The sensitive galVanometers, noWfast taking the place of all others, arc those designed by sir William' Thomson. In fig 5, which represents the dead-beat galvanometer, the general action of these is shown. C is a bobbin filled with fine covered wire set on a stand. In the tube A, Which forms the center of the bobbin, a tubular box fits, in the middle of which hangs. a circular mirror made of microscopic glass, slightly concave, suspended by a fine silk fiber. To the back of this mirror' are stuck four fine magnetic needles. The mirror and needles weigh of a gram grain). The fiber by which the mirror hangs is too feeble to give anything like a quick set to it.
To accomplish this, a powerful magnet is laid outside the instrument, and placed so that the mirror lies in the way required for observation, About a meter in front of the mirror, a graduated scale, es, is placed, and immediately below 4 a lamp, L. A pencil of light, I, is sent from L,"and reflected (R) back to the scale, where it comes to a focus at the zero point. A slight deflection of the needles, caused by a current in the coil, makes a decided displacement in the spot of light on the scale. The compartment in which the mirror is hung is very limited in size. and behind is so close to the mirror as to allow it no more play than the small range the instrument requires, and the mirror, when at rest, just clears the sides of the box, and nothing more. In other galvanometers, the mass of the mova ble part, and its comparatively weak magnetism, make the indications of the needle tediously slow. In this instrument, the moving mass is small, and its magnetism com paratively great, and this, combined with the viscosity of the air iu the narrow cham ber and the powerful directive force of the external magnet, gives an immensely greater rapidity of indication, with even increased sensitiveness.
Tangent instrument is shown in fig 6. It consists essentially of a thick strip of copper, bent into the form of a circle, from 1 to 2 ft. in diameter, with a small magnetic needle, moving on a graduated circle, at its center. When the needle is small compared with the ring, it may be assumed that the needle, in any direction it lies, holds the same relative position to the disturbing power of the ring. This the case, it is easy to prove that the strengths of currents circulating in the ring are proportionate to the tangents of the angles of deviation of the needle.. Thus, if the deflection caused by one galvanic couple was 45°, and of another 60°, the relative strengths of the currents sent by each would be as the tangent of 45° to the tangent of viz., as 1 to 1.73. The needle can never be deflected 60°, for as the' tangent of 904is infinitely large, the strength of the • deviating current must be infinitely great, a strength mani festly unattainable. The tangent galvanometer 'can conse
quently be used to measure the strongest curreutg.
•YroWerneter.—This was invented by Faraday for testing the strength of a current. In this apparatus two platinum plates,.
each about half a square inch in size. are placed in a bottle con taining water acidulated with sulphuric acid; the plates are soldered to wires Which pass up through the cork of the bot tle; binding screws are attached to the upper ends of these wires; a glass tube fixed into the cork serves to discharge the gas formed within. When the binding screws are connected with the poles of a battery, the water in the bottle begins to be decomposed, and hydrogen and oxygen rise to the surface. If, now, the outer end of the discharging tube be placed in a trough of mercury (mer cury does not dissolve the gases), and a graduated tube, likewise filled with mercury, be placed over it, the combined gases rise into the tube, and the quantity of gas given off in a given time measures the strength of the current. The voltameter .chooses as a test the work which the current can actually perform, and establishes a uniform \standard of comparison. The indications of the tangent galvanometer are comparable only with its own, but the quantity of gas discharged by the voltameter, corrected for pressure and temperature is something quite absolute. However, by conTaring the indications of both instruments with each other when placed in the same circuit, an absolute standard may likewise'be got for the tangent galvanometer. - If, for instance, the current given by a battery should give 60 cubic centimeters of gas in a minute, and produced at the same time a deflection of 45° hi the galvanometer, the ratio of 60 to the tangent of 45° 60 to 1 = 60, is constant, for correct measurements of the strength of currents, however taken, must bear to each other a constant ratio. If the angle of deviation for another current was 30°, we have therefore only to multiply 60 by the tangent of 30°, to ascertain the amount of gas that would be liberated by a current of that strength in a minute. This found, we know a deflection of galvanomo ter in question in a perfectly comparable standard. The plates of the voltameter must be small, for when they are large, a small quantity of electricity is found to pass with out decomposing the water. It is found also that a minute quantity of the oxygen forms hydric peroxide with the water, and remains in solution, so that when very great accuracy is required, the hydrogen alone ought to be measured.