Old Sparky.


A Wimshurst Machine



A Wimshurst Machine

The Wimshurst Machine consists of two varnished glass plates revolving in opposite directions. On the outside of each of these plates are cemented a number of tinfoil "sectors" arranged radially. Two conductors at right angles to each other extend obliquely across the plates, one at the back and the other at the front. These conductors each terminate in brushes of tinsel which electrically excite the "sectors" as the plates revolve. The electricity is collected by a set of "collectors" arranged as shown in Fig. 42. The Glass Plates are each eighteen inches in diameter. Purchase two panes of clear glass twenty inches square from a glass-dealer. The white glass is far preferable to the green glass and will make the best electric machine. The plates should be of the thickness known as "single light" and should be perfectly free from wavy places, bubbles, or other imperfections.

The work is first laid out using a piece of stiff paper twenty inches square as a pattern. Describe a circle four inches in diameter. Using the same centre, draw other circles, making them respectively eight, sixteen, and eighteen inches in diameter. Then mark sixteen radial lines, from the centre, making them equal distances apart, as shown in Fig. 33. Lay one of the glass panels over the pattern and cut out a glass circle eighteen inches in diameter, or perhaps you may be able to get a glazier to do the cutting for you and so save considerable trouble and possible breakage. Two such plates should be made.

The sectors are cut from heavy flat tinfoil according to the pattern shown in Fig. 35. They should be made one inch and one-half wide at the wide end and three-quarters of an inch at the other end. They are each four inches long. Thirty-two such sectors are required. The easiest way to make them is to cut out a pattern from heavy cardboard to serve as a guide. Clean and dry both the glass plates very carefully and then give them each two thin coats of white shellac. After they have dried, lay one of the plates on the paper pattern so that the outside of the plate will coincide with the largest circle on the paper. Then place a weight in the centre of the plate so that it will not move, and stick sixteen of the tinfoil sectors on the plate with thick shellac. The sectors are arranged symmetrically on the plate, using the eight-inch and sixteen-inch circles and the radial lines as guides. Both plates should be treated in this manner. Each sector should be carefully pressed down on the glass, so that it will stick smoothly without air-bubbles or creases. When all sectors are in place the plates will appear like that shown in Fig. 35. The Bosses will have to be turned out at a wood-working mill or some place where they have a turning-lathe. The bosses are four inches in diameter at the large end and one inch and one-half at the other. A groove is turned near the small end of each to accommo- date a round leather belt. A hole should be made in each boss about half-way through the small end. These holes should be bushed with a piece brass tubing having an inside diameter of one-half inch. The tubing should go into the hole very snugly and be a "driven fit." As shown in Fig. 36. The bosses should both be given a coat of shellac, and after this is dry, fastened to the glass plates on the same side to which the foil sectors are attached. The best plan is to lay the disks on the paper pattern and adjust them until the outer edge coincides with the largest circle. Then apply some bichromate glue to the flat surface of one of the bosses and place the latter in the centre of the plate in line with the smallest circle. Place a weight on the boss to hold it down firmly against the plate and leave it overnight, or for ten or twelve hours, until thoroughly dry.

The glue is prepared by placing some good quality glue in a tin cup and covering it with cold water. Allow it to stand until the glue absorbs all the water it will and becomes soft. Then pour the water off and add enough glacial acetic acid to cover the glue. Heat the mixture until it is reduced to a liquid, stirring it until it is perfectly smooth. Add a teaspoonful of powdered bichromate of potash to the glue. The glue must now be kept in the dark, for sunlight will "set" the glue so that it becomes insoluble.

The Frame of the machine is composed of two strips twenty five inches long, three inches wide, and an inch and one-half in thickness, and two cross-pieces of the same thickness and fifteen inches wide. Notches are cut at both sides of the base to admit the feet of the uprights. The Uprights are seventeen inches long, three inches wide, and one and one-half inches thick. The notch at the foot is cut the same width as the thickness of the long members of the frame, and is arranged so that when fitted in place the foot of the upright will rest on the table in line with the bottom of the cross-pieces. As shown in Fig. 37. The Driving-wheels are turned out of wood on a lathe. They are seven inches in diameter and seven-eighths of an inch thick. A groove should be turned in the edge to carry a small round leather belt. The wheels are mounted on a wooden axle made from a round curtain-pole. They are glued to the axle and arranged so that the grooves will fall directly underneath the pulleys turned in the bosses. The ends of the axle pass through the uprights, five inches above the bottom.As shown in Fig. 39. The front end of the axle is fitted with a crank and a handle. The plates are mounted on short iron axles passing through the top of the upright into the brass bushings. One end of each of the axles is filed flat where it passes through the wood upright, so that it may be held firmly by a set-screw and prevented from revolving. Fasten a small fibre washer to the centre of one glass disk so that it will separate the plates and prevent them from touching when revolving.

The Collectors, quadrant rods, etc., are mounted on glass rods one inch in diameter. The bottoms of the rods fit in holes "(H H)" bored in the cross-pieces of the base (Fig. 37). The upper ends are each fitted with a brass ball two inches in diameter. The balls are mounted on the rods by soldering a piece of brass tubing to each ball and slipping it over the rod. The rods should be of the proper length to bring the centre of the balls on a line with the centre of the plates. (Fig. 41). Make two forks, as shown in Fig. 42. , out of brass rod, three- sixteenths of an inch in diameter, and solder brass balls at the ends. The forks are eleven inches long. A number of small holes must be bored in the "prongs," and pins made by cutting ordinary dressmakers' pins in half and soldering them in place. These pins, mounted on the forks, form the combs, or collectors. Bore a horizontal hole through each of the brass balls on the tops of the glass rods and pass the shanks of the forks through and solder them in place. One of the shanks may be provided with a discharge ball at the end, as shown by "D B" in The other is provided with a hard rubber handle made from a piece of rod. Bore a three-eighths of an inch hole directly in the top of each brass ball to receive the quadrant rods forming the spark gap.

