| This is an example of Sturgeon's
"If the wire transmitting the electrical current, after passing over the needle, is bent and returned under it ...., it might be supposed that as the electricity which flows from [the right-hand terminal to the one at the left] in the upper part of the wire, must pass in a contrary direction, in returning from [the left-hand terminal to the middle one] (the right-hand terminal being connected with the positive pole of the battery and [the middle one] with the negative), the influence of the one part of the wire would neutralize that of the other, for it has already been stated that the needle is deflected to one side or the other according to the direction of the electrical circuit. And this would in fact be the case, if the returning part of the wire were upon the same side of the needle with the other part, and at an equal distance from it. But a wire transmitting an electrical current, while passing below the needle, will produce an effect the reverse of that produced by one passing above, if the current in both cases flows in the same direction." (from the 1842 edition of the Manual of Magnetism, pg 48)
This galvanometer is at Harvard University.
| This double galvanometer is at Harvard University, but
does not seem to be listed in the Davis catalogues.
The double windings on each side suggests that this is an astatic galvanometer, with a pair of oppositely-magnetized compass needles suspected by a fine fiber from the adjusting screws at the top. One needle was placed in the center of the coil and one above it, giving no net magnetic torque due to the magnetic field of the earth.
One set of coils has approximately twice the number of turns as the other, giving twice the sensitivity.
These uncalibrated devices could also be considered to be galvanoscopes.
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