ROYAL SOCIETY MODEL.

The base is a piece of Alusett extrusion 50x150 mm. and 1.5 m. long. It was 3 m. long but was too difficult to transport in ordinary cars. It has four small plastic feet stuck on underneath, two close together on a spacer plate about 370 mm. from one end, and two at the ends of a crossbar about 300 mm. long for stability, the same distance from the other

Fringe camera is Baxall CD 6242 , it has a microscope objective 10x/.25 the back of which is 60 mm. from the front of the camera body. There is a dark red filter behind the lens, and then a tube to the camera.

The main lens is f=96 mm. unequal bi-convex meant as a laser beam expander and it's centre is 110 mm. from the front of the objective.

The rotating hole array consists of three holes 0.7 mm. in diameter in a brass disk soldered to the front of an 8 mm. O.D. brass tube which rotates in two 8x16 ball races mounted in an aluminium block milled away between the bearings for the drive belt. The belt is an 0.08 in. pitch miniature timing belt, and the pulley is plastic, the smallest size with flanges which will fit on 8 mm. diameter. It is driven by a slightly larger pulley on a small DC motor/tacho unit with a proper speed controller, but that is probably overkill - I just had it to hand. The hole centres are 2.1 , 3.4 , and 5.3 mm. which just clear the inside of the tube.

Aperture camera which looks at the rotating holes is Baxall CD 9230 - we first had another CD 6242, but the 9230 is smaller lighter and cheaper. It might even be sensitive enough for the fringes, but we haven't tried. The lens is a standard TV lens f=50 mm. and looks at the holes from about 300 mm. away.

"Atmosphere" is a disk of clear thin plastic mounted on a small geared DC motor which rotates at a few rpm. It is about 500 mm. from the hole array.

"Star". The star is made in the same way as the hole array, but with a knurled black plastic knob pressed on so it can be rotated by hand in a plastic bush on the front of the lamp house. The holes were 0.4 mm. at 1 mm. centres drilled in a brass disc, but now that the model has been shortened they are 0.2 mm. diameter at 0.5 mm. centres made by photography and glued on.

Lamp is a 6v 10w quartz halogen with a small condenser lens ( unknown spec. ! ) mounted in a diecast box with a plastic bush on the front to hold the star. The condenser focusses an image of the lamp filament on the hole array when the star is removed.

Quite a bit of trial and error was involved in getting acceptable results, but have a go and let us know how you get on!

All the best!

Donald.

Peter Lawson
Updated 26 October 2001

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