The quadrant rods extend over the top of the plates and are three- eighths of an inch in diameter. They are loose in the tops of the balls so that they may be moved about or removed entirely. A small brass ball three-quarters of an inch in diameter should be soldered to the top of one of the quadrant rods, and a similar ball two inches in diameter to the other. Fig. 41. Two large brass balls, two inches in diameter, are fitted over the ends of the axles, which project through the uprights. Bore a one quarter-inch hole through each ball at right angles to the axle and slip a one-quarter-inch brass rod through and solder it fast. Fig. 43. The ends of the rods should be tipped with a bunch of tinsel or fine copper wires and be curved so that the brushes will just touch the sectors on the disks when the latter are revolved. These are the neutralisers, and are arranged in the approximate positions shown in Fig. 44. The driving-wheels are connected to the bosses by means of small round leather belts. The belt at the rear of the machine is crossed in order to make the plates revolve in opposite directions. If the machine has been properly built it is now ready for operation. It may be necessary to charge the machine the first time that it is used, by touching several of the sectors with the charged cover of an electrophorus. Then if the handle is turned the accumulated electricity should discharge across the spark-gap at the top of the machine in the form of bright blue sparks.



APPENDUM...
The above text comes from an old text book called..
The Boy Electrician.
Written by J. W. Simms M.I.E.E. M.I.Mech.E.
Senior Lecturer Electrical Engineering Department
Imperial College of Science and Technology South Kensington.
First published by George G Harrup & Co. Ltd. July 1920.


Tips for users of modern materials....

1: Flake shellac is sold at many hardware stores, it is dissolved in methylated spirits and best applied with a lint free pad.

2: A good substitute for the bichromate glue is "high strength" Araldite. Don't be tempted to use the five minute type as it will fail.

3: The brass balls are virtually unobtainable, the only option if you really want brass balls is to turn them your self. If you do machine them, they must be smooth as static electricity tends to accumulate at any irregular or raised points. (point discharge). I dispensed with brass balls and used discarded ball bearings from a coal dredge. These have to be heat treated in the following way in order to drill them.... First up fill a bucket with lime, (I used garden lime). Next, using an oxy. or blow torch, heat the ball to close to white heat, maintaining the temperature for about four minutes to ensure the ball is heated right through. Then keeping the flame on the ball as much as possible, bury it in the lime. Do this for all balls and do all balls in one batch so as all balls are in the lime at the one time. This ensures very slow cooling which is what is required to soften them. Even using this process you may encounter balls that are still fairly hard and require a slow drill speed, or in some cases further heat treatment.

4: The glass rods are also hard to come by and I in my wisdom decided to use plastic conduit "disaster"!!! The conduit is too whippy and the collector pins hit the rotating plates. I overcame this by filling the conduit with an epoxy filling resin. This was only partly successful. I recommend that some other method is used. A solid high insulating rod of some kind. It should be noted also that the better the insulating materials used, the better the machine will perform.

5: For the brushes use slot car motor brushes soldered to the neutraliser rods. These are obtainable from most hobby shops.

6: For the collector pins use map pins. The type with round coloured plastic heads. The drilling of the holes for these pins is very fiddly and requires a very high speed drill, because of the small drill size. Again these drills are not a standard hardware shop item and may have to be specially ordered.

7: The material for the sectors could just be alfoil. I thought this would be a bit flimsy and cut up some of those disposable aluminium pie plates and they proved quite successful.

8: Ensure that the wood used for the frame is very dry and evenly coated with shellac as this also affects the performance of the machine.

9: For the belt I tried large rubber "O" rings, no good, so don't bother. They slip too much. The best material I found to be is a circular section synthetic belting that I obtained from a shop that sold farm equipment (pumps etc.). Unfortunately I can't remember what it was called. I can only attempt to describe it as amber in colour and about 5/16 inches in diameter. It was be joined by heating the ends and pressing them together to make a butt joint. This joint was as strong as the parent material. I am sure if you can obtain round section leather belting this will do the job.



GETTING THE THING GOING...

I have built a machine of this design and it worked like a beauty, at times throwing sparks up to 100 mm long. I say at times, because a few things can affect the operation of the machine. Most of all is humidity. On a dry day the machine should fire up as soon as you turn the crank. On a humid day you can crank away and achieve nothing. Under these circumstances a hair drier works wonders, just play it around the plates and the insulators and your machine will arc up. Another thing I noticed (and never did work out why) is that the machine I built would only work if turned in one direction. So if it does not arc up, try rotating in the other direction. The position of the neutraliser rods on the sectors also affects operation and these should be adjusted for maximum spark.

Using the rule of thumb of a million volts to the metre for a spark gap I reckon that my machine could generate about 100 kV.

NB: This voltage is in no way harmful as the current generated is very small. In fact some of the most impressive effects you can see with this machine are seen when you bring yourself into contact with it.



Download Assembly Instructions. (157kB)


NOW GO BUILD THE THING!




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