Foucault Testing and Theory
Foucault Testing Device. The requisites are a light source
contained in an opaque shield in which there is a pinhole perforation, a knife-edge, and a scale for making measurements
illustrates a practical testing device. The baseboard is a flat well-seasoned board, such as ¾"-thick plywood, about 6" x 9" in size. The guide cleat, a straight-edged stick 1" wide and 6" long, is attached to the baseboard about 2½" in from one edge.
Fastened to this cleat in a suitable position is a strip of metal or plastic material on which has been scribed the scale. The scale should be made under a reading glass for best accuracy. The horizontal lines are spaced 0.1" apart, and the vertical lines 0.2" apart. The diagonal lines divide the verticals into fiftieths of an
inch, and interpolation can be made for readings to hundredths of an inch.
The knife-edge, of rectangular shape, and with a perfectly smooth edge, is cut from a thin piece of sheet metal, and tacked to a block of wood about 1½" x 3" x 3" in size. If the lower part of the piece of sheet metal is cut and bent at right angles, and attached to the block so that this edge just skims the surface of the scale, it will also serve as the indicator. This indicator edge must be exactly parallel to the horizontal divisions of the scale when the block bears against the cleat.
For the light source, an ordinary 50-walt frosted lamp and socket are fastened to a 4" x 4" block of wood. A section of small stovepipe, or a tall can with both ends cut away, is set over the lamp. A "window" or hole about 1/8 in diameter is drilled into the side of the can at a height opposite the bright spot of the lamp. A band of thin sheet metal, containing two or three pinholes of different sizes, is clamped around the can so that it can be slipped down over the window.
Two pin-holes, placed one above the other and about 14" apart, will suffice. To make these, thin an area of the metal band with a file and emery paper until it is only one or two thousandths of an inch thick; lay it on a piece of glass, and, twirling a needle between the finger tips, pierce the first hole. Stop when the needle just breaks through, and examine the hole against a light with the magnifier to see that it is perfectly round. Any burrs should be removed with fine emery paper.
For a finer hole, sharpen the needle on a fine stone, twirling and drawing it out at the same time under the finger tip, until it appears perfectly sharp under the magnifier. Pierce the hole as before, using less pressure. Holes about 0.005" to 0.010" in diameter are probably most satisfactory, but the size will depend on the amount of illumination used, which can, of course, be concentrated and intensified on the pinhole through the introduction of a condensing lens. Too small a pinhole may not admit sufficient light; a large hole is lacking in sensitivity. Foucault used a pinhole 1/12 of a millimeter (about 1/300 of an inch) in diameter.
To avoid discomfort from the heat of the lamp, and to bring the pinhole and knife-edge into closer proximity, a small right-angle prism may be used. Instead of the window of the lamp-container facing the mirror, turn it 90° to the left, so that it faces the knife-edge, and mount the prism as in the picture, on a block of wood, directly in front of the window or pinhole opening. A small chipped prism can be picked up for a few cents from dealers in optical goods.
A great many rulinemenls arc often added in an effort to increase the efficiency of this device, including an arc-light source, condensing lenses, extremely small pinholes or an adjustable slit, and knife-edge motions controlled in two directions by micrometer screws. But the seemingly crude device described should enable the reader to produce a mirror capable of the finest possible definition.
Of course, the accuracy of the scale and one's ability to read it accurately determine its worth. Nearly as simple to construct, and requiring no scale, is a small table, on which the knife-edge can be mounted, that rolls between
guides on three ball bearings. A screw, which substitutes for the scale, controls the motion, with rubber bands or tension springs taking up the slack by an opposing pull. As the tension will have a tendency to lift the table from the ball bearings, the table should be weighted. Almost any thumbscrew will be suitable, or a piece of threaded rod into one end of which a pin is inserted at right angles, making a T-handle.
The number of turns and the number of threads per inch determine the extent of longitudinal motion. For example, consider a screw having 24 threads per inch, a standard number. This may be regarded as approximately 25 per inch, with a single revolution equal to about 0.04", and a quarter revolution (easily estimated
from the change in position of the T-handle), about 0.01". Two and a quarter turns of a 24-thread screw will therefore yield an exact measurement of 0.094", the full amount of correction called for on the 6-inch f/8 mirror (see Chapter VI for an explanation of correction), probably with a greater degree of accuracy than could be obtained from the first device.
Foucault Theory
