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The magneto-electricity developed in the coils of the revolving electro-magnet was measured by an accurate galvanometer; and the temperature of the water was taken before and after each experiment by a very delicate thermometer.
But in the case of the twirling planet the galvanometer wire would necessarily be carried along with the earth; there would be no relative motion. What must be the consequence? Take the case of a telegraph wire with its two terminal plates dipped into the earth, and suppose the wire to lie in the magnetic meridian.
We will now remove the copper plate, as we did the iron one, and increase the speed of the reversals to the same as in the experiment with the iron, and you observe the deflection on the galvanometer is about the same as it was on that occasion. Now, by replacing the copper plate to its former position you will note how rapidly the deflection falls.
The small weight of the mirror and magnets which form the moving part of this instrument, and the range to which the minute motions of the mirror can be magnified on the screen by the reflected beam of light, which acts as a long impalpable hand or pointer, render the mirror galvanometer marvellously sensitive to the current, especially when compared with other forms of receiving instruments.
These results were of much utility in their bearing on the working of submarine cables, and it is not too much to affirm that it was to Sir William Thomson's counsel that the success of the Atlantic Cable is in great part due. His mirror galvanometer was the first instrument that could be applied with anything like satisfactory results to submarine telegraphy.
If the galvanometer be replaced by a telephone, no matter how the spiral be moved, no sound will be heard, simply because the induced currents produced consist of comparatively slow undulations, and not of sharp variations suitable for a telephone.
By dividing the range of the galvanometer thus obtained into 100 equal parts, the requisite actinometric scale will be established. In practice, the Clamond battery is used to supply the constant current required.
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.
It is a wooden cylinder fixed against the edge of a table; on the table two vessels filled with salt and water are so placed that, as you close your hands on the cylinder, the forefinger of each hand can drop into the water; each of the vessels has a metallic plate, and communicates by wires with a galvanometer with its needle.
One after another, he was studying the emotions of each of his audience. Suddenly the scene changed. A door was burst in, cards and gambling paraphernalia were scattered about and hidden, men rushed to escape, and the sounds were much like those on the night of the raid. Garrick was still engrossed in the study of what the galvanometer was showing. The film stopped.
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