| If you unwrap a helix, you get an inclined
plane, as shown by this model in the University Museum at the University
The earliest reference to this demonstration I have found
is in the lectures of Hauksbee and Whiston, published in 1713 in London.
They wrote "It is a Paper Wedge, ..., coiled round a Cylinder, and so representing
a Screw; and shows that its force must be increas'd in Proportion to the
Progress along its Cylinder, when it is compar'd with the Circumferences
on the same Cylindrical Surface. (from A Course of Mechanical, Optical,
Hydrostatical and Pneumatical Experiments, To be perform'd by Francis Hauksbee;
and the Explanatory Lectures read by William Whiston.)
| Students at Denison University probably made this model
in the eighteen nineties as part of their practical laboratory work. It
is copied directly from a piece of apparatus sold by Benjamin Pike, Jr.
of New York, and shown in the 1856 first volume of his Illustrated Descriptive
Catalogue of Optical, Mathematical and Philosophical Instruments. The
accompanying text points out that the length of the [missing] handle on
the screw increases the mechanical advantage, and gives a numerical example
to show how a one pound force exerted tangentially on the handle can raise
a weight of 152 pounds.
The brass helix in the form of the thread of a screw is used to explain the screw as an inclined plane, used in connection with the lever.
| This is another student-built helix from Denison University.
It does show the geometrical shape, and is large enough to allow a direct
measurement of the net distance around its outer edge from the bottom to
the top. Dividing this number by the vertical rise gives the ideal mechanical
advantage of the device.
| The demonstration screw at the right is in regular use
at the physics lecture halls at the University of Texas in Austin. Some
of the original wood can be seen on the left side of the front of the base.
Fortunately, the device could be brought nearly back to its condition when
it was produced in the workshop of an unknown American apparatus manufacturer
in the early years of the twentieth century.