected to see the balance destroyed to an extent which would be indicated by the deflection of the needle of the galvanometer.
Knowing the latter to be extremely sensitive to minute changes of pressure, for example, those of sonorous vibrations, he conceived the idea of measuring radiant heat by causing it to elongate a thin bar or strip of metal or vulcanite, bearing at one end on the button. To indicate the effect, he included a galvanometer in the circuit of the battery and the button.
Messages have been sent from England to America through one Atlantic cable and back again to England through another, and there received on the mirror galvanometer, the electric current used being that from a toy battery made out of a lady's silver thimble, a grain of zinc, and a drop of acidulated water.
The resulting iron helix contained two hundred and eight feet; but whether the current from the trough was passed through the copper or the iron helix, no effect upon the other could be perceived at the galvanometer. "In these and many similar experiments no difference in action of any kind appeared between iron and other metals.
For instance, by connecting the small flat spiral of copper wire in direct circuit with the galvanometer, you will perceive that the slightest movement of the spiral generates a current of sufficient strength to very sensibly affect the galvanometer; and as you observe, the amplitude of the deflection depends upon the speed and direction in which the spiral is moved.
The latter comprises a ring of German silver wire, forming two branches. A contact lever P moves over the ring, and is used as a battery key. A small galvanometer G shows the indications of the testing current. A brass sliding piece S puts the galvanometer needle in and out of action.
Bose found that when he attached the galvanometer to bars of various metals they gave a similar response when struck or twisted. The greater the irritation applied to the metal, the greater the response registered by the instrument. The analogy between the response of the metal and that of the living muscle was startling.
Thus, when this galvanometer is placed as the receiving-instrument at the end of a long submarine cable, the movement of the spot of light, consequent on the completion of a circuit through the battery, cable, and earth, can be so observed as to furnish a curve representing very accurately the arrival of an electric current.
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