"The magnetizing power will be greatly increased if the wire be coiled in the manner of a cork-screw, so as to form a hollow cylinder into which the body to be magnetized can be inserted. Such a coil is denominated a Helix...
In using the coil, the following rule will indicate the extremity at which the north pole will be found. If the helix be placed between the observer with one of its ends toward him, and the current of electricity in passing from the positive to the negative pole of the battery, circulates in the coil in a direction similar to that of the hand of a watch or the threads of a common screw; then the north pole will be from the observer, and the south pole towards him. If it passes round in the contrary direction, the poles will be reversed." (from the 1842 edition of the Manual of Magnetism, pg 71)
Davis then gives nine experiments to be done with the coil, including: Magnetizing a soft iron bar and showing that its magnetism disappears when the current is cut off; repeating the same experiment with a steel rod and showing the the magnetism persists after the cessation of the current; negligible magnetic field outside the middle of the helix; with the helix held vertically and a bar of soft iron inside, the magnetic forces will balance the gravitational forces and the bar will not drop out.
The Middlebury instrument, which looks like the others,
is marked with the name of E. S. Ritchie of Boston. The only apparatus
which I am sure is made by Davis is that from Washington and Jefferson
College; the shape of the pillar resembles that in the cut in the Davis
Washington and Jefferson College
Hampden-Sydney College Middlebury College
Another example is in the Greenslade collection, and is
listed with the electromagnets.
| Electro-Magnet, with Three Poles.
instrument ... consists of an iron rod, wound with insulated wire, which
is carried in one direction around half the length of the rod, and then
turns and it wound in the other direction. The effect of this arrangement
is, that, when connection is made to the battery by means of the screw-cups
on the stand, the two extremities of the bar become similar poles, while
the middle acquires a polarity opposite to that of the ends. The middle,
as well as the ends, will sustain a considerable weight of iron. By reversing
the direction of the current, all of the poles are reversed. The arrangement
of the poles may be shown by passing a magnetic needle along the bar."
(from the 1851 edition of the Manual of Magnetism, pg 188)
This demonstration device is at Grinnell College in Iowa.
The shape of the base is typical of Davis' work, but the pillar is the
wrong shape. It is likely by another maker, possibly Palmer and Hall of