Today we would call the separable helix a dissectible transformer.
The helix consisted of a primary, wound with a relatively few turns of heavy
wire, with a secondary (many turns of fine wire) placed over it. The core was
a parallel bundle of varnished iron wires which prevented eddy current heating.
The primary had to be supplied with some sort of alternating current, which
was a battery driving an external interrupter or electrotome.
| The only problem with identifying the apparatus at the
left (at the University of Vermont, and clearly labelled as being made
by Daniel Davis) and the one above it from Union College, is that they
do not appear in Davis catalogues! Only the separable helices with electrotome
(the rasp device shown on the example below) are shown in the catalogue.
A hint may be in the 1848 edition of Davis's Manual of Magnetism,
where a price range is given from $10.00 to $12.00; the extra $2.00 may
be for the electrotome.
The base of this apparatus has been painted black. When the paint was removed, the wood appeared to have been sanded (the Davis varnish was high gloss, making adhesion difficult). However, the wood had the usual wild Davis grain, and the apparatus will soon be returned to its original finish.
| The simplest possible interrupter for direct current is
a mechanical make and break produced when the end of the wire is dragged
along the end of a rasp: a bar of brass with serrations cut across it.
This was proposed by E. M. Clarke in 1837 and used by Joseph Henry. This
example is at Middlebury College and cost $12.00 to $18.00, depending on
the amount of wire in the outer helix.
The inner helix has about 75 ft of coarse copper wire, while the outer helix is wound with one to three thousand feet of fine wire. The core of iron wires can be removed and replaced with a solid iron rod, resulting in a decrease in the intensity of the shocks produced by the instrument.
The two pairs of separable helices below have rudimentary
electromagnetic make-and-break arrangements on the right-hand side. The
apparatus at the left is from Transylvania University and at the right
is one from Grinnell College.
|This pair of separable helices at Dartmouth College has lost the covering material to its removable coil. However, the general shape, plus the spherical feet, mark it as a product of Daniel Davis. In its original form, it looked like the example from Middlebury College at the top of this page.|
| The Rolls-Royce of separable helices is this Separable
Helix and Electrotome in the Smithsonian Institution collection. In the
Manual of Magnetism this is listed at $20.00 to $25.00. The
texts notes that "If a voltaic pair, consisting of a silver dollar and
a piece of zinc the same size be used [as a source of EMF], and the helix
filled with soft iron wires, the shock is quite severe."
This instrument is provided with a clockwork electrotome
or interrupter, as well as a rasp. All of the elements of Davis design
are here: the circular ball feet, the turned brass finials and the wooden
base with bold grain